Knowledge

The Big Bang

About 13.8 billion years ago, everything in the observable universe was packed into a state hotter and denser than anything that exists today. The Big Bang was not an explosion in space, like a firework bursting in a dark room. It was the expansion of space itself, and it happened everywhere at once. Ask where the Big Bang took place and the honest answer is: right here, and everywhere else too.

In the first sliver of a second, most cosmologists think the universe went through inflation, a burst of runaway expansion that stretched a region far smaller than an atom to astronomical size. As it went on expanding and cooling, energy condensed into particles. Within about a second, protons and neutrons had formed. Between roughly three and twenty minutes, they fused into the nuclei of the lightest elements: about three-quarters hydrogen and one-quarter helium by mass, with a trace of lithium. Everything heavier, from the carbon in your cells to the iron in your blood, would have to wait for stars.

For the next 380,000 years the universe was a glowing fog of charged particles that scattered light the way a cloud scatters sunlight. When the temperature fell to about 3,000 kelvin, electrons settled into atoms and the fog cleared. The light released at that moment is still travelling. Stretched by the expansion of space into microwaves, it arrives from every direction as the cosmic microwave background, now just 2.7 degrees above absolute zero. Arno Penzias and Robert Wilson found it by accident in 1965 while trying to remove a stubborn hiss from a radio antenna in New Jersey.

Did you know? A small part of the static on an old analogue television tuned between channels was the afterglow of the Big Bang.

Two further clues hold the theory up. In 1929 Edwin Hubble showed that distant galaxies are moving away from us, and that the farther away they are, the faster they recede, like dots on an inflating balloon drifting apart. The Belgian priest and physicist Georges Lemaître had predicted exactly this two years earlier. And the measured amounts of hydrogen and helium in the oldest stars match the predictions for those first minutes remarkably well.

The name came from the astronomer Fred Hoyle on BBC radio in 1949. He favoured a rival "steady state" universe with no beginning, and the phrase is often called a jibe, although Hoyle said he meant it only to be vivid. What came before remains an open question, and it may not even be a meaningful one if time itself began then. Since 1998 astronomers have known that the expansion is speeding up, driven by an unexplained dark energy. The universe began with a bang, and it has not stopped growing since.

Galaxies

A galaxy is a vast system of stars, gas, dust and invisible dark matter, all held together by gravity. Our own, the Milky Way, contains somewhere between 100 and 400 billion stars arranged in a flattened disc roughly 100,000 light-years across. On a dark night, away from street lights, its pale band arcs across the sky. The ancient Greeks saw spilt milk, which is why the word galaxy comes from gala, Greek for milk.

The Sun sits in a quiet suburb about 26,000 light-years from the centre and takes around 230 million years to complete one orbit. The last time Earth was in its present position, the first dinosaurs were just appearing. At the very heart of the galaxy lies Sagittarius A*, a black hole of about four million solar masses, photographed for the first time by the Event Horizon Telescope in 2022.

In 1926 Edwin Hubble sorted galaxies by shape, and his scheme is still used. Elliptical galaxies are smooth, rugby-ball swarms of mostly old, reddish stars. Spiral galaxies have bright arms where new stars are being born, and many, including the Milky Way, have a straight bar of stars across the middle. Irregular galaxies, such as the two Magellanic Clouds visible from the southern hemisphere, have no tidy form at all.

Did you know? The Andromeda Galaxy can be seen with the naked eye, and the light reaching you left it about 2.5 million years ago, long before modern humans existed.

Galaxies are not scattered evenly. The Milky Way, Andromeda and dozens of smaller galaxies form the Local Group, which belongs to larger clusters and superclusters strung along vast filaments around nearly empty voids. The observable universe holds hundreds of billions of galaxies at the very least, and some estimates run far higher.

Galaxies also hold the strongest evidence for dark matter. In the 1970s the American astronomer Vera Rubin measured how fast stars orbit in spiral galaxies and found that the outer stars move far too quickly for the visible matter to hold them. Something unseen, outweighing ordinary matter roughly five to one, must supply the extra gravity. What it is made of remains unknown.

Galaxies collide, too. Because they are mostly empty space, their stars almost never hit one another; instead gravity reshapes both into a new, larger galaxy. Andromeda is approaching at about 110 kilometres per second, and the two may merge in roughly five billion years, though the latest measurements suggest the outcome is less certain than once thought. Even islands of stars drift.

Stars

A star is a giant ball of hot gas that makes its own light by nuclear fusion. Every one begins inside a cold, dark cloud of gas and dust called a nebula. When part of the cloud is squeezed, perhaps by the shock wave from a nearby exploding star, gravity takes over and the clump collapses, heating up as it shrinks. The Orion Nebula, about 1,300 light-years away and just visible to the naked eye, is a stellar nursery doing exactly this today.

When the core of the collapsing clump reaches around 10 million degrees, hydrogen nuclei begin to fuse into helium, releasing energy. A star is born. For most of its life it holds a delicate balance: gravity tries to crush it inward while the pressure of hot gas and light pushes outward. Astronomers call this long, stable stage the main sequence.

Mass decides everything. Small, cool red dwarfs sip their fuel so slowly they could shine for trillions of years, far longer than the universe has existed. The Sun will last about 10 billion years in total. A star 20 times heavier blazes through its fuel in only a few million. Colour reveals temperature: red stars have surfaces near 3,000 °C, while blue-white giants top 20,000 °C.

When the hydrogen in the core runs low, a star like the Sun swells into a red giant and begins fusing helium into carbon and oxygen. Eventually it puffs its outer layers into space as a glowing shell, misleadingly named a planetary nebula, leaving behind a white dwarf about the size of Earth. Stars of more than about eight solar masses go further, fusing ever heavier elements in onion-like layers until the core is iron. Fusing iron absorbs energy rather than releasing it, so the core collapses in less than a second and the star explodes as a supernova, leaving a neutron star or a black hole.

Did you know? Stars do not really twinkle. The flicker comes from turbulence in Earth's atmosphere bending their light, which is why astronauts in orbit see steady points.

These deaths are how the universe gets its chemistry. The carbon in your cells and the oxygen in every breath were forged inside stars that died before the Sun was born. Supernovae and the violent collisions of neutron stars, one of which was observed in 2017, scatter heavier elements such as gold and uranium through space, ready for the next generation of stars and planets. People are, quite literally, made of recycled starlight.

The Sun

The Sun is an ordinary star that happens to be very close. It is about 150 million kilometres away, so its light takes around eight minutes and twenty seconds to reach us. It is 109 times wider than Earth, and it holds about 99.8 per cent of all the mass in the Solar System. Formed around 4.6 billion years ago, it is roughly halfway through its life.

Its power comes from the core, where the temperature is about 15 million °C and the pressure is immense. There, every second, the Sun fuses around 600 million tonnes of hydrogen into helium. A few million tonnes of that mass vanish each second, converted into energy exactly as Einstein's equation E = mc² describes.

That energy then makes a remarkably slow escape. In the radiative zone, light is absorbed and re-emitted countless times, zigzagging so aimlessly that the journey can take many thousands of years. Farther out, in the convection zone, the gas boils like porridge in a pan: hot plumes rise, cool and sink again. Their tops show on the visible surface, the photosphere, as a pattern of granules, many of them about as wide as a small country, at a temperature of about 5,500 °C.

Did you know? The Sun is white, not yellow. Earth's atmosphere scatters away some blue light, which makes it look yellowish from the ground; astronauts in space see it as brilliant white.

The surface is marked by sunspots, cooler and darker patches where tangled magnetic fields hold back the heat. Their numbers rise and fall over a cycle of about 11 years; the current cycle peaked around late 2024. Above the photosphere lies the thin chromosphere, then the corona, a wispy outer atmosphere seen during a total solar eclipse. Strangely, the corona is hotter than the surface, above a million degrees, and exactly why is still debated.

From the corona streams the solar wind, a flow of charged particles that blows past every planet. Large eruptions can disturb Earth's magnetic field, triggering auroras and threatening satellites and power grids. The Carrington Event of 1859 set telegraph wires sparking. In December 2024 NASA's Parker Solar Probe passed about 6.1 million kilometres from the surface, closer than any spacecraft before it.

In about five billion years the Sun will run short of hydrogen in its core and swell into a red giant, probably engulfing Mercury and Venus. Until then it will go on doing what it has done since before Earth existed, quietly powering almost all life on the planet.

The Planets

Around 4.6 billion years ago, a spinning disc of gas and dust surrounded the newborn Sun. Grains stuck together into pebbles, pebbles into boulders, and boulders into planetesimals that collided and merged. Close to the Sun it was too hot for ice, so only rock and metal could gather, making small worlds. Beyond a boundary called the frost line, ice was plentiful, and cores grew big enough to sweep up huge envelopes of gas.

The result is two families. The four inner rocky planets are Mercury, Venus, Earth and Mars. Mercury is the smallest and fastest, circling the Sun in 88 days. Venus is the hottest planet, at about 465 °C, even though Mercury is closer: its thick carbon dioxide atmosphere traps heat in a runaway greenhouse effect. Mars, half Earth's width, has the tallest volcano known, Olympus Mons, more than twice the height of Everest.

Did you know? A day on Venus lasts longer than its year: it spins once in 243 Earth days but orbits the Sun in about 225.

Past the asteroid belt come the giant planets. Jupiter is more than twice as massive as all the other planets combined, and its Great Red Spot is a storm wider than Earth that has raged for at least two centuries. Saturn, famous for rings made mostly of water ice, is the least dense planet, lighter overall than the same volume of water, and holds the record for known moons, with well over two hundred. These two are gas giants, made mainly of hydrogen and helium.

The outer pair, Uranus and Neptune, are called ice giants because much of their interior is water, ammonia and methane. Uranus is tipped on its side, perhaps by an ancient collision, so each pole gets about 42 years of continuous sunlight followed by 42 of darkness. Neptune was found in 1846 by mathematics before telescopes: Urbain Le Verrier predicted its position from wobbles in Uranus's orbit, and Johann Galle spotted it within a degree of the forecast.

Pluto was counted as the ninth planet from its discovery in 1930 until 2006, when the International Astronomical Union ruled that a planet must also clear its orbit of other debris. Pluto shares its path with the Kuiper belt, so it became a dwarf planet.

The distances are hard to imagine. If the Sun were the size of a front door, Earth would be a pea about 200 metres away, and Neptune would orbit some six kilometres off. The Solar System is overwhelmingly empty space, with eight bright exceptions.

The Moon

The Moon is 3,474 kilometres across, a little over a quarter the width of Earth, which makes it unusually large compared with its planet. It orbits at an average distance of about 384,400 kilometres, so roughly thirty Earths could line up in the gap. Most scientists think it formed about 4.5 billion years ago in the giant impact, when a body about the size of Mars, often called Theia, struck the young Earth and threw out a ring of molten debris that gathered into a moon.

The Moon always shows the same face to Earth because it spins exactly once for every orbit, a state called tidal locking. The far side is often called the dark side, but that is a myth: it receives just as much sunlight, only never towards us. The phases happen because we see different amounts of the sunlit half as the Moon travels around us, from new moon to full and back again in about 29.5 days.

The dark patches that make the "man in the Moon" are maria, Latin for seas. Galileo saw them through his telescope in 1609, but they hold no water: they are plains of hardened lava from eruptions more than three billion years ago. The brighter highlands are older still and battered by craters, preserved because the Moon has no weather to wear them away.

Did you know? The Sun is about 400 times wider than the Moon and also about 400 times farther away, which is why the Moon can cover it so exactly in a total solar eclipse.

The Moon's gravity raises the tides. It pulls hardest on the side of Earth facing it and least on the far side, stretching the oceans into two bulges. As Earth rotates through them, most coasts get two high tides roughly every 24 hours and 50 minutes. When Sun and Moon line up, at new and full moon, their pulls combine into extra-large spring tides.

That tidal tug works both ways. It is slowly braking Earth's spin, lengthening the day, and pushing the Moon outward by about 3.8 centimetres a year, measured by bouncing lasers off mirrors left by Apollo astronauts. Twelve men walked on the Moon between 1969 and 1972 and brought home 382 kilograms of rock. In 2024 China's Chang'e 6 returned the first samples from the far side, and water ice in permanently shadowed polar craters has made the south pole the target for the next human landings. The Moon is drifting away, but only very slowly.

Black Holes

A black hole is a region of space where matter is packed so tightly that its gravity lets nothing escape, not even light. The boundary of no return is the event horizon. Outside it, an object can still break free if it moves fast enough; inside, every possible path leads inward. In 1916, months after Einstein published his theory of general relativity, the German physicist Karl Schwarzschild worked out the size of this horizon for any given mass.

The numbers are startling. To turn the Sun into a black hole, it would have to be crushed into a ball about six kilometres across. Earth would need to be squeezed to the size of a marble. Nature manages this when a very massive star dies. Once its nuclear fuel is spent, nothing can hold its core up, and it collapses into a stellar black hole a few to a few dozen times the mass of the Sun. The first strong candidate, Cygnus X-1, was identified in the early 1970s from the X-rays of gas falling into it.

At the centres of most large galaxies sit supermassive black holes, millions to billions of times the Sun's mass. The Milky Way's, Sagittarius A*, is about four million solar masses. In 2019 the Event Horizon Telescope, a network of radio dishes spanning the globe, released the first image of a black hole's shadow: the giant in galaxy M87, about 6.5 billion times the Sun's mass.

Did you know? If the Sun were replaced by a black hole of exactly the same mass, Earth would carry on orbiting just as before. Black holes do not suck things in like cosmic vacuum cleaners.

A black hole itself is invisible, but its surroundings are not. Gas spiralling inward forms an accretion disc, heated by friction to millions of degrees, and some black holes fire narrow jets of particles outward at nearly the speed of light. The brightest of these, quasars, can outshine entire galaxies. On 14 September 2015 the LIGO detectors in the United States picked up gravitational waves, ripples in space itself, from two black holes of about 36 and 29 solar masses colliding some 1.3 billion light-years away.

Falling into a small black hole would be a grim affair: the pull on your feet would be so much stronger than on your head that you would be stretched thin, a process physicists cheerfully call spaghettification. In 1974 Stephen Hawking showed that black holes should slowly leak energy, now called Hawking radiation, though it is far too faint ever to have been detected. Even gravity's greatest victory may not last for ever.

Comets and Asteroids

When the planets formed 4.6 billion years ago, not all the building material was used. The leftovers still circle the Sun as asteroids and comets, and because many have barely changed since, they are time capsules from the birth of the Solar System.

Asteroids are rocky or metallic bodies, most of them in the main belt between Mars and Jupiter, where Jupiter's gravity stopped them from ever gathering into a planet. The first and largest, Ceres, about 940 kilometres across, was found by Giuseppe Piazzi in 1801 and is now classed as a dwarf planet. Science fiction likes to show the belt as a crowded field of tumbling boulders, but that is a myth. Its objects are typically hundreds of thousands of kilometres apart, and spacecraft cross it without having to dodge anything.

Comets are icy bodies from the cold outer Solar System, famously described by the astronomer Fred Whipple in 1950 as dirty snowballs. A comet's nucleus is usually only a few kilometres across. Short-period comets come mostly from the Kuiper belt beyond Neptune; long-period comets are thought to fall inward from the Oort cloud, a vast, never directly observed shell of icy bodies perhaps a light-year or more from the Sun. As a comet nears the Sun its ice turns to gas, swelling into a glowing coma and two tails: a curved one of dust and a straight blue one of ionised gas. Halley's Comet returns every 75 to 76 years and is next due in 2061.

Did you know? A comet's tails always point away from the Sun, pushed by sunlight and the solar wind, so a comet heading back out into space travels tail first.

A meteor, or shooting star, is a grain of dust burning up in the atmosphere; if a piece survives to the ground, it is a meteorite. The Perseid shower each August is Earth ploughing through dust shed by Comet Swift-Tuttle. Larger impacts are rare but decisive. About 66 million years ago an asteroid some 10 kilometres wide struck what is now Mexico's Yucatán Peninsula, at Chicxulub, helping to end the age of the dinosaurs.

Today astronomers track tens of thousands of near-Earth objects. In 2022 NASA's DART spacecraft deliberately rammed the small asteroid Dimorphos and shortened its orbit by about 32 minutes, the first test of planetary defence. Missions have landed on comet 67P (Rosetta, 2014) and returned samples of asteroids Ryugu and Bennu, which contain organic molecules including amino acids. The Solar System's rubble may also have helped deliver some of the ingredients for life.

Exoplanets

For centuries people wondered whether other stars had planets. The answer arrived only in the 1990s. In 1992 Aleksander Wolszczan and Dale Frail found planets orbiting a pulsar, the dead core of an exploded star. In 1995 Michel Mayor and Didier Queloz discovered 51 Pegasi b, the first planet found around a Sun-like star, work that won them a Nobel Prize in 2019. Today the count of confirmed exoplanets has passed 6,000, and statistical surveys suggest the Milky Way holds more planets than stars.

Exoplanets are almost impossible to see directly, because their stars outshine them millions or billions of times. Astronomers mostly detect them indirectly. The transit method watches for the tiny dip in a star's brightness when a planet passes in front of it; NASA's Kepler telescope (2009 to 2018) and its successor TESS have found thousands this way. The radial velocity method measures how a star wobbles as an orbiting planet tugs it to and fro, detected as a shift in the colour of its light. Rarer techniques include gravitational microlensing, when a star's gravity briefly magnifies a more distant star, and direct imaging, which blocks the starlight to reveal the faint planet beside it.

The first discoveries were a shock. 51 Pegasi b is a hot Jupiter, a gas giant orbiting so close to its star that its year lasts just over four days. Nothing like it exists in our Solar System, and its existence showed that planets can migrate far from where they formed.

Did you know? The most common kinds of planet found so far, sized between Earth and Neptune, have no example at all in our own Solar System.

Astronomers are especially interested in rocky planets in a star's habitable zone, the band of orbits where water could stay liquid on the surface. The nearest known exoplanet, Proxima Centauri b, found in 2016, orbits within the habitable zone of the closest star to the Sun, about 4.2 light-years away. In 2017 the TRAPPIST-1 system, around 40 light-years away, was found to host seven Earth-sized planets, several of them potentially temperate.

The next step is reading atmospheres. When a planet transits, a little starlight filters through its air, and the gases leave fingerprints in the spectrum. In 2022 the James Webb Space Telescope made the first clear detection of carbon dioxide in an exoplanet atmosphere, on the gas giant WASP-39b. Claims of possible signs of life on other worlds have so far proved contested. The question of whether we are alone has moved from philosophy into the observatory.

Exploring Space

Space is surprisingly close. By a widely used convention it begins at the Kármán line, 100 kilometres up, a distance you could drive in about an hour. The hard part is not height but speed. To stay in orbit, a spacecraft must travel sideways at about 7.8 kilometres per second, fast enough that as it falls towards Earth, the curved ground falls away beneath it. To escape Earth's gravity entirely takes 11.2 kilometres per second.

Reaching such speeds requires the rocket, which works by throwing mass backwards very fast so that the vehicle is pushed forwards, as Newton's third law describes. Because rockets must carry all their own fuel, most of their launch weight is propellant, and they shed empty stages as they climb. The Russian schoolteacher Konstantin Tsiolkovsky worked out the mathematics in 1903; Robert Goddard launched the first liquid-fuelled rocket in Massachusetts in 1926.

The Space Age began on 4 October 1957, when the Soviet Union launched Sputnik 1, a beeping metal sphere the size of a beach ball. On 12 April 1961 Yuri Gagarin became the first person in space, circling Earth once in 108 minutes, and in 1963 Valentina Tereshkova became the first woman. The race culminated on 20 July 1969, when Neil Armstrong and Buzz Aldrin of Apollo 11 walked on the Moon; by 1972 twelve astronauts had done so.

Did you know? Voyager 1, launched in 1977, is the most distant human-made object, more than 24 billion kilometres away, and still sends faint signals home on about the power of a refrigerator light bulb.

Robotic probes have since visited every planet. The Voyagers toured the giant planets, and Voyager 1 crossed into interstellar space in 2012. Rovers such as Curiosity and Perseverance explore Mars, and in 2015 New Horizons gave the first close look at Pluto. Space telescopes see what the atmosphere hides: Hubble, launched in 1990, and the James Webb Space Telescope, launched on Christmas Day 2021, which observes from 1.5 million kilometres away.

People now live in orbit permanently. The International Space Station, built by the United States, Russia, Europe, Japan and Canada, has been continuously crewed since November 2000. It circles about 400 kilometres up at roughly 28,000 kilometres per hour, so its crew sees about sixteen sunrises a day. China operates its own station, Tiangong.

Costs have fallen sharply since 2015, when SpaceX first landed an orbital-class booster so it could fly again. Reusable rockets, thousands of small satellites and NASA's Artemis programme, which aims to return astronauts to the Moon, have opened a new era. Exploration still begins the same way, with a controlled explosion pointed at the sky.

Inside the Earth

Dig straight down and, after 6,371 kilometres, you would reach the centre of the Earth. Nobody has come close. The deepest hole ever drilled, the Kola Superdeep Borehole in Arctic Russia, stopped in 1989 at 12,262 metres, where rock at around 180°C had become too hot and soft for the drill. Almost everything known about the deep Earth comes from indirect evidence, above all from earthquakes.

When a large earthquake strikes, its waves travel through the whole planet, bending and bouncing wherever the material changes. In 1909 the Croatian seismologist Andrija Mohorovičić noticed waves suddenly speeding up a few tens of kilometres down, marking the base of the crust, a boundary now called the Moho. Seismologists also found that one kind of wave, the S-wave, cannot pass through liquid, and that it vanishes on the far side of the planet. The conclusion: part of the core is molten. In 1936 the Danish seismologist Inge Lehmann spotted faint echoes revealing a solid ball inside the liquid.

The result is a layered planet, sorted roughly by density, like a jar of sand, water and oil left to settle. The crust is a thin, brittle skin: about 7 kilometres thick under the oceans and 30 to 70 kilometres under the continents. Scaled to an apple, it would be thinner than the peel. Below lies the mantle, stretching to a depth of about 2,900 kilometres and holding around 84% of Earth's volume. It is a common myth that the mantle is a sea of molten rock. It is almost entirely solid, yet so hot that over millions of years it creeps like very stiff toffee, slowly carrying heat upwards.

At 2,900 kilometres the rock gives way to the outer core, a churning ocean of liquid iron and nickel about 2,200 kilometres deep. As this metal swirls, it acts as a giant dynamo, generating the magnetic field that steers compass needles and shields the planet from the solar wind. At the very centre sits the inner core, a ball about 1,220 kilometres in radius, roughly 70% the size of the Moon. It is thought to be around 5,000 to 6,000°C, comparable to the surface of the Sun, yet it stays solid because the pressure there is more than three million times that at sea level.

Did you know? The Kola Superdeep Borehole, still the deepest hole humans have drilled, reached barely 0.2% of the way to the centre of the Earth.

The heat that drives all this comes partly from the planet's formation 4.5 billion years ago and partly from radioactive elements such as uranium and thorium decaying in the rock. That slow leak of inner warmth powers moving continents, volcanoes and the magnetic shield alike. The ground may feel still, but it rests on a planet that is very much alive.

Plate Tectonics

Look at a map of the Atlantic and the coasts of South America and Africa seem to fit like torn pieces of paper. In 1912 the German meteorologist Alfred Wegener argued that this was no coincidence: the continents had once been joined in a supercontinent he called Pangaea, and had since drifted apart. He pointed to matching fossils and rock layers on opposite shores. Most geologists dismissed him, because he could not explain what force could shove whole continents across the globe. He died on the Greenland ice in 1930, decades before he was proved right.

The answer came from the seafloor. After the Second World War, ships mapping the ocean bottom traced a mountain range running down the middle of the Atlantic, the Mid-Atlantic Ridge. In the early 1960s the American geologist Harry Hess proposed that new seafloor is made at such ridges, and soon afterwards scientists found stripes of rock along each side, magnetised in alternating directions as Earth's magnetic field flipped over time. The stripes were mirror images on either side of the ridge: proof that the seafloor was spreading like two conveyor belts.

The modern theory, plate tectonics, took shape by the late 1960s. Earth's rigid outer shell, the crust together with the top of the mantle, is broken into about a dozen large plates and many smaller ones. They ride on hotter, softer mantle rock beneath, moving between about 2 and 10 centimetres a year, roughly the speed at which fingernails grow. The action happens at the edges. At divergent boundaries, such as the ridge splitting Iceland, plates pull apart and magma wells up to fill the gap. At convergent boundaries they collide. Where dense ocean plate meets another plate, it dives beneath it in subduction, carving deep trenches like the Mariana Trench and feeding chains of volcanoes. Where two continents meet, neither sinks easily, so the crust crumples upwards. India's collision with Asia, beginning around 50 million years ago, raised the Himalaya and is still lifting it. At transform boundaries, like the San Andreas Fault in California, plates grind sideways past each other, storing strain that is released in earthquakes.

Did you know? The summit of Mount Everest is made of limestone that formed on the floor of an ancient sea, and it contains fossils of marine creatures.

Pangaea itself was only the latest of several supercontinents. It assembled more than 300 million years ago and began breaking apart about 200 million years ago, opening the Atlantic. The process has not stopped. The Atlantic is still widening by a few centimetres a year, while the Pacific is slowly shrinking. Geologists expect the continents to gather again in a few hundred million years. The map on the classroom wall is a single frame from a very long film.

Volcanoes

A volcano is an opening in Earth's crust through which molten rock, gas and ash escape from below. The molten rock, called magma while underground and lava once it reaches the surface, forms where the mantle partly melts. Being lighter than the solid rock around it, magma rises and collects in a magma chamber a few kilometres beneath the surface. When the pressure there grows great enough, it forces its way up a central pipe, or conduit, and bursts out of a vent.

The style of an eruption depends largely on how sticky the magma is and how much gas it holds. Runny, hot basalt lets gas bubble out gently, so Hawaiian volcanoes pour out rivers of lava at around 1,150°C that build broad, gently sloping shield volcanoes. Mauna Loa is the largest active volcano on Earth by volume. Thick, silica-rich magma traps its gas, like a shaken bottle of fizzy drink with the cap on, until it erupts explosively. These eruptions build steep, layered cones of ash and lava called stratovolcanoes, such as Mount Fuji, Vesuvius and Mount St Helens. Small cinder cones, built from loose fragments, can grow in a matter of weeks.

The deadliest dangers are often not lava flows, which usually move slowly enough to walk away from. Pyroclastic flows, avalanches of hot gas, ash and rock, can race downhill at well over 100 kilometres an hour. Such flows buried Pompeii and Herculaneum when Vesuvius erupted in AD 79. Volcanic mudflows called lahars, ash clouds that ground aircraft and gases that poison the air add to the hazard. Large eruptions can even cool the whole planet by injecting sulphur gases high into the stratosphere. After Tambora in Indonesia erupted in 1815, the most powerful eruption in recorded history, Europe and North America suffered 1816 as the "Year Without a Summer". Pinatubo in the Philippines lowered global temperatures by around half a degree after 1991.

Most volcanoes sit along plate boundaries. About three-quarters of the world's active volcanoes line the Ring of Fire around the Pacific, where ocean plates sink and melt. Others, like Iceland's, sit on spreading ridges. A few, like Hawaii, rise over hotspots, plumes of hot mantle that punch through the middle of a plate, building a chain of islands as the plate slides over them. Around 1,350 volcanoes on land are considered potentially active, and many more lie hidden under the sea.

Did you know? Measured from its base on the seabed, Hawaii's Mauna Kea is over 10,000 metres tall, higher than Mount Everest.

Volcanoes are destroyers, but also makers. Their gases helped to form the early atmosphere and oceans, and their ash weathers into some of the most fertile soils on Earth. The most common gas they release is not smoke at all, but water vapour. The planet really does let off steam.

Earthquakes

Rocks seem rigid, but under enough force they bend a little, like a stick flexed between two hands. Along a fault, a fracture where two blocks of crust meet, moving plates drag on the rock, but friction locks the two sides together. Strain builds for decades or centuries. Eventually the rock's strength is exceeded, the fault slips in seconds, and the stored energy is released as vibrations. This idea, called elastic rebound, was proposed by the American geologist Harry Fielding Reid after studying the San Francisco earthquake of 1906, when the ground on either side of the San Andreas Fault shifted by up to about 6 metres.

The point underground where the rupture begins is the focus, or hypocentre; the spot on the surface directly above it is the epicentre. From the focus, energy spreads as seismic waves. P-waves (primary waves) push and pull the rock like a stretched spring and travel fastest, at around 6 kilometres a second in the crust. S-waves (secondary waves) shake it from side to side and arrive later. Last come the surface waves, which roll along the ground and usually do the most damage. By timing the gap between P- and S-waves at three or more seismometers, scientists can pinpoint where an earthquake began.

Earthquake size is measured today on the moment magnitude scale, which has replaced the Richter scale, devised in 1935, for large quakes. The scale is logarithmic: each whole step means about ten times more ground motion and roughly 32 times more energy. A magnitude 7 releases about a thousand times the energy of a magnitude 5. Small tremors happen constantly, with many thousands detected every year, while a magnitude 8 or greater strikes about once a year on average. The largest ever recorded, magnitude 9.5, struck Valdivia in Chile in 1960.

The shaking itself rarely kills; falling buildings do. That is why strict building codes, flexible frames and foundations that isolate a structure from the ground save so many lives in places such as Japan and Chile. Undersea earthquakes bring another danger. When a fault lifts the seafloor, it shoves the ocean above into a tsunami. The magnitude 9.1 Indian Ocean earthquake of 2004 sent waves across an entire ocean, killing around 230,000 people in more than a dozen countries.

Did you know? The magnitude 9 Tōhoku earthquake of 2011 moved part of Japan's main island about 2.4 metres east and shortened Earth's day by an estimated 1.8 millionths of a second.

Despite popular claims, no one can yet predict the exact time and place of an earthquake. Scientists can, however, map faults, estimate how likely a strong quake is over the coming decades and, in some countries, send warnings seconds before the shaking arrives. Seconds are enough to stop a train or take cover under a table.

Rocks and Minerals

Every rock is a mixture of minerals, and every mineral is a naturally occurring solid with a definite chemical make-up and an orderly internal arrangement of atoms, usually a crystal. Around 6,000 minerals are known, but only a handful build most of the crust. Feldspars make up roughly half of it, and quartz, pure silicon dioxide, is among the commonest of the rest. Granite is essentially feldspar, quartz and mica locked together, and the speckles in a granite worktop are those three minerals side by side.

Minerals are identified by colour, lustre, the way they split and, above all, hardness. In 1812 the German mineralogist Friedrich Mohs ranked ten minerals by which could scratch which, from soft talc at 1 to diamond at 10. A fingernail sits at about 2.5, a steel knife at around 5.5. Diamond and graphite, the soft grey stuff in pencils, are both pure carbon; only the arrangement of their atoms differs.

Geologists sort rocks into three families according to how they formed. Igneous rocks crystallise from molten rock. Cooled slowly deep underground, magma grows large crystals and becomes granite; erupted and chilled quickly at the surface, it forms fine-grained basalt, the rock of the ocean floor. Sedimentary rocks form from fragments laid down in layers, usually under water, then squeezed and cemented over time. Sand becomes sandstone, mud becomes shale, and the shells and skeletons of sea creatures become limestone. These are the rocks that hold fossils. Metamorphic rocks are older rocks transformed by heat and pressure without melting. Shale becomes slate; limestone becomes marble, the stone of the Taj Mahal and of Michelangelo's David.

None of these states is permanent. The rock cycle links them in an endless loop driven by two engines: the Sun's energy at the surface and Earth's internal heat below. Wind, rain and ice wear mountains into grains, rivers carry them to the sea, and burial turns them into new rock. Plate movements drag rocks deep down, where they are cooked into metamorphic rocks or melted back into magma, and then lift them up again to be eroded once more. The Scottish geologist James Hutton read this cycle in the tilted layers of Siccar Point in 1788 and realised how immensely old the Earth must be.

Did you know? The White Cliffs of Dover are made of chalk, a limestone built from the skeletons of countless microscopic algae that lived in a warm sea more than 66 million years ago.

The cycle runs slowly, over millions of years, but it is never finished. The oldest known fragments of Earth, tiny zircon crystals from the Jack Hills of Western Australia, are about 4.4 billion years old. Every pebble on a beach is a moment in a story that began long before the first living thing.

The Water Cycle

Earth holds about 1.4 billion cubic kilometres of water, and almost none of it is new. The same water has been circulating for billions of years, endlessly moving between sea, sky, land and living things. This loop is the water cycle, and its engine is the Sun.

The journey usually begins at the sea, which holds about 97% of all Earth's water. Sunlight warms the surface and water molecules escape into the air as invisible vapour, a process called evaporation. The salt stays behind, which is why rain is fresh even though most of it started in the ocean. Plants add more vapour through transpiration, drawing water from the soil and releasing it through tiny pores in their leaves. A single large oak can give off hundreds of litres on a hot summer day.

Warm, moist air rises and, as it climbs, it expands and cools. Cool air holds less vapour, so the excess condenses onto specks of dust, salt or smoke, forming the tiny droplets and ice crystals that make up clouds. A typical cloud droplet is so small that around a million must merge to make a single raindrop. When drops or snowflakes grow heavy enough to fall, the result is precipitation: rain, snow, sleet or hail. On average, a water molecule spends only about nine days in the atmosphere before falling back to Earth.

What happens next depends on where it lands. Much of it falls straight back into the sea. On land, some flows over the surface as runoff, gathering into streams and rivers that carry it back to the ocean. Some soaks into the ground by infiltration, filling the spaces in soil and rock to become groundwater, stored in layers of porous rock called aquifers. Some is locked up in glaciers and ice sheets. The time spent in each store varies enormously: days or weeks in a river, a few thousand years in the deep ocean, and hundreds of thousands of years in the oldest Antarctic ice.

Did you know? Falling raindrops are not teardrop-shaped. Small ones are nearly round, and larger ones flatten into something like the top of a hamburger bun.

Only about 2.5% of Earth's water is fresh, and most of that is frozen in ice caps and glaciers or hidden underground. The lakes and rivers that people depend on hold well under 1% of the total. The cycle also moves heat around the planet: evaporation absorbs energy from the sea, and condensation releases it into the air, fuelling storms and hurricanes. Because the water is so old and so well travelled, a glass of it almost certainly contains molecules that have passed through clouds, glaciers and dinosaurs. Every drop is a veteran.

Oceans

Seen from space, Earth is a blue planet. The oceans cover about 71% of its surface and hold about 97% of its water, with an average depth of around 3,700 metres. Although people speak of five oceans, the Pacific, Atlantic, Indian, Southern and Arctic, they form one connected world ocean. The Pacific alone is larger than all the land on Earth put together.

The ocean is layered like a tall building with the lights switched off floor by floor. In the sunlit zone, the top 200 metres, there is enough light for photosynthesis. Here drift countless phytoplankton, microscopic algae that form the base of almost every marine food chain and produce a large share of the oxygen in the air, often estimated at around half. Below, in the twilight zone down to about 1,000 metres, light fades to a dim blue. Many animals there make their own light by bioluminescence, and every night vast numbers rise towards the surface to feed, then sink again at dawn, the largest daily migration on the planet. Beneath lies the midnight zone, pitch dark and near freezing, then the abyss, and finally the hadal zone of the deep trenches.

The deepest point of all is the Challenger Deep in the Mariana Trench in the western Pacific, nearly 11,000 metres down. Mount Everest could sink into it with more than 2 kilometres of water to spare. Jacques Piccard and Don Walsh first reached it in the bathyscaphe Trieste in 1960. Since 2019, a series of dives has taken more people to the Challenger Deep than have ever walked on the Moon. Even so, only a modest fraction of the seafloor has been mapped in fine detail.

The ocean is also the planet's great engine of heat. Water stores far more heat than air, and currents carry it around the globe. Surface currents such as the Gulf Stream, pushed by the winds, carry warm tropical water north-east across the Atlantic and help keep western Europe mild for its latitude. Deeper down, a slower circulation is driven by differences in temperature and saltiness. Near Greenland and Antarctica, cold, salty water becomes dense and sinks, then creeps along the seabed through the world's oceans. This global conveyor belt takes around a thousand years to complete a circuit.

Did you know? At the bottom of the Challenger Deep the water presses down with more than 1,000 times the pressure of the air at sea level.

The ocean also cushions the effects of a warming world. It has absorbed around 90% of the extra heat trapped by greenhouse gases and roughly a quarter of the carbon dioxide released by human activity. That help has a cost: warmer, more acidic water bleaches coral reefs and pushes species towards the poles. The ocean shapes the weather, the air and the food on billions of plates. It is the planet's largest living space, and it is still mostly unexplored.

Weather

Weather is the state of the air at a particular place and time: its temperature, pressure, wind, cloud and moisture. Almost all of it happens in the troposphere, the lowest layer of the atmosphere, which is about 8 kilometres thick over the poles and around 18 kilometres over the equator. Above it the air is too thin and dry for most clouds to form, which is why airliners cruise near its top, above most of the bumps below.

The driving force is uneven heating by the Sun. Tropical regions receive far more energy than the poles, and air warmed at the ground expands, becomes lighter and rises, leaving lower pressure beneath it. Cooler, denser air sinks and creates high pressure. Air flows from high to low pressure, and that flow is wind. Because the Earth is spinning, moving air is deflected by the Coriolis effect, to the right in the Northern Hemisphere and to the left in the Southern. Instead of blowing straight from high to low, winds spiral around pressure centres, anticlockwise around lows in the north.

On a global scale this produces three great loops of circulating air in each hemisphere. In the Hadley cell, air rises near the equator, dropping heavy rain on the tropics, and sinks again around 30 degrees north and south, where dry, descending air creates the world's great hot deserts, among them the Sahara. Surface air returning towards the equator becomes the steady trade winds that once carried sailing ships across the Atlantic. Between about 30 and 60 degrees, the prevailing westerlies carry weather systems from west to east, which is why Britain's weather usually arrives from the Atlantic. Near the top of the troposphere, fast rivers of air called jet streams steer storms along their paths.

Where cold and warm air masses meet, they form boundaries called fronts, a term coined by Norwegian meteorologists just after the First World War. Warm, moist air pushed upwards cools, its vapour condenses, and clouds and rain follow. Low-pressure systems, or depressions, bring changeable, wet and windy weather; high-pressure systems bring settled, clear conditions. Over warm tropical seas, the same physics can build hurricanes, also called typhoons or cyclones, storms hundreds of kilometres across.

Did you know? A lightning bolt heats the air around it to roughly 30,000°C, about five times hotter than the surface of the Sun.

Forecasting began in earnest in the nineteenth century with the telegraph, which let observers share readings faster than storms could travel. Today supercomputers divide the atmosphere into millions of grid boxes and solve the laws of physics for each one, fed by satellites, weather balloons and ground stations. A five-day forecast now is about as reliable as a one-day forecast was around 1980. And lightning does strike twice: the Empire State Building is hit around twenty times a year.

Climate

Weather is what happens today; climate is the pattern of weather over a long period, usually averaged over 30 years. A cold day in July says little about climate. A run of ever-warmer decades says a great deal. A climate is defined by averages and extremes: how hot the summers are, how wet the winters, how often droughts or storms arrive.

Earth's climate is set by a balance between energy arriving from the Sun and heat radiating back to space. Some gases in the atmosphere, chiefly water vapour, carbon dioxide and methane, let sunlight through but absorb the outgoing heat and send part of it back down. This greenhouse effect is natural and vital: without it, Earth's average surface temperature would be around -18°C rather than about 15°C. Scientists worked this out long ago. In the 1820s the French mathematician Joseph Fourier suggested that the atmosphere keeps the planet warm. In the 1850s the American scientist Eunice Foote and the Irish physicist John Tyndall showed that carbon dioxide and water vapour absorb heat. In 1896 the Swedish chemist Svante Arrhenius calculated that burning coal could eventually warm the world.

Climate has always changed naturally. Slow wobbles in Earth's orbit, known as Milankovitch cycles, have paced a series of ice ages over the past few million years. At the peak of the last one, around 20,000 years ago, ice sheets covered much of North America and northern Europe, and sea level was about 120 metres lower than today. Volcanic eruptions, changes in the Sun and shifting continents have all played a part over longer timescales.

What is different now is the speed and the cause. Since the Industrial Revolution, burning coal, oil and gas and clearing forests have raised the concentration of carbon dioxide from about 280 parts per million to over 420. The American scientist Charles David Keeling began measuring it on Mauna Loa in Hawaii in 1958, producing the steadily climbing Keeling Curve. Air bubbles trapped in Antarctic ice show that levels are now higher than at any time in at least 800,000 years. Global average temperature has risen by about 1.3°C since the late nineteenth century, and 2024 was the warmest year on record, the first calendar year more than 1.5°C above pre-industrial levels.

Did you know? In 1856 Eunice Foote filled glass cylinders with different gases, set them in sunlight and found that the one holding carbon dioxide grew hottest.

The effects are already measurable: shrinking glaciers and Arctic sea ice, sea level up by roughly 20 centimetres since 1900, and more frequent heatwaves and heavy downpours. Under the Paris Agreement of 2015, nearly every nation agreed to try to hold warming well below 2°C and to aim for 1.5°C. How much further the world warms depends mainly on how quickly emissions fall. The physics is settled; the choices are not.

Biomes

Travel from the equator towards either pole, or climb a high mountain, and the living landscape changes in a predictable order: steaming forest gives way to grassland, desert, woodland, conifers and finally bare, frozen ground. These great communities of plants and animals, shaped by climate, are called biomes. The German naturalist Alexander von Humboldt was one of the first to see the pattern. Climbing the volcano Chimborazo in Ecuador in 1802, he noted how vegetation changed with altitude in bands that mirrored the changes with latitude, and later drew them in a famous cross-section of the mountain.

Two factors matter most: temperature and rainfall. Plot one against the other and each biome occupies its own region of the chart. Hot and wet produces tropical rainforest, which covers only around 6% of the land yet is thought to hold about half of all land species. The Amazon is the largest. Hot with a long dry season produces savanna, open grassland dotted with trees, home to Africa's great herds of grazing animals. Where rain is scarcest, under the sinking air near 30 degrees north and south, lie the hot deserts, such as the Sahara and the Namib, whose plants and animals survive by storing water or hiding from the sun.

In the mid-latitudes, moderate rain supports temperate forest, where broadleaved trees such as oak, beech and maple drop their leaves in winter. The drier interiors of continents become temperate grassland: the prairies of North America, the steppes of Eurasia and the pampas of South America, now among the world's great grain-growing regions. Lands with hot, dry summers and mild, wet winters, around the Mediterranean, in California and in parts of Australia, grow tough, fire-adapted scrub.

Further towards the poles stretches the taiga, or boreal forest, the largest land biome on Earth. It forms a band of spruce, pine, fir and larch across Canada, Scandinavia and Russia, surviving long winters far below freezing. Beyond the tree line lies the tundra, where the ground below the surface stays frozen all year as permafrost. Only mosses, lichens, grasses and dwarf shrubs can grow there, during a short summer of almost endless daylight.

Did you know? The largest desert on Earth is not the Sahara but Antarctica, which receives so little snow each year that it counts as a desert.

Biomes are not fixed. Mountains compress them into a few kilometres of height, because the temperature drops by roughly 6.5°C for every 1,000 metres climbed. Over the past few thousand years, farms, cities and pastures have replaced large parts of the natural grasslands and forests. And as the climate warms, biomes are beginning to shift, with shrubs advancing into the Arctic tundra. The map of life is drawn by climate, and it is being redrawn.

What Is Life?

Biologists have argued for centuries about where the living ends and the non-living begins, yet everyone recognises life when they see it. A sparrow is alive, a pebble is not. The trouble starts at the edges, and the edges are surprisingly crowded.

School textbooks offer a handy checklist, often remembered as MRS GREN: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition. A living thing moves, even if only by turning its leaves towards the light. It releases energy from food through respiration, a chemical process running in every cell. It senses its surroundings and responds. It grows, it produces offspring, it gets rid of waste and it takes in raw materials to build and fuel itself.

The checklist works well for oak trees and otters, but it is a description rather than a definition. Fire grows, feeds, consumes oxygen and spreads by throwing off sparks, yet nobody calls a bonfire alive. A mule cannot reproduce, yet it is plainly a living animal. Seeds can sit dormant for centuries, doing almost nothing at all, and still sprout when conditions are right; a date palm grown in 2005 from a 2,000-year-old seed found at Masada is the famous example.

Viruses are the hardest case. A virus is a strand of genetic material wrapped in a protein coat, far smaller than a bacterium. Outside a cell it is inert: it does not feed, respire or grow. Inside a cell it hijacks the machinery and makes thousands of copies of itself. Many biologists class viruses as being on the borderline of life, and the question remains open.

Did you know? The giant mimivirus, found in a cooling tower in Bradford in 1992, was so large it was first mistaken for a bacterium, and only recognised as a virus in 2003.

A working definition used by NASA's astrobiologists cuts through the list: life is a self-sustaining chemical system capable of evolution. That puts the stress on two deep features. Living things keep themselves going by harnessing energy, and they carry inherited instructions, written in DNA or RNA, that can change from one generation to the next. Everything on Earth that clearly counts as alive shares the same genetic code, a sign that all of it descends from a single common ancestor that lived more than 3.5 billion years ago.

Life, in the end, is less a thing than a process: chemistry that copies itself and keeps going.

Cells

In 1665 the English scientist Robert Hooke peered through his microscope at a thin slice of cork and saw rows of tiny empty boxes. They reminded him of the small rooms in which monks slept, so he called them cells. He was looking at the dead walls of plant cells, but the name stuck, and it came to mean the basic building block of every living thing.

By 1839 the German biologists Matthias Schleiden and Theodor Schwann had concluded that all plants and animals are made of cells. In 1855 Rudolf Virchow added the final piece: every cell comes from another cell. Together these ideas form cell theory, one of the foundations of modern biology.

Cells come in two great designs. Prokaryotes, the bacteria and archaea, are small and simple, with their DNA floating loose inside. Eukaryotes, the cells of animals, plants and fungi, are typically ten times wider and packed with internal compartments called organelles. The nucleus holds the DNA, the instruction manual. Ribosomes read those instructions and assemble proteins. The endoplasmic reticulum and Golgi apparatus fold, package and ship them. Mitochondria, the power stations, release energy from sugar. A thin, oily cell membrane wraps the whole thing and controls what enters and leaves.

Plant cells add three features of their own: a stiff wall of cellulose, a large water-filled vacuole that keeps them firm, and green chloroplasts that capture sunlight. Mitochondria and chloroplasts both have their own small loops of DNA, because they were once free-living bacteria. Somewhere between 1.5 and 2 billion years ago they were swallowed by a larger cell and never left, an idea championed by the American biologist Lynn Margulis in the 1960s and now widely accepted.

Did you know? A human red blood cell has no nucleus at all: it pushes it out while maturing, leaving more room for the haemoglobin that carries oxygen.

Some living things are a single cell; others are vast cooperatives. A human body contains around 30 trillion cells of some 200 types, from long nerve cells that stretch the length of a leg to flat skin cells that are shed by the million every day. Every one of them began as a single fertilised egg that divided again and again.

Most cells are far too small to see. Dozens of typical human cells would fit across the full stop at the end of this sentence.

Evolution

In 1831 a 22-year-old Charles Darwin set sail on HMS Beagle for a voyage around the world that would last almost five years. He collected fossils in Argentina, watched earthquakes in Chile and, in 1835, spent five weeks on the Galápagos Islands, where mockingbirds and tortoises differed subtly from island to island. Back in England he spent more than two decades working out why.

His answer was natural selection, and its logic is simple. Living things produce more young than can possibly survive. Those young vary: some are faster, darker, taller or better at digesting a particular food. Much of that variation is inherited. Individuals whose features suit their surroundings are more likely to survive and breed, so those features become more common in the next generation. Repeat this for thousands or millions of generations and populations change, sometimes into entirely new species.

Darwin was not alone. In 1858 the naturalist Alfred Russel Wallace, feverish in the islands of what is now Indonesia, reached the same idea and sent it to Darwin. Their work was presented together that year, and Darwin's book On the Origin of Species followed in 1859. Neither man knew how inheritance worked. The answer was hiding in the pea experiments of the monk Gregor Mendel, published in 1866 and largely ignored until 1900. Today we know that traits are carried by genes, and that new variation arises from random changes in DNA called mutations.

Evolution can be watched in action. On the tiny Galápagos island of Daphne Major, the biologists Peter and Rosemary Grant measured every finch for decades. During a severe drought in 1977, small soft seeds ran out, and most medium ground finches died. The survivors tended to have deeper beaks, strong enough to crack the tough seeds that remained, and their offspring inherited them. Bacteria evolving resistance to antibiotics are a faster, more worrying example.

Did you know? The phrase "survival of the fittest" was coined not by Darwin but by the philosopher Herbert Spencer in 1864; Darwin adopted it later.

A common myth says humans descended from chimpanzees. In fact both species descend from a shared ancestor that lived around six or seven million years ago, and both have been evolving ever since. Evolution has no goal and no ladder of progress; "fittest" simply means best suited to the conditions of the moment.

Every living thing alive today is the tip of an unbroken line of survivors stretching back billions of years.

The Tree of Life

Scientists have named and described around two million species, and new ones are added at a rate of roughly 15,000 to 20,000 a year. Estimates of the true total vary wildly, but one widely cited study in 2011 put the number of plant, animal and fungus species alone at around 8.7 million. Keeping track of them all needs a filing system, and biology has had one since the 18th century.

Its architect was the Swedish botanist Carl Linnaeus. In 1753 for plants and 1758 for animals he gave every species a two-part Latin name, a system called binomial nomenclature. The first part is the genus, the second the species, so the lion is Panthera leo and its cousin the tiger Panthera tigris. A scientific name works in every language, which is why a biologist in Japan and one in Brazil can be certain they mean the same frog.

Linnaeus nested his names inside a ladder of ever larger groups. In its modern form the ladder runs: domain, kingdom, phylum, class, order, family, genus, species. The lion belongs to the animal kingdom, the phylum of chordates, the class of mammals, the order of carnivores and the cat family. Generations of students have remembered the sequence with mnemonics such as "King Philip came over for good soup".

Linnaeus sorted living things by how they looked. Since Darwin, biologists have tried instead to sort them by ancestry, so that each branch, or clade, contains an ancestor and all of its descendants. DNA sequencing has transformed the picture. In 1977 the American microbiologist Carl Woese compared genetic sequences from microbes and discovered that some single-celled organisms that looked like bacteria were in fact as different from them as bacteria are from humans. He named them archaea, and in 1990 proposed that all life falls into three great domains: Bacteria, Archaea and Eukarya, the last including every animal, plant and fungus.

Did you know? The official reference specimen for our own species, Homo sapiens, is Linnaeus himself, formally chosen in 1959 because he never named one.

The result is not a ladder but a branching tree of life, its root in a single last universal common ancestor and its twigs in the species alive today. DNA keeps redrawing the branches: fungi turn out to be closer relatives of animals than of plants, and whales sit within the same group as hippos.

Every name in the system is a hypothesis about family history, and it can change when new evidence arrives.

Microbes

In the 1670s a Dutch draper named Antonie van Leeuwenhoek, grinding tiny glass lenses in his spare time in Delft, looked at drops of lake and rain water and found it swarming with what he called "animalcules". He had discovered microbes, living things too small to see with the naked eye, and with them a hidden world that outnumbers and outweighs the visible one.

Microbes are not a single group but a size class. Bacteria and archaea are single cells without a nucleus, typically a few thousandths of a millimetre long. Protists such as amoebas and paramecia are larger single cells with a nucleus. Many fungi, including yeasts, are microscopic too. Viruses are smaller still, often around a ten-thousandth of a millimetre across, and are not cells at all: they can only multiply inside a host.

For most of history nobody connected these creatures with disease. In the 1860s Louis Pasteur showed that microbes cause food and wine to spoil, and that boiling, later called pasteurisation, kills them. In 1882 Robert Koch identified the bacterium behind tuberculosis. In 1928 Alexander Fleming noticed that a mould, Penicillium, had killed the bacteria growing on a dish in his London laboratory, a chance observation that led to penicillin, the first widely used antibiotic.

Yet disease is a tiny part of the story. Microbes made the air breathable: cyanobacteria began releasing oxygen into the atmosphere more than 2.4 billion years ago, and microscopic ocean plankton still produce around half of the oxygen made on Earth each year. Bacteria in soil and roots turn nitrogen from the air into forms plants can use. Microbes rot dead leaves and bodies, returning their nutrients to the soil. They also make bread rise, turn milk into yoghurt and cheese, and brew beer and wine.

Did you know? Bacteria contain roughly 35 times as much carbon as all the world's animals put together, including every human, whale and ant.

A human body is home to trillions of microbes, mostly in the gut, roughly equal in number to the body's own cells. This community, the microbiome, helps digest food, trains the immune system and crowds out harmful invaders. The popular claim that bacterial cells outnumber human cells ten to one has been revised; the ratio is closer to one to one.

The world belongs to microbes. Larger living things are, in a sense, the visitors.

Plants

A green leaf is a solar-powered factory. Using nothing but sunlight, water and a gas that makes up less than a twentieth of one per cent of the air, it builds sugar, and with it almost every meal ever eaten on Earth. The process is photosynthesis, and it has been running for well over two billion years.

The chemistry can be written in one line: carbon dioxide plus water, powered by light, become glucose plus oxygen. Water rises from the roots through pipes called xylem. Carbon dioxide enters the leaf through tiny pores on its underside called stomata, which open and close using a pair of guard cells. Inside the leaf, tightly packed palisade cells are crammed with chloroplasts containing the green pigment chlorophyll. Chlorophyll absorbs red and blue light strongly but reflects green, which is why leaves look green. The captured energy splits water molecules, releasing oxygen as a by-product, and is used to fix carbon into sugar. Another set of pipes, the phloem, carries that sugar to wherever the plant needs it.

Did you know? In the 1640s Jan Baptist van Helmont grew a willow in a pot for five years: it gained about 74 kilograms, while the soil lost only around 57 grams.

Van Helmont thought the extra mass came from water. In fact most of it came from the air. A tree is, in large part, carbon dioxide turned solid by sunlight.

There are about 390,000 known species of plant, and they dominate the living world by weight: plants hold roughly four-fifths of all the carbon in Earth's living things. Mosses and liverworts, the earliest land plants, have no internal pipes and stay small and damp. Ferns and their relatives evolved xylem and phloem and grew into forests more than 300 million years ago, forests whose buried remains became much of the coal burned in the Industrial Revolution. Conifers then wrapped their seeds in protective coats, and flowering plants, which appeared roughly 140 million years ago, recruited insects, birds and bats as couriers for their pollen. Today flowering plants make up the vast majority of plant species.

The records are remarkable. A coast redwood in California named Hyperion stands around 116 metres tall, the tallest known living tree. A Great Basin bristlecone pine called Methuselah has been growing for more than 4,800 years. The largest single flower, Rafflesia arnoldii, can reach a metre across and smells of rotting meat.

Every breath taken on land owes something to a leaf.

Invertebrates

Pick an animal species at random and the odds are overwhelming that it has no backbone. Invertebrates make up around 97 per cent of all known animal species, from sponges and jellyfish to snails, spiders, crabs, worms and starfish. Backboned animals, the vertebrates, are a single, rather recent offshoot of this enormous crowd.

The group is defined by what it lacks, so it holds astonishingly different body plans. Sponges, among the oldest animal lineages, have no muscles, nerves or organs at all: water simply flows through their pores and cells pick out food. Cnidarians, the jellyfish, sea anemones and corals, have stinging cells and a single opening for food in and waste out. Tiny coral polyps, building limestone skeletons generation after generation, have created the Great Barrier Reef, which stretches for some 2,300 kilometres off north-east Australia.

Molluscs include snails, clams and the cephalopods, the octopus, squid and cuttlefish, which are among the most intelligent of all invertebrates. An octopus can open screw-top jars, squeeze through any gap larger than its beak and change colour in a fraction of a second. Annelids, the segmented worms, include the earthworm, which Charles Darwin admired so much that his last book, in 1881, was about them. Echinoderms, the starfish and sea urchins, have bodies arranged in fives.

The most successful of all are the arthropods: insects, spiders, scorpions, crabs, lobsters, centipedes and their relatives. They wear their skeletons on the outside, a tough jointed exoskeleton of chitin, and must moult it in order to grow. Insects alone account for about a million described species, more than all other animals combined, and beetles for around 400,000 of those. Flight, which insects mastered more than 300 million years ago, and metamorphosis, the transformation from caterpillar to butterfly or grub to beetle, help explain their success.

Did you know? An octopus has three hearts and blue blood, coloured by a copper-based molecule called haemocyanin that carries oxygen well in cold water.

Invertebrates come in every size. Some mites are smaller than a full stop. The colossal squid, the heaviest invertebrate known, can weigh around 500 kilograms and has the largest eyes of any animal, about the size of a dinner plate. The Japanese spider crab has legs spanning more than three metres.

Bees pollinate crops, worms aerate soil and krill feed whales. Without a backbone, it turns out, life runs the planet.

Vertebrates

A blue whale, a hummingbird, a frog and a human share one defining feature: a backbone. Vertebrates have a skull protecting a brain and a spine of linked bones, the vertebrae, guarding the spinal cord. That internal skeleton grows with the animal, unlike the exoskeleton of an insect, and it has allowed vertebrates to become the largest animals that have ever lived.

The first vertebrates were small, jawless, fish-like creatures swimming more than 500 million years ago; fossils from Chengjiang in China, such as Haikouichthys, are among the earliest. Lampreys and hagfish are their jawless descendants today. Then came jaws, hinged from the bones that once supported gills, and a burst of predatory fish. Fish remain the largest vertebrate group by far, with more than 35,000 species, from sharks with skeletons of cartilage to bony fish such as cod, salmon and seahorses.

Around 375 million years ago some lobe-finned fish began hauling themselves into shallow water and onto land. Tiktaalik, discovered in the Canadian Arctic in 2004, had a fish's scales and gills but a neck and sturdy, wrist-like fins. Its relatives became the first four-limbed animals, the tetrapods. Amphibians, today's frogs, newts and salamanders, still lay soft eggs in water and many breathe partly through their moist skin.

The next leap was the amniotic egg, an egg with its own waterproof membranes and food store, which freed animals from the water's edge more than 300 million years ago. Its owners split into two great lines. One gave rise to reptiles, the lizards, snakes, turtles and crocodiles, and to the dinosaurs, whose surviving branch is the birds, about 11,000 species of feathered, warm-blooded flyers. The other line led to mammals, which grow hair, feed their young on milk and number around 6,500 species, from the tiny Etruscan shrew to the blue whale, at up to 30 metres long the heaviest animal known to have lived.

Did you know? The smallest known vertebrate is a frog from Papua New Guinea, Paedophryne amauensis, described in 2012: adults are about 7.7 millimetres long.

Warm-bloodedness evolved twice, separately, in birds and in mammals. Both burn food to hold their bodies at a steady temperature, which costs a great deal of energy but lets them stay active at night and in the cold.

Vertebrates are only a small slice of animal life, yet in that slice are the fastest, largest and cleverest animals on Earth.

Dinosaurs

For about 165 million years, dinosaurs were the dominant land animals on Earth. They appeared around 230 million years ago in the Triassic period, flourished through the Jurassic and Cretaceous, and vanished, mostly, 66 million years ago. Together these three periods form the Mesozoic era, the age of reptiles.

The earliest dinosaurs, such as Eoraptor and Herrerasaurus from Argentina, were nimble two-legged animals no bigger than a large dog. What set them apart from other reptiles was their stance: their legs were tucked directly beneath the body, like pillars, rather than sprawling sideways like a lizard's. That upright posture made them efficient walkers and runners. After a mass extinction at the end of the Triassic, around 201 million years ago, they took over.

The Jurassic was the age of giants. Long-necked sauropods such as Brachiosaurus and Diplodocus browsed on conifers and could weigh as much as several elephants. The later Argentinosaurus, from the Cretaceous, may have reached 35 metres and 70 tonnes or more. Plated Stegosaurus and the predator Allosaurus roamed what is now the western United States around 150 million years ago. At about the same time, in what is now Germany, Archaeopteryx glided between trees: part reptile, part bird.

Several popular ideas need correcting. Pterosaurs, the flying reptiles, and ichthyosaurs and plesiosaurs, the sea reptiles, were not dinosaurs, though they lived alongside them. Velociraptor was about the size of a turkey and covered in feathers. And no human ever saw a living non-bird dinosaur; our own species appeared more than 65 million years after they died out.

Did you know? Tyrannosaurus rex lived closer in time to us than to Stegosaurus: around 82 million years separate the two dinosaurs, but only 66 million separate T. rex from today.

The end came suddenly. Around 66 million years ago an asteroid roughly 10 kilometres wide struck what is now the Yucatán Peninsula in Mexico, leaving the Chicxulub crater, about 180 kilometres across. Fires, a blast of debris and years of darkened skies followed, and around three-quarters of all species disappeared. The English anatomist Richard Owen had coined the name Dinosauria, meaning "terrible lizards", in 1842, long before anyone knew how they died.

But they did not all go. Birds are the descendants of small feathered theropods, the same group as T. rex, and scientists count them as living dinosaurs. With around 11,000 species, the dinosaurs are still very much with us.

Ecosystems

A pond, a forest, a coral reef and a rotting log are all ecosystems: communities of living things bound together with the soil, water, air and sunlight around them. The English botanist Arthur Tansley coined the word in 1935 to capture the idea that organisms and their surroundings work as a single system, with energy flowing through it and materials cycling round.

Almost every ecosystem runs on sunlight. Producers, plants on land and microscopic algae in water, capture that light and build it into sugar. Primary consumers, the plant-eaters, from caterpillars to zebras, eat the producers. Secondary consumers eat the plant-eaters, and top predators such as eagles, sharks and wolves eat them in turn. Each of these feeding levels is called a trophic level, and a simple line linking them, grass to rabbit to fox, is a food chain. In reality the lines cross and branch into a tangled food web, because most animals eat several things and are eaten by several others.

Energy is lost at every step. An animal burns most of what it eats to move, keep warm and stay alive, and much of its food passes out undigested. As a rough rule, only about a tenth of the energy at one level becomes body mass at the next. That is why food chains rarely have more than four or five links, why big fierce predators are always rare, and why a field of wheat can feed far more people than the same field turned over to cattle.

Matter, unlike energy, is recycled. Decomposers, the fungi, bacteria, worms and beetles, break down dead bodies and waste and return carbon, nitrogen and phosphorus to the soil and air, ready to be used again.

Did you know? When the ecologist Robert Paine removed the starfish from a stretch of rocky shore in Washington State in the 1960s, mussels took over and the number of species there roughly halved.

Paine called such animals keystone species: like the central stone of an arch, their removal brings the structure down. Sea otters are another example. They eat sea urchins, which graze on kelp; where otters were hunted out along the Pacific coast, urchins multiplied and stripped whole kelp forests bare. When grey wolves were reintroduced to Yellowstone National Park in 1995, they changed how elk grazed, an effect scientists are still measuring.

The balance of nature is less a fixed balance than a constant negotiation. Remove one thread and the whole web shifts.

Building a Body

Every human body begins as a single cell, a fertilised egg about a tenth of a millimetre across, just visible to the naked eye. By adulthood that one cell has become roughly 37 trillion. None of this growth is haphazard. The body is built in levels, each made from the level below, rather as letters form words, words form sentences and sentences form a book.

The cell is the basic unit of life. Each has an outer membrane, a watery interior called cytoplasm and, in almost every case, a nucleus holding a full copy of the body's DNA. Inside are tiny working parts called organelles: mitochondria release energy from food, while ribosomes assemble proteins. Biologists recognise some 200 cell types, and their shapes suit their jobs. A red blood cell is a flexible disc about 8 micrometres wide, built to squeeze through the narrowest vessels. A motor neuron running from the base of the spine to the big toe can be a metre long.

Cells of the same kind gather into tissues, and there are only four basic sorts. Epithelial tissue covers surfaces and lines tubes. Connective tissue, which includes bone, fat, cartilage and even blood, supports and binds. Muscle tissue contracts. Nervous tissue carries signals. Two or more tissues working together form an organ. The stomach, for instance, has an epithelial lining, muscular walls that churn, connective tissue holding everything in place and nerves that set the rhythm.

Organs cooperate in organ systems. Anatomists usually count eleven: skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary, reproductive and integumentary, the last being skin, hair and nails. Skin is the largest organ of all, covering about two square metres in an adult. No system works alone. Breathing is useless without blood to carry oxygen, and blood is useless without a heart to push it.

How does one cell produce such variety? Every cell inherits the same genes, but each switches on only a selection of them. In the early embryo, stem cells can still become many things; as development continues, chemical signals from neighbouring cells steer them down narrower paths until a cell commits to being skin, liver or nerve. A popular claim says the body replaces all its cells every seven years. It is a myth: the gut lining renews within days, but most neurons of the cerebral cortex and the cells at the core of the eye's lens last a lifetime.

Did you know? The old claim that bacteria outnumber human cells ten to one is wrong; a 2016 estimate put the ratio at roughly one to one.

Thirty-seven trillion parts, one owner.

The Skeleton

An adult skeleton has 206 bones, but nobody is born with that many. A newborn has around 270 to 300 bony pieces, many still partly made of cartilage. Over childhood several fuse: the plates of the skull knit together, five vertebrae merge into the sacrum, and each hip bone forms from three. Growth carries on at the growth plates, discs of cartilage near the ends of long bones, until they harden into bone in the late teens or early twenties.

The skeleton does far more than hold the body up. It protects: the skull encases the brain, and the ribs cage the heart and lungs. It anchors muscles, giving them levers to pull on. It stores about 99 per cent of the body's calcium, releasing it into the blood when needed. And inside many bones lies red marrow, a soft tissue that manufactures blood cells, hundreds of billions of them every day.

Cut across a long bone such as the femur, the thigh bone, and its structure turns out to be clever engineering. A tough living skin called the periosteum wraps the outside. Beneath lies dense compact bone, built from microscopic cylinders called osteons, each with a central canal carrying blood vessels and nerves. Towards the ends the bone becomes spongy bone, a honeycomb of struts lined up along the directions of stress, strong but light. The hollow shaft holds marrow, red in young children and mostly yellow, fatty marrow in adults.

Bone is alive and constantly rebuilt. Cells called osteoblasts lay down new bone, osteoclasts dissolve old bone, and osteocytes, trapped inside the mineral, sense strain and direct the work. Roughly a tenth of the adult skeleton is replaced each year, so the whole frame is renewed over about a decade. Load drives the process. Astronauts in orbit lose bone mass because their skeletons no longer carry weight, while professional tennis players develop measurably thicker bones in their racquet arm.

Sizes vary enormously. The femur is the longest and strongest bone, about a quarter of a person's height. The smallest is the stapes, or stirrup, in the middle ear, around three millimetres long. More than half of all the bones are in the hands and feet: 54 in the two hands and 52 in the two feet, which helps explain their remarkable dexterity.

Did you know? The hyoid bone in the throat, which anchors the tongue, is the only bone in the body that does not form a joint with any other bone.

A skeleton in a museum looks dry and still; the one inside a living body is anything but.

Muscles

Every movement, from a sprint to a blink to a heartbeat, is made by muscle. The body has more than 600 named skeletal muscles, and together they typically make up between a third and two-fifths of body weight. Yet muscle has only one trick: it contracts. It can pull but never push, which is why skeletal muscles work in antagonistic pairs. The biceps bends the elbow; to straighten it, the triceps on the back of the arm pulls the other way.

There are three kinds of muscle tissue. Skeletal muscle, attached to bones by tough cords called tendons, is under conscious control and looks striped under a microscope. Cardiac muscle, found only in the heart, is also striped, but its branching cells contract rhythmically without any conscious command and are remarkably resistant to fatigue. Smooth muscle lines the walls of the gut, blood vessels, bladder and airways, squeezing slowly and automatically.

The secret of contraction lies in the stripes. A skeletal muscle is a bundle of long cells called fibres, each packed with threads called myofibrils, which are divided into repeating units called sarcomeres. Inside each sarcomere, thick filaments of the protein myosin sit between thin filaments of actin. When a nerve signal arrives, calcium is released inside the fibre, and the myosin heads grab the actin, swivel, let go and grab again, hauling the filaments past each other. This is the sliding filament model, published in 1954 by two independent teams that included Andrew Huxley and Hugh Huxley, who were not related. Every stroke uses a molecule of ATP, the cell's energy currency.

Not all fibres are alike. Slow-twitch fibres contract steadily and resist fatigue, ideal for posture and marathon running. Fast-twitch fibres contract powerfully but tire quickly, ideal for sprinting and jumping. The mix is partly inherited and partly shaped by training. Muscle also keeps the body warm: shivering is simply rapid contraction whose only purpose is to make heat.

The largest muscle is the gluteus maximus in the buttock, which keeps the body upright and powers climbing. The smallest is the stapedius in the middle ear, about a millimetre long, which tightens to protect the ear from loud noise. The popular saying that frowning takes more muscles than smiling has no solid basis; it depends entirely on the expression.

Did you know? Rigor mortis, the stiffening after death, happens because muscle needs ATP to release its grip; without fresh ATP, myosin stays locked onto actin.

The whole repertoire of human movement rests on one tiny, endlessly repeated tug.

Heart and Blood

The heart is a muscular pump about the size of a clenched fist, and it never rests. At around 70 beats a minute it beats some 100,000 times a day and roughly two and a half billion times in a long life. At rest it moves about five litres of blood a minute, close to the body's entire supply. During hard exercise that figure can rise four- or fivefold.

The heart is really two pumps side by side, and the blood follows a figure of eight. The right side receives used blood from the body and sends it a short distance to the lungs, where it drops off carbon dioxide and picks up oxygen. The left side receives this freshly oxygenated blood and drives it out through the aorta to every other tissue. Each side has two chambers: a thin-walled atrium that collects blood and a powerful ventricle that ejects it. Four valves act as one-way doors, and their closing makes the familiar "lub-dub" sound. The English physician William Harvey showed in 1628 that blood circulates in a closed loop, overturning ideas that had stood since antiquity.

The beat starts itself. A cluster of cells in the right atrium, the sinoatrial node, fires an electrical pulse that spreads across both atria, pauses briefly at a second relay station, and then sweeps through the ventricles. This built-in pacemaker is why a heart can keep beating for a while even after it has been removed from the body.

The network the heart feeds is vast. Arteries carry blood away under pressure, veins bring it back with the help of valves, and between them lie capillaries, vessels so narrow that red blood cells pass through in single file. The total length of all the vessels in an adult is often estimated at around 100,000 kilometres, enough to circle the Earth more than twice.

An adult carries about five litres of blood. Just over half is plasma, a straw-coloured liquid carrying salts, nutrients, hormones and waste. Most of the rest is red blood cells, packed with the iron-rich protein haemoglobin, which binds oxygen. White blood cells and platelets, which plug leaks, make up only a tiny fraction. A red cell lives for about 120 days, and bone marrow replaces them at a rate of roughly two million every second. Veins are often drawn in blue, but blood is never blue: it is always red, merely darker when low in oxygen.

Did you know? At rest, a single red blood cell takes only about a minute to complete a full circuit of the body.

Every heartbeat is a delivery, and the round never ends.

Lungs and Breathing

An adult at rest breathes about 12 to 20 times a minute, which adds up to more than twenty thousand breaths a day. Each quiet breath moves around half a litre of air. The purpose is a swap: oxygen in, carbon dioxide out. Every cell needs oxygen to release energy from food, and every cell produces carbon dioxide as waste, which turns the blood acidic if it is allowed to build up.

Breathing is powered mainly by the diaphragm, a dome-shaped sheet of muscle beneath the lungs. When it contracts, it flattens and pulls downwards, while the intercostal muscles between the ribs lift the ribcage up and out. The chest expands, the pressure inside falls and air rushes in. Breathing out at rest needs no effort at all: the muscles relax, and the elastic lungs recoil like a balloon letting go.

Air travels down a branching tree. It enters through the nose, where hairs and sticky mucus trap dust and the air is warmed and moistened. It passes the larynx, or voice box, into the trachea, a tube held open by C-shaped rings of cartilage. The trachea splits into two bronchi, one for each lung, and these divide again and again, about 23 times in all, into ever finer bronchioles. The two lungs are not identical: the right has three lobes and the left only two, leaving room for the heart.

The tree ends in clusters of tiny air sacs called alveoli, around 480 million of them in an adult. Each is wrapped in a mesh of capillaries. The barrier between air and blood is less than a thousandth of a millimetre thick, so oxygen simply diffuses across into the blood, where haemoglobin grabs it, while carbon dioxide diffuses the other way. Spread flat, the alveoli would cover something like 70 square metres, many times the area of the skin.

Keeping such delicate surfaces clean is a constant job. Cells lining the airways carry microscopic hairs called cilia that beat in coordinated waves, sweeping a carpet of mucus and trapped particles up towards the throat to be swallowed. Tobacco smoke damages these cilia, which is one reason smokers cough: the body falls back on coughing to clear what the cilia no longer can.

Did you know? The urge to breathe is triggered mainly by rising carbon dioxide in the blood, not by falling oxygen, which is why holding a breath becomes unbearable long before oxygen runs short.

More than twenty thousand breaths a day, and almost none of them with a thought.

Digestion

Digestion turns a meal into something the body can actually use: molecules small enough to cross the gut wall into the blood. It does so by two methods at once. Mechanical digestion tears and churns food into smaller pieces. Chemical digestion relies on enzymes, protein catalysts that snip large molecules into small ones: proteins into amino acids, starches into sugars, fats into fatty acids.

The work starts in the mouth. Teeth grind, and saliva, typically a litre or more a day, moistens food and begins breaking down starch with the enzyme amylase. Swallowing pushes a soft ball of food into the oesophagus, where waves of muscular contraction called peristalsis carry it down to the stomach in a matter of seconds. Peristalsis works even upside down, which is why astronauts can eat in orbit.

The stomach is a muscular bag that churns food with hydrochloric acid, typically at a pH of around 1.5 to 3.5, as acidic as lemon juice or more so. The acid kills most microbes and helps the enzyme pepsin unravel proteins. The stomach protects itself with a thick layer of mucus and by constantly renewing its lining. After a few hours, the meal has become a thick liquid called chyme, released in small squirts into the small intestine.

The small intestine is where most digestion and almost all absorption take place. It is around six to seven metres long. Bile from the liver breaks fat into tiny droplets, and juice from the pancreas adds enzymes that finish the job. The inner wall is folded and carpeted with finger-like villi, which are themselves covered in microscopic microvilli. This was long said to give an area the size of a tennis court; a careful study in 2014 found closer to 30 square metres, about half a badminton court. Still impressive.

What remains passes into the large intestine, about 1.5 metres long, which reclaims water and salts. It houses trillions of bacteria, the gut microbiome, which ferment fibre the body cannot digest and produce some vitamins, including vitamin K. The whole journey from mouth to exit usually takes between one and three days, and most of that time is spent in the large intestine.

Did you know? Much early knowledge of the stomach came from Alexis St Martin, a fur trader shot in 1822 whose wound healed with an opening into his stomach, which the surgeon William Beaumont studied for years.

The gut is a tube running through the body, and only what crosses its wall is truly inside.

The Brain

The human brain weighs about 1.4 kilograms, roughly 2 per cent of body weight, yet it uses around a fifth of the body's energy at rest. A living brain is soft, closer to firm jelly than to the rubbery specimens in jars, which have been hardened by preservatives. It contains about 86 billion neurons, a figure established in 2009 by the Brazilian neuroscientist Suzana Herculano-Houzel, who dissolved brains into a countable soup of cell nuclei. It replaced the round, much-repeated guess of 100 billion.

A neuron is a cell built for signalling. Branching dendrites receive messages, the cell body adds them up, and if the total crosses a threshold the neuron fires an electrical pulse along its axon. In axons wrapped in fatty myelin, the pulse can travel at more than 100 metres a second. At the far end, the signal crosses a tiny gap, the synapse, as a burst of chemical messengers called neurotransmitters. Each neuron may connect with thousands of others, giving an estimated hundred trillion or so synapses.

The large, wrinkled cerebrum is split into left and right hemispheres, linked by a thick bridge of fibres, the corpus callosum. Each hemisphere has four lobes. The frontal lobe handles planning, decisions and movement; the parietal lobe processes touch and the sense of space; the temporal lobe deals with hearing, language and memory; and the occipital lobe, at the back, is devoted to vision. Each hemisphere mostly controls the opposite side of the body.

Tucked beneath the back of the cerebrum sits the cerebellum, which fine-tunes movement and balance. It takes up only about a tenth of the brain's volume yet holds roughly four out of every five of its neurons. The brainstem connects the brain to the spinal cord and runs the automatic essentials: breathing, heart rate, sleeping and waking.

Much of this map was first drawn from injuries. In 1848 a railway foreman, Phineas Gage, survived an iron rod driven through his frontal lobe, and his changed behaviour hinted at that region's role. In 1861 the French physician Paul Broca linked a patch of the left frontal lobe to producing speech. Today scanners show brain activity directly, and they confirm that the idea that people use only 10 per cent of their brains is a myth: over a day, virtually every region is active.

Did you know? The brain itself has no pain receptors, which is why some brain surgery can be carried out while the patient is awake and talking.

The brain never stops rewiring. Every skill learned strengthens some connections and prunes others, a property called neuroplasticity. The organ reading this sentence is slightly different for having done so.

The Senses

Schoolchildren learn that there are five senses: sight, hearing, touch, taste and smell. The list goes back to Aristotle, and it is far too short. The body also senses balance, temperature, pain, the position of its own limbs and the state of its insides, from a full bladder to a racing heart. Many scientists count at least nine senses, and some count more than twenty, depending on how they divide them.

Every sense works the same way at heart. Specialised receptor cells convert one kind of energy, whether light, sound waves, pressure or chemicals, into electrical nerve signals, a process called transduction. The brain never receives light or sound directly, only coded pulses, and it builds the experience of the world from them.

In the eye, the cornea and lens focus light onto the retina, which holds about 120 million rods, sensitive to dim light, and about 6 million cones, which detect colour. Three types of cone, tuned to long, medium and short wavelengths, combine to produce every colour a person sees. Where the optic nerve leaves the eye there are no receptors at all, creating a blind spot that the brain quietly fills in.

In the ear, sound waves vibrate the eardrum, and three tiny bones, the hammer, anvil and stirrup, pass the vibration to the fluid-filled cochlea. There, around 15,000 hair cells respond to different pitches, from about 20 to 20,000 hertz in a young adult. Next door, the semicircular canals of the inner ear detect rotation and help keep the body balanced.

Smell and taste are chemical senses. The nose carries around 400 kinds of smell receptor, and combinations of them let people tell apart an enormous number of odours. The tongue detects five basic tastes: sweet, sour, salty, bitter and umami, the savoury taste identified by the Japanese chemist Kikunae Ikeda in 1908. Much of what is called flavour actually comes from smell, which is why food tastes bland with a blocked nose. Touch relies on several kinds of receptor in the skin, packed most densely in the fingertips and lips.

Did you know? The famous "tongue map", with sweet at the tip and bitter at the back, is a myth that grew from a misreading of a 1901 German study; every area with taste buds can detect all five tastes.

The world arrives as electricity, and the brain does the rest.

The Immune System

Every day the body is approached by bacteria, viruses, fungi and parasites, and nearly all of them are turned away without anyone noticing. The immune system is not a single organ but a distributed defence force of cells, proteins and tissues spread through the blood, the lymph, the bone marrow, the thymus, the spleen and hundreds of lymph nodes. It works in layers, each slower but more precise than the one before.

The first layer is a set of barriers. Skin is a tough, dry wall of dead cells. Mucus in the nose and airways traps invaders, tears and saliva contain lysozyme, an enzyme that breaks open bacteria, and stomach acid destroys most of what is swallowed.

Anything that slips through meets the innate immune system, which responds within minutes to hours to general signs of infection. Neutrophils, the most abundant white blood cells, rush to the scene and engulf bacteria. Macrophages, whose name means "big eaters", swallow microbes and debris and raise the alarm with chemical signals called cytokines. Natural killer cells destroy body cells infected by viruses. The heat, redness and swelling of inflammation are signs of this response at work, and so is fever, which makes the body less hospitable to many germs.

The third layer, the adaptive immune system, takes days to get going but is highly specific. Dendritic cells carry fragments of the invader to the lymph nodes and show them to lymphocytes. B cells produce antibodies, Y-shaped proteins that lock onto one particular target and tag it for destruction. Helper T cells coordinate the campaign, and cytotoxic T cells kill infected cells directly. Afterwards, long-lived memory cells remain, ready to respond far faster if the same invader returns.

That memory is what vaccination exploits. In 1796 the English doctor Edward Jenner inoculated a boy with material from a cowpox sore and showed that it protected him against smallpox. In 1980 the World Health Organization declared smallpox eradicated, the first human disease to be wiped out. The system can also misfire. In allergies it attacks harmless substances such as pollen, and in autoimmune diseases such as type 1 diabetes it turns on the body's own tissues.

Did you know? B cells are named after the bursa of Fabricius, an organ in birds where they were first identified, not after bone marrow, although in humans they do mature in the marrow.

An army with no headquarters, it still remembers the battles it has won.

DNA and Genes

Almost every cell in the body carries a complete set of instructions for building and running a human being. The instructions are written in DNA, deoxyribonucleic acid, a long molecule shaped like a twisted ladder, the famous double helix. Its structure was published in 1953 by James Watson and Francis Crick in Cambridge, who relied heavily on X-ray images made in Rosalind Franklin's laboratory at King's College London, above all the celebrated Photograph 51.

The rungs of the ladder carry the code. Each rung is a pair of chemical bases, and there are only four: adenine, thymine, cytosine and guanine, written A, T, C and G. A always pairs with T, and C with G. Because each strand dictates its partner, a cell can unzip the helix and build two identical copies, which is how DNA is copied every time a cell divides.

The human genome, one full set of DNA, runs to about 3.1 billion base pairs. Stretched out, the DNA in a single cell would be around two metres long, yet it is wound around proteins and coiled into 46 chromosomes, 23 inherited from each parent, inside a nucleus only a few thousandths of a millimetre wide. The Human Genome Project published an essentially complete sequence in 2003, and the last stubborn gaps were closed in 2022.

A gene is a stretch of DNA that holds the recipe for a product, usually a protein. Humans have about 20,000 protein-coding genes, far fewer than biologists once expected, and together they make up only around 1 to 2 per cent of the genome. To use a gene, the cell first copies it into a messenger molecule, RNA, which travels to a ribosome. There the code is read three letters at a time; each triplet, or codon, specifies one of 20 amino acids, which are strung together into a protein. Almost all life on Earth uses the same code, from bacteria to blue whales.

The rest of the genome was once dismissed as "junk DNA". That label has proved too simple: some of it controls when genes switch on and off, and some produces useful RNA, although much may indeed have no known function. Any two people share around 99.9 per cent of their DNA sequence. The remaining differences, together with environment, help shape height, eye colour and the risk of many diseases. Changes in the code, called mutations, are mostly harmless, occasionally harmful and, over vast spans of time, the raw material of evolution.

Did you know? Printed as letters, a single copy of the human genome would fill roughly a thousand books of a thousand pages each.

Four letters, three billion times over, and out comes a person.

Atoms

Every solid, liquid and gas, from a grain of sand to the Sun, is built from atoms. They are almost absurdly small. A typical atom measures about a tenth of a billionth of a metre across, so the full stop at the end of this sentence is roughly a million atoms wide. A human body contains around seven billion billion billion of them, and most of its heavier atoms were forged inside stars long before the Earth existed.

The idea is ancient. Around 400 BC the Greek thinkers Leucippus and Democritus argued that matter could not be cut in half forever and must end in tiny indivisible grains, which they called atomos, "uncuttable". It remained a philosophical hunch until 1808, when the English chemist John Dalton showed that elements combine in fixed whole-number ratios, exactly as they would if each element were made of identical atoms with a characteristic weight.

Then the atom was cut after all. In 1897 J. J. Thomson discovered the electron, a particle nearly two thousand times lighter than the lightest atom. In 1909 Hans Geiger and Ernest Marsden, working for Ernest Rutherford in Manchester, fired alpha particles at gold foil thinner than a page. Most passed straight through, but a tiny fraction, roughly one in eight thousand, bounced sharply back. Rutherford concluded in 1911 that almost all of an atom's mass is packed into a tiny, dense, positively charged nucleus. Its uncharged partner particle, the neutron, was found by James Chadwick in 1932.

The picture that emerged is strange. The nucleus, made of protons and neutrons, is roughly 100,000 times smaller across than the atom itself. If an atom were blown up to the size of a football stadium, its nucleus would be a pea on the centre spot, yet it would hold more than 99.9 per cent of the mass. Electrons do not circle the nucleus like tiny planets, whatever the famous logo suggests. They occupy fuzzy orbitals, clouds of probability that describe where each electron is likely to be found. Nor are protons and neutrons the end of the story: each is made of three smaller particles called quarks.

Did you know? Atoms are mostly empty space, yet a hand never sinks into a table: the electrons in both push each other away, and that electrical repulsion is what the sense of touch actually feels.

The number of protons, the atomic number, fixes the element: one for hydrogen, six for carbon, seventy-nine for gold. The number of neutrons can vary, giving different isotopes of the same element, while the outer electrons do all of the chemistry. Ninety-odd kinds of atom, endlessly rearranged, make every rock, river and living thing.

The Periodic Table

In 1869 the Russian chemist Dmitri Mendeleev wrote out the 63 elements then known, with their atomic weights and properties, and shuffled them until a pattern appeared. Similar elements recurred at regular intervals, like notes repeating an octave apart. The result was the periodic table, chemistry's single most useful page and probably the most recognisable chart in all of science.

Mendeleev's boldness lay in the gaps. Where no known element fitted, he left an empty space and predicted what would one day fill it. He described an "eka-aluminium" in detail, down to its density; gallium, discovered in France in 1875, matched closely. Scandium followed in 1879 and germanium in 1886. A table that could forecast substances nobody had yet seen was no mere filing system. It was a theory, and it had passed its test.

The modern table orders elements by atomic number, the count of protons in each nucleus, a principle the young British physicist Henry Moseley established in 1913 using X-rays. Its seven horizontal rows are periods; its eighteen vertical columns are groups. Elements in the same group have similar arrangements of outer electrons and so behave alike. Group 1, the alkali metals, are so reactive that sodium skates fizzing across water and caesium explodes in it. Group 18, the noble gases, barely react at all, which is why helium is safe in a party balloon and argon is used to protect hot metal during welding.

The table also divides into four blocks, named s, p, d and f after the type of electron orbital being filled. The f-block, home of the lanthanides and actinides, is usually printed as a separate strip underneath, purely so that the chart fits on a page; drawn in its proper place, the table would be 32 columns wide. Metals occupy the left and centre, non-metals the upper right, and a staircase of in-between metalloids such as silicon and germanium divides them.

Today the table holds 118 confirmed elements. Around ninety occur naturally on Earth; the rest have been made in reactors and particle accelerators, often a handful of atoms at a time, some lasting only fractions of a second. The four newest names, nihonium, moscovium, tennessine and oganesson, were approved in 2016, completing the seventh row.

Did you know? Gallium, the element Mendeleev predicted, melts at about 30 °C, so a lump will melt in a warm hand and a teaspoon cast from it softens and slumps in a cup of hot tea.

Every substance ever handled, from salt to steel, is assembled from this single sheet of squares.

States of Matter

Ice, water and steam are the same substance, H₂O, in three different states of matter. What separates them is not the molecules themselves but how fast they move and how tightly they cling to one another. Heat is, at bottom, the jiggling of particles, and adding or removing it changes the character of the crowd.

In a solid, particles are locked in place, vibrating around fixed positions like spectators in assigned seats. Many solids, such as salt, metals and ice, form orderly crystal lattices; others, such as glass, are frozen in a disordered jumble. Contrary to a popular story, old cathedral windows are not thicker at the bottom because glass has slowly flowed over the centuries. At room temperature glass is effectively rigid, and the uneven panes came from the way they were made.

Warm a solid to its melting point and its particles gain enough energy to slide past one another. A liquid keeps its volume but takes the shape of its container. Heat it further, to its boiling point, and particles escape entirely into a gas, flying freely and filling any space available. Steam at 100 °C occupies around 1,700 times the volume of the water it came from, which is why it could drive the engines of the Industrial Revolution.

These changes swallow or release energy without changing temperature, a hidden quantity called latent heat. A pan of boiling water stays at 100 °C however fiercely the hob burns, because the extra energy goes into pulling molecules apart. Pressure matters too. On the summit of Everest, where the air is thin, water boils at only about 70 °C, too cool for a decent cup of tea. Some substances skip a stage altogether: solid carbon dioxide, known as dry ice, turns straight into gas at −78.5 °C, a process called sublimation.

Heat a gas to many thousands of degrees and electrons are stripped from their atoms, creating plasma, a glowing soup of charged particles. It is rare on Earth, found in lightning, neon signs and the aurora, yet it is the most common state of visible matter in the universe, because stars are made of it. At the opposite extreme, in 1995 physicists cooled a cloud of rubidium atoms to within a whisker of absolute zero and created a Bose–Einstein condensate, in which thousands of atoms behave as one single quantum object.

Did you know? Water is unusual in expanding as it freezes. Ice is about 9 per cent less dense than liquid water, so it floats, and lakes freeze from the top down, sheltering the fish beneath.

The same molecules, given more or less energy, can become a glacier, a river or a cloud.

Chemical Reactions

A chemical reaction is a rearrangement. Bonds between atoms break, the atoms regroup, and new bonds form, producing substances with entirely different properties. Sodium, a soft metal that bursts into flame on water, and chlorine, a poisonous green gas, combine to make ordinary table salt. No atom is created or destroyed along the way. They are simply reshuffled, like letters spelling new words.

That principle, the conservation of mass, was nailed down by the French chemist Antoine Lavoisier, who weighed everything before and after reactions in sealed vessels and set out his findings in 1789. He showed that burning is a combination with oxygen, not the escape of a mysterious substance called phlogiston, and he helped devise the system of chemical names still used today.

Burning natural gas is a textbook case. One molecule of methane, CH₄, meets two molecules of oxygen, O₂. Their bonds break and the atoms reassemble as one molecule of carbon dioxide and two of water. Count the atoms on each side and they match exactly: one carbon, four hydrogen, four oxygen. What changes is energy. The new bonds are more stable than the old ones, so the surplus escapes as heat and light. Reactions that give out energy are exothermic; those that soak it up, such as photosynthesis storing sunlight in sugar, are endothermic.

Even an energy-releasing reaction usually needs a nudge to begin. A gas hob does not light until it is sparked, because existing bonds must first be strained to breaking point. This hurdle is the activation energy. Temperature helps: hotter molecules collide harder and more often, which is why food keeps longer in a fridge. A catalyst lowers the hurdle by offering an easier route, and emerges unchanged at the end. A car's catalytic converter uses platinum-group metals to clean its exhaust, and the human body runs on thousands of natural catalysts called enzymes, some of which speed reactions up by factors of millions or more.

Some reactions have reshaped history. The Haber–Bosch process, developed in Germany between 1909 and 1913, combines nitrogen from the air with hydrogen to make ammonia for fertiliser. Food grown with its help is thought to sustain around half of the world's population. Rusting, digestion, baking and the slow yellowing of old paper are reactions too, simply running at very different speeds.

Did you know? Antoine Lavoisier, often called the father of modern chemistry, was executed by guillotine in 1794 during the French Revolution, condemned not for his science but for his career as a tax collector.

Chemistry never makes new matter; it only teaches old atoms new partnerships.

Forces and Motion

For around two thousand years, educated Europeans followed Aristotle in believing that a moving object needs a constant push to keep going, and naturally comes to rest when the push stops. Everyday life seems to agree: a cart halts when the horse does. The flaw was invisible. Friction and air resistance are forces too, quietly slowing everything down.

Galileo Galilei saw through it in the early 1600s by rolling bronze balls down gentle ramps and imagining a perfectly smooth, endless floor on which a ball would roll forever. Isaac Newton turned the insight into a system. In 1687 he published the Principia Mathematica, and at its heart are three laws of motion that still guide the design of bridges, cars and spacecraft.

The first law says that an object stays at rest, or keeps moving in a straight line at a steady speed, unless a force acts on it. This stubbornness is called inertia. It explains why passengers lurch forwards when a bus brakes hard: their bodies simply carry on. A seatbelt supplies the force that stops them safely. In deep space, with almost nothing to slow it, the Voyager 1 probe, launched in 1977, has coasted onwards for decades without needing its engines to keep it moving.

The second law links force, mass and acceleration: force equals mass times acceleration, written F = ma. Push an empty supermarket trolley and it leaps away; push a fully loaded one just as hard and it barely shifts. The unit of force is called the newton in his honour. One newton is roughly the weight of a small apple, a neat coincidence given the famous story.

The third law states that forces always come in pairs: for every action there is an equal and opposite reaction. When a swimmer pushes off the wall of a pool, the wall pushes back and sends the swimmer gliding away. A rocket does not need air to push against. It hurls hot gas backwards, and the gas shoves the rocket forwards, which is why rockets work perfectly well in the vacuum of space. Even standing still involves the third law: the floor pushes up on the feet exactly as hard as the body presses down.

Did you know? Newton's laws were precise enough to steer the Apollo astronauts to the Moon and back in 1969. At those speeds the corrections from Einstein's relativity were far too small to matter.

Every journey, from a single footstep to a lunar landing, is three short rules applied over and over again.

Gravity

Gravity is by far the weakest of nature's four fundamental forces, yet it is the one that builds the universe. A small fridge magnet can hold a sheet of paper against the pull of an entire planet. But gravity has two great advantages: it reaches across any distance, and it only ever attracts. Pile up enough mass and it always wins, gathering gas into stars, stars into galaxies and galaxies into vast clusters.

Isaac Newton described it in 1687 with his law of universal gravitation: every mass attracts every other, with a force that grows with their masses and falls with the square of the distance between them. Double the distance and the pull drops to a quarter. A falling apple did set him thinking, by his own account late in life, though there is no evidence that it hit him on the head. His real leap was to see that the force tugging the apple also holds the Moon in its orbit. The Moon is constantly falling towards the Earth; it simply moves sideways fast enough to keep missing.

The strength of gravity, the constant G, is so tiny that it was first measured only in 1798, when Henry Cavendish used a delicate twisting balance and heavy lead spheres in an outbuilding of his London home. His experiment effectively weighed the Earth, at about six million billion billion kilograms.

A common misconception holds that astronauts float because there is no gravity in space. On the International Space Station, about 400 kilometres up, gravity is still roughly 90 per cent as strong as on the ground. Astronauts are weightless because they and their station are falling together around the planet, endlessly, exactly as Newton's Moon does.

In 1915 Albert Einstein offered a deeper picture. In his general theory of relativity, mass and energy curve the fabric of spacetime, and objects follow the straightest possible paths through that curved geometry. The theory explained a slow drift in Mercury's orbit that had puzzled astronomers, and predicted that starlight would bend as it passed the Sun, which was confirmed during the total solar eclipse of 1919. It also implied black holes and ripples in spacetime called gravitational waves.

Did you know? Gravitational waves were first detected on 14 September 2015, from two black holes that merged over a billion years ago. The passing ripple changed the length of LIGO's four-kilometre detector arms by a few thousandths of the width of a proton.

Gravity also raises the ocean tides and keeps the atmosphere wrapped around the Earth. It is the faintest of the forces, and the one that nothing can escape.

Energy

Energy is the capacity to make things happen: to move, heat, light or change something. It comes in many forms, from the motion of a cyclist, called kinetic energy, to the height of a raised hammer, called potential energy, the chemical bonds in a slice of toast and the nuclei at the hearts of atoms. Its most important property is a simple rule of bookkeeping. The total amount never changes.

This is the law of conservation of energy, also known as the first law of thermodynamics. Energy cannot be created or destroyed, only moved around or transformed. The English brewer and physicist James Prescott Joule demonstrated it in the 1840s with a falling weight that turned a paddle wheel inside a tank of water, warming it by a carefully measured fraction of a degree. Mechanical work had become heat, at a fixed rate of exchange. The unit of energy, the joule, honours him: it is roughly the energy needed to lift an apple one metre.

Follow energy through a single day and the chain is long. Nuclear fusion in the Sun turns mass into energy, around four million tonnes of it every second, as Einstein's E = mc² describes. Sunlight reaches a wheat field, where photosynthesis stores it in chemical bonds. A slice of bread delivers it to muscles, which turn it into motion, and every step leaks some away as heat. Food labels count this energy in kilocalories; one kilocalorie is 4,184 joules.

Why, if energy is never lost, do batteries run flat and engines need refuelling? Because each transformation leaves energy more spread out and less useful, usually as low-grade warmth. This is the second law of thermodynamics: in an isolated system, disorder, or entropy, tends to increase. It is why no perpetual-motion machine has ever worked, and why a dropped cup shatters but never leaps back together.

How tightly energy is packed matters enormously. Petrol stores about 46 megajoules per kilogram, ten times more than TNT. A good lithium-ion battery holds only around one megajoule per kilogram, which is why electric cars need heavy battery packs. Nuclear fuel is in another league entirely: the fission of a kilogram of uranium-235 releases millions of times more energy than burning a kilogram of coal.

Did you know? Weight for weight, a bar of chocolate holds about five times more energy than TNT. The explosive is dangerous not because it stores more, but because it releases everything in a few millionths of a second.

In 1918 the mathematician Emmy Noether proved that conservation of energy follows from a deeper truth: the laws of physics do not change with time. The universe keeps honest accounts because its rules never alter.

Electricity and Magnetism

For most of history, electricity and magnetism looked like separate curiosities. Rubbed amber attracted feathers; lodestones pulled on iron. In 1600 the English physician William Gilbert published De Magnete, arguing that the Earth itself behaves as a giant magnet, which is why compass needles point north. Yet nobody suspected that the spark and the compass were related.

Electricity is the behaviour of electric charge. Electrons carry negative charge and protons positive; like charges repel and opposites attract. A flow of charge is an electric current, measured in amperes, driven along by a voltage, measured in volts. In 1800 the Italian physicist Alessandro Volta built the first battery, a pile of zinc and copper discs separated by brine-soaked cloth, giving scientists a steady current for the first time.

In 1820 the Danish physicist Hans Christian Ørsted noticed that a wire carrying current made a nearby compass needle swing. Moving charge creates a magnetic field. In 1831 Michael Faraday showed the reverse: moving a magnet near a coil of wire makes a current flow. This is electromagnetic induction, and it powers the modern world. Almost every power station, whether burning gas, splitting atoms or turned by wind or water, spins coils past magnets to generate electricity. Electric motors run the same trick backwards, turning current into motion.

The Scottish physicist James Clerk Maxwell gathered all of this into a handful of equations, published in the 1860s. They showed that a changing electric field creates a magnetic field and vice versa, so the two can sustain each other as a travelling wave. Maxwell calculated the speed of that wave and found it matched the measured speed of light. Light, he concluded, is an electromagnetic wave. Heinrich Hertz produced radio waves in his laboratory in 1887, proving him right and opening the way to radio, television and wireless networks.

Electric and magnetic effects are two aspects of a single electromagnetic force, which also holds atoms together and governs all of chemistry. The Earth's own magnetic field is generated by swirling molten iron in its outer core, and it shields the planet from much of the solar wind. That field has flipped its north and south poles many times; the last full reversal was around 780,000 years ago.

Did you know? Electrons in a household wire drift along at only a fraction of a millimetre per second. A light comes on instantly because the electrical signal itself races through the wire at a large fraction of the speed of light.

Every switched-on bulb, every phone call and every ship's compass rests on one force wearing two faces.

Light and Sound

Almost everything known about the world arrives as one of two kinds of wave. Light is an electromagnetic wave, a ripple of electric and magnetic fields that needs no medium and crosses empty space at 299,792 kilometres per second. Sound is a mechanical wave, a travelling pattern of squeezes and stretches in air, water or rock. Without matter to carry it there is no sound at all, which is why real explosions in space are silent, whatever films suggest.

Visible light is a narrow slice of the electromagnetic spectrum, with wavelengths from about 380 nanometres for violet to about 700 for red. Beyond red lie infrared, microwaves and radio waves, some of them longer than a football pitch; beyond violet come ultraviolet, X-rays and gamma rays, far shorter than an atom is wide. In the 1660s Isaac Newton passed sunlight through a glass prism and showed that white light is a blend of all the colours of the rainbow. Around 1801 Thomas Young shone light through two narrow slits and saw bands of light and dark, strong evidence that light behaves as a wave.

Light is fast enough to circle the Earth about seven and a half times in a single second, yet sunlight still takes about eight minutes and twenty seconds to reach the Earth. When light passes from air into water or glass it slows down and changes direction, a bending called refraction that makes lenses, spectacles and rainbows possible.

Sound is slower by a factor of nearly a million. In air at 20 °C it travels at about 343 metres per second; in water at around 1,480; in steel at roughly 5,900. That gap explains the familiar rule for thunderstorms: count the seconds between the flash and the thunder, and every three seconds means about a kilometre. The pitch of a sound depends on its frequency. Young human ears detect roughly 20 to 20,000 vibrations per second, or hertz, a range that narrows with age. Bats and dolphins use higher-pitched ultrasound to find their way by echo, and doctors use it to scan unborn babies.

Loudness is measured in decibels, a logarithmic scale on which every rise of ten means ten times the sound intensity. A quiet library sits at around 40 decibels and a pneumatic drill at around 100, and long exposure to anything above about 85 can damage hearing.

Did you know? The light leaving the Sun's surface takes only minutes to arrive, but the energy it carries was released in the core many thousands of years earlier, and spent most of that time slowly working its way out.

Light and sound are the messengers of the senses: one crosses the cosmos in silence, the other needs the air to speak.

Relativity and Quantum

Around 1900 physics seemed close to complete. Newton's mechanics described motion, Maxwell's equations described light, and only a few loose ends remained. Within thirty years those loose ends had unravelled into two revolutions, relativity and quantum mechanics, which govern the very fast and the very small and remain the twin foundations of modern physics.

In 1905 Albert Einstein, then a 26-year-old clerk at the patent office in Bern, published his special theory of relativity. It rests on two ideas: the laws of physics are the same for everyone moving steadily, and the speed of light is the same for every observer, however fast they travel. The consequences are startling. Moving clocks run slow, moving objects shrink along their direction of travel, and nothing with mass can reach the speed of light. Mass and energy turn out to be interchangeable, through E = mc². These effects are real: particles called muons, created by cosmic rays high in the atmosphere, survive long enough to reach the ground only because their clocks are slowed. Einstein's general theory of 1915 brought in gravity, and with it the discovery that clocks tick slightly faster high above the Earth than on its surface.

Quantum theory began in 1900, when Max Planck found he could explain the glow of hot objects only by assuming that energy comes in tiny packets, or quanta. In 1905 Einstein proposed that light itself is made of such packets, later called photons, work that won him the Nobel Prize. By the late 1920s Werner Heisenberg, Erwin Schrödinger, Niels Bohr and others had built quantum mechanics, in which particles behave like waves, can exist in a superposition of states, and have properties that are genuinely undecided until measured. Heisenberg's uncertainty principle of 1927 states that a particle's position and momentum can never both be known exactly.

Strange as it is, quantum mechanics is among the most precisely tested theories in science, and it is everywhere. Transistors in every computer chip, lasers, LED lights and MRI scanners all depend on it. So does sunshine: protons in the Sun's core rarely have enough energy to fuse, and manage it only by quantum tunnelling through the barrier that ought to keep them apart.

Did you know? Satellite navigation needs both of Einstein's theories. Clocks on GPS satellites run about 38 millionths of a second a day fast compared with clocks on the ground, and without correction position errors would build up by roughly ten kilometres a day.

The great unfinished task is to unite the two. General relativity describes the large and smooth, quantum theory the small and grainy, and inside black holes or at the Big Bang they disagree. A working theory of quantum gravity remains the most wanted prize in physics, a century after both revolutions began.

Simple Machines

Long before engines, people moved the immovable with nothing but clever geometry. The ancient Greeks recognised a handful of devices that change the size or direction of a force, and Renaissance scholars later settled on a list of six simple machines: the lever, the wheel and axle, the pulley, the inclined plane, the wedge and the screw. Almost every mechanical gadget, from a tin opener to a tower crane, is built from combinations of them.

The secret is a trade. A simple machine never creates energy; it swaps force for distance. Push a crowbar's long handle down by 50 centimetres and its short tip may rise only 5, but it lifts with roughly ten times the force. Engineers call this ratio mechanical advantage. The work done, force multiplied by distance, stays the same at both ends, minus whatever friction steals.

The lever is the purest example. It needs only a rigid bar and a pivot, or fulcrum. Physicists sort levers into three classes by what sits in the middle. A seesaw or a pair of pliers has the fulcrum between effort and load. A wheelbarrow puts the load in the middle, so a gardener can raise a heavy barrowful. Tweezers and fishing rods put the effort in the middle, sacrificing force to gain speed and reach. Archimedes, who worked out the law of the lever in the third century BC, is said to have boasted that with a long enough lever and somewhere to stand he could move the Earth.

Did you know? Your forearm is a third-class lever. The biceps attaches just a few centimetres from the elbow, so it must pull with several times the weight of anything held in the hand.

The other machines are levers and ramps in disguise. A wheel and axle is a lever that can keep turning: a large steering wheel lets small hands turn a stiff shaft. A pulley redirects a rope's pull, and a block of several pulleys shares the load among many lengths of rope, so a sailor can hoist a heavy sail. The inclined plane lets a load climb gradually instead of straight up, which is how heavy stone was hauled onto ancient monuments. A wedge is two ramps back to back, driven into wood to split it, and every axe and knife is one. A screw is a ramp wrapped round a cylinder, turning a twist into a powerful push.

Archimedes is credited with a screw that lifts water, and versions of it still drain fields and move grain today. The simple machines are the alphabet of engineering, and they have never gone out of print.

The Wheel

The wheel feels so obvious that its lateness is a surprise. People had farmed, woven cloth, sailed boats and smelted copper for thousands of years before anyone put a load on rolling discs. The earliest wheels appear around 3500 BC, and they were not for travel at all: Mesopotamian potters spun clay on a heavy turntable called the potter's wheel.

The hard part was never the circle; it was the axle. A wheel only works if a smooth, round axle fits a round hole snugly enough to stay true but loosely enough to turn. That demanded precise carving and, ideally, metal tools. Once solved, the payoff was enormous, because rolling friction is far smaller than sliding friction. A loaded sledge drags its whole underside across the ground; a wheel touches the road along a narrow strip and simply peels away from it as it turns.

Wheeled vehicles spread with remarkable speed between about 3500 and 3000 BC, and archaeologists still debate where they began, with early traces from Mesopotamia, the steppes north of the Black Sea and central Europe. The first were solid discs of planks pegged together, heavy and slow, pulled by oxen. Around 2000 BC, chariot builders on the steppes east of the Urals hit on the spoked wheel: lighter, springier and fast enough to change warfare across Eurasia.

Did you know? The oldest wooden wheel yet found, pulled from the Ljubljana Marshes in Slovenia, is around 5,150 years old, and its axle survived alongside it.

The wheel was not inevitable. In the Americas, Mesoamerican craftspeople made little wheeled figurines, yet wheels were never used for transport there. Without horses or oxen to pull carts, and with steep or forested terrain, rollers, canoes and human porters made more sense. Invention depends on circumstance as much as on cleverness.

Later centuries kept refining the idea. Iron tyres shrunk onto wooden rims made cartwheels last. Ball bearings, sketched by Leonardo da Vinci and perfected in the nineteenth century, let axles spin almost freely. In 1888 John Boyd Dunlop, a Belfast vet, fitted air-filled rubber tyres to his son's tricycle, and the pneumatic tyre soon carried bicycles and then cars. Meanwhile the wheel had turned into something else entirely: gears, waterwheels, turbines, clockwork and hard drives all rotate on the same principle.

Today the world rolls on well over a billion cars, and every one of them depends on that ancient partnership of hub and axle. The circle was easy. The hole in the middle changed history.

The Printing Press

Before printing, every book in Europe was copied by hand. A scribe might spend many months on a single Bible, so books were rare, costly and mostly locked away in monasteries, universities and the libraries of the rich. Around 1440, in the German city of Mainz, a goldsmith named Johannes Gutenberg began work on a system that would change that within a lifetime.

Gutenberg did not invent printing. Chinese printers were carving whole pages into wooden blocks well over a thousand years ago, and around 1040 an artisan named Bi Sheng made movable type from baked clay. Korean printers were casting type in metal by the thirteenth century. But Chinese writing needs thousands of different characters, which made movable type laborious. The Latin alphabet, with a couple of dozen letters plus punctuation, was ideal for it.

Did you know? The oldest surviving book printed with movable metal type is the Korean Buddhist text Jikji, printed in 1377, roughly 78 years before the Gutenberg Bible.

Gutenberg's genius was to combine several pieces into one working system. A hand mould let a worker cast identical letters quickly from an alloy of lead, tin and antimony, which melted easily and set hard. An oil-based ink, closer to varnish than to the watery inks of scribes, clung to metal. And a screw press, adapted from the presses that squeezed grapes and olives, pushed paper evenly onto the inked type. A compositor set letters into lines, locked the lines into a frame, inked them, pressed, and repeated; afterwards the type could be broken up and reused.

His great showpiece, the Gutenberg Bible, was completed in about 1455. Around 180 copies were printed, and about 49 survive, some on vellum, some on paper. The technology then spread like wildfire. By 1500 there were presses in more than 200 European towns, and they had produced millions of books.

The consequences were vast. Texts became standardised, so scholars across Europe could cite the same page of the same edition. When Martin Luther challenged the Church in 1517, printed copies of his arguments spread across Germany within weeks, fuelling the Reformation. Printed maps, scientific diagrams and tables helped drive the Scientific Revolution, and cheaper books made reading worth learning for ordinary people. Newspapers, pamphlets and, in time, novels followed.

Many historians rank the press alongside writing itself as a turning point in human communication. It made ideas cheap to copy and hard to destroy. Once knowledge could be multiplied, it could no longer be contained.

The Steam Engine

Steam power is older than it looks. In the first century AD, Hero of Alexandria described the aeolipile, a hollow ball that spun as jets of steam escaped from bent nozzles. It was a curiosity, not a machine. Real steam power had to wait for a pressing problem, and in Britain that problem was water flooding the coal and tin mines.

In 1712 the ironmonger Thomas Newcomen set up an engine near Dudley, in the English Midlands, to pump water from a mine. His design used steam at little more than normal pressure. Steam filled a large cylinder under a piston; a spray of cold water condensed it, leaving a partial vacuum, and the weight of the atmosphere pushed the piston down. A great wooden beam rocked like a seesaw, working a pump deep in the shaft. It was slow and ferociously hungry for coal, but it worked, and hundreds were built.

The breakthrough came from a Glasgow instrument maker repairing a model Newcomen engine in 1765. James Watt saw that heating and then cooling the same cylinder with every stroke wasted most of the fuel. His answer was a separate condenser: steam was drawn off into a second, permanently cold chamber, so the cylinder could stay hot. Watt's engines used around a quarter of the coal. With his business partner Matthew Boulton, he later added rotary motion, double action and a spinning governor that controlled speed automatically.

Did you know? Watt popularised the term horsepower to sell his engines by comparing them with the horses they replaced, and the unit of power, the watt, is named after him.

Rotary motion released steam from the mines. Engines now turned the machinery of cotton mills, so factories no longer had to sit beside fast rivers. Richard Trevithick built high-pressure engines compact enough to move themselves, and in 1804 his locomotive hauled iron along a tramway in South Wales. George and Robert Stephenson's Rocket won the Rainhill Trials in 1829, and railways soon threaded across Britain and then the world. Steamships crossed oceans regardless of the wind.

Together these engines powered the Industrial Revolution: cities swelled, goods became cheap, and coal smoke darkened the skies. Burning coal on that scale also began the long rise in atmospheric carbon dioxide that drives climate change today.

Steam has not gone away. Charles Parsons' steam turbine of 1884 swapped pistons for spinning blades, and turbines driven by steam still generate a large share of the world's electricity in coal, gas and nuclear power stations. The kettle's breath still runs the world.

Electric Power

Electricity was a parlour trick for most of history: a spark from rubbed amber, a jolt from a Leyden jar. Turning it into a dependable supply took a chain of discoveries. In 1800 Alessandro Volta stacked discs of zinc and copper separated by brine-soaked card to build the first battery, giving scientists a steady current to study.

The key to large-scale power came in 1831, when Michael Faraday found that moving a magnet near a coil of wire makes a current flow in it. This is electromagnetic induction, and it is how nearly all electricity is still generated. Spin a magnet inside coils, or coils inside a magnet, and current flows. The spinning can come from steam, falling water, wind or a diesel engine; a power station is, at heart, a machine for turning something round.

Light gave electricity its first mass market. Joseph Swan in England and Thomas Edison in the United States both developed practical incandescent bulbs around 1879 and 1880, and Edison's genius lay as much in the system as in the bulb: generators, cables, meters and switches. His Pearl Street Station opened in New York in 1882, supplying direct current to customers in lower Manhattan. The popular belief that Edison invented the light bulb from scratch is a myth; dozens of inventors had tried before him.

Did you know? One of the world's first public electricity supplies lit the streets of Godalming in Surrey in 1881, powered by a waterwheel on the River Wey.

Direct current had a flaw: it could not travel far without heavy losses. Alternating current, championed by Nikola Tesla and George Westinghouse, could be pushed through a transformer to very high voltage. At high voltage the same power flows with a smaller current, and smaller currents waste far less energy heating the wires. After the bitter "war of the currents" of the late 1880s and 1890s, AC won. Britain's National Grid was built in the late 1920s and early 1930s, linking power stations into a single network.

The modern grid is a balancing act at continental scale. Generators spin in step, at 50 cycles per second in Britain and Europe and 60 in North America. Pylons carry current at hundreds of thousands of volts; substations step it down; a final transformer delivers 230 volts to British homes. Supply must match demand every second, which is why controllers watch for the surge when millions of kettles go on at once.

Electricity changed the shape of the day. Factories could run at night, refrigerators kept food fresh, and lifts made skyscrapers practical. Now wind and solar farms are joining the grid, and the challenge is storing their power. The flick of a switch hides one of the largest machines ever built.

Flight

For thousands of years flight belonged to birds and myths. The first people to leave the ground did so not on wings but beneath a bag of hot air. On 21 November 1783, in Paris, Jean-François Pilâtre de Rozier and the Marquis d'Arlandes drifted over the rooftops in a balloon built by the Montgolfier brothers, papermakers from Annonay. Hot air is less dense than cold, so the envelope floated like a cork in water.

Balloons go where the wind takes them. Controlled, winged flight needed a theory, and the English baronet Sir George Cayley supplied it around 1799. He separated the problem into four forces: lift and weight, thrust and drag. A fixed wing would provide lift, and a separate engine thrust. In 1853 one of his gliders carried a reluctant coachman across a Yorkshire valley. In the 1890s Otto Lilienthal made around 2,000 glides in Germany, publishing data that inspired two bicycle makers from Ohio.

On 17 December 1903, at Kill Devil Hills near Kitty Hawk, North Carolina, Orville Wright flew the Flyer for 12 seconds and about 37 metres. The Wright brothers' real breakthrough was control. They built a wind tunnel, carved efficient propellers and invented wing-warping, twisting the wings to roll and steer, the ancestor of the ailerons on every aircraft today.

Did you know? The Wrights' first flight covered about 37 metres, shorter than the wingspan of a Boeing 747.

A wing produces lift by turning the airflow downwards. Its curved shape and slight upward tilt, the angle of attack, make air flow faster over the top and leave lower pressure there; by pushing air down, the wing is pushed up. The tidy schoolbook story that air over the top must "catch up" with air underneath is a myth. In reality it reaches the trailing edge well before.

Progress afterwards was startling. Within a dozen years aircraft were fighting in the First World War. In 1919 John Alcock and Arthur Brown flew nonstop across the Atlantic. Frank Whittle in Britain and Hans von Ohain in Germany independently developed the jet engine, and a German jet first flew in 1939. In 1952 the de Havilland Comet began the world's first jet airliner service, and in 1970 the Boeing 747 brought wide-bodied air travel within reach of millions.

Today something like 100,000 commercial flights take off every day, and airliners are among the safest ways to travel. Only sixty-six years separate the Wright Flyer from the first flight of the jumbo jet. Few technologies have climbed so far, so fast.

Medicine

For most of history, doctors fought diseases they could not see. Illness was blamed on bad air, called miasma, or on imbalances of bodily fluids. Surgery was a desperate gamble, and infection often killed patients whose operations had gone well. In little more than a century, a handful of discoveries transformed medicine more than the previous two thousand years combined.

The first came from the countryside. Milkmaids who caught mild cowpox seemed to escape smallpox, one of history's deadliest diseases. In 1796 the Gloucestershire doctor Edward Jenner scratched material from a cowpox sore into the arm of eight-year-old James Phipps, then showed that the boy resisted smallpox. The word vaccine comes from vacca, Latin for cow. A vaccine trains the immune system with a harmless stand-in, so that it recognises the real invader quickly.

Next came germ theory: the idea that many diseases are caused by microscopic organisms. In the 1840s Ignaz Semmelweis cut deaths in a Vienna maternity ward by making doctors wash their hands. In 1854 John Snow traced a London cholera outbreak to a single water pump on Broad Street. In the 1860s Louis Pasteur showed that microbes cause fermentation and decay, and Joseph Lister used carbolic acid to keep surgical wounds clean, sharply reducing deaths. Robert Koch then identified the bacteria behind anthrax, tuberculosis and cholera.

Antibiotics arrived almost by accident. In 1928 Alexander Fleming returned from holiday to find that a mould, Penicillium, had killed bacteria growing on one of his dishes. Turning the observation into a drug took more than a decade, until Howard Florey, Ernst Chain and their Oxford team purified penicillin in the early 1940s, in time to save countless lives in the Second World War. Antibiotics work by attacking structures that bacteria have and human cells lack, such as their tough cell walls.

Did you know? Smallpox is the only human disease ever eradicated. The World Health Organization declared it gone in 1980, after a global vaccination campaign.

Together with clean water, sanitation and better food, these advances helped more than double average human life expectancy in little more than a century. Childhood diseases that once filled cemeteries became rare wherever vaccines were used. Modern vaccines can be designed at record speed: during the Covid-19 pandemic, mRNA vaccines went from genetic sequence to approved jabs in under a year.

The battle is not over. Bacteria evolve, and overuse of antibiotics has bred resistant strains that shrug off once-reliable drugs. Medicine's great lesson is that the smallest enemies demand the most patient science.

Computers

A computer is a machine that follows instructions, and what makes it remarkable is that the instructions can be changed. A loom or a calculator does one job; a computer can become a word processor, a flight simulator or a weather model just by loading a different program.

The idea is older than electronics. In the 1830s the English mathematician Charles Babbage designed the Analytical Engine, a steam-age computer of brass gears with a memory store, a processing "mill" and instructions fed in on punched cards borrowed from the Jacquard loom. It was never completed. Yet in 1843 Ada Lovelace published notes on it that included a method for calculating Bernoulli numbers, often called the first computer program, and she foresaw that such a machine might one day handle music and symbols as well as numbers.

In 1936 Alan Turing described an imaginary machine that reads and writes symbols on a tape, and showed that one such device could, in principle, imitate any other. This universal machine is the theoretical heart of every computer. The Second World War made the theory urgent. At Bletchley Park, the codebreakers' Colossus, working from 1944, was the world's first programmable electronic digital computer. In the United States, ENIAC, completed in 1945, filled a large room, weighed about 30 tonnes and used some 18,000 vacuum tubes.

Inside, every digital computer works in binary: everything is reduced to ones and zeros, on and off. Tiny switches combine into logic gates that perform operations such as AND, OR and NOT; gates combine into circuits that add numbers and store them; and billions of those circuits make a processor. The transistor, invented at Bell Labs in 1947, replaced fragile, hot vacuum tubes with a sliver of semiconductor. The integrated circuit, developed around 1958, put many transistors on one chip, and in 1971 Intel's 4004 microprocessor held about 2,300 of them.

Did you know? Engineers called faults "bugs" long before 1947, when a moth was found in the Harvard Mark II computer and taped into its logbook as the "first actual case of bug being found".

Gordon Moore observed in 1965 that the number of transistors on a chip was doubling at a steady pace, about every two years by his revised estimate of 1975. That trend held for decades. The most advanced chips today carry tens of billions of transistors, each far too small to see with an ordinary microscope, and a phone outperforms the supercomputers of the early 1990s.

Computers now sit in cars, washing machines and pockets, and run most of the world's money, mail and maps. Underneath it all, a vast crowd of switches is still flicking between zero and one.

The Internet

The internet is not one thing but a network of networks: tens of thousands of separate systems, owned by universities, companies and governments, that have agreed to speak the same language. No one switches it on, and no single person controls it.

Its roots lie in the Cold War. In the 1960s Paul Baran in the United States and Donald Davies in Britain independently proposed packet switching. Instead of holding open a single line, as a telephone call does, a message is chopped into small chunks, which Davies called packets, each labelled with its destination. Packets can travel by different routes and are reassembled at the far end, so the network survives if one link fails. On 29 October 1969 the US-funded ARPANET sent its first message, from the University of California, Los Angeles, to Stanford. The operator meant to type LOGIN; the system crashed after two letters.

Did you know? The first message ever sent over the ARPANET was "LO", because the receiving computer crashed before the rest of LOGIN arrived.

Different networks needed a common tongue. In 1974 Vint Cerf and Bob Kahn described what became TCP/IP, the rules for addressing packets and making sure they all arrive. The ARPANET switched over on 1 January 1983, a date often treated as the internet's birthday. Every connected device has an IP address, a number that works like a postal address, and the Domain Name System translates friendly names into those numbers.

The internet and the World Wide Web are not the same thing. In 1989, at CERN near Geneva, the British scientist Tim Berners-Lee proposed a system of linked pages, and by 1991 the first website was online. The web runs on top of the internet, just as email, video calls and online games do. CERN released the web's software free for anyone to use, which helped it spread explosively.

Physically, the internet is surprisingly solid. Data travels as pulses of light through glass fibres, and the vast majority of intercontinental traffic crosses the seabed in cables about as thick as a garden hose. Routers pass each packet from network to network, often making a dozen or more hops in a fraction of a second. Enormous data centres store and serve web pages, videos and messages.

By the mid-2020s around two-thirds of humanity was online, roughly five and a half billion people. The internet has reshaped shopping, news, friendship, work and politics, and brought new problems from misinformation to cybercrime. It began with a crashed computer and two letters. It now carries a large share of everything people say to one another.

Artificial Intelligence

Most software follows rules that a programmer wrote down. Artificial intelligence is the attempt to make machines do things that seem to require intelligence: recognising faces, translating languages, playing games, answering questions. Its most powerful modern form, machine learning, does not hand the computer the rules at all. It shows the machine examples and lets it work out the rules itself.

The question is old. In 1950 Alan Turing asked whether machines could think and proposed an "imitation game", now called the Turing test, in which a judge chats by text with a hidden human and a hidden machine. The term artificial intelligence was coined by John McCarthy for a workshop at Dartmouth College in 1956, whose organisers hoped that key problems might be cracked in a summer. They were not. Early successes were followed by long, disappointing lulls known as AI winters.

The idea that eventually won was inspired, loosely, by the brain. An artificial neural network is layers of simple units connected by adjustable numbers called weights. Data enters at one end, each unit adds up its inputs and passes a signal on, and an answer emerges at the other. During training, the network's answer is compared with the right one, and a method called backpropagation nudges every weight slightly to reduce the error. Repeat this over millions of examples and the network tunes itself.

For decades networks were too small and data too scarce. Then cheap graphics chips, built for video games, and the vast stock of images and text on the internet changed everything. In 2012 a deep network called AlexNet won an image-recognition contest by a wide margin. In 2016 DeepMind's AlphaGo beat the champion Lee Sedol at Go, a game long thought too subtle for machines. In 2017 Google researchers published the transformer design, which powers today's large language models, trained to predict the next word across enormous libraries of text. ChatGPT, released in late 2022, brought them to hundreds of millions of people.

Did you know? In 2024 the Nobel Prizes in both physics and chemistry honoured AI work, including neural network pioneers and AlphaFold's prediction of protein structures.

These systems are impressive but strange. A language model can write a sonnet or fix a bug in code, yet state falsehoods with total confidence, because it predicts plausible text rather than checking facts. Questions about bias, jobs, copyright, energy use and safety are now debated by governments worldwide.

Every earlier invention in this part extended human muscles or senses. AI is the first that aims to extend thought itself. How it is used will be decided by people, not machines.

Human Origins

Every person alive today belongs to a family tree whose roots lie in Africa. Around six to seven million years ago, the line that led to humans split from the line that led to chimpanzees, and for most of the time since, our ancestors and their many cousins lived only on that continent. Fossils such as Sahelanthropus from Chad, about seven million years old, mark the first uncertain steps.

Walking upright came long before big brains. In 1974, in the Afar region of Ethiopia, a team led by Donald Johanson found Lucy, a 3.2-million-year-old Australopithecus afarensis who stood about 1.1 metres tall, with a brain no bigger than a chimpanzee's and hips built for walking. At Laetoli in Tanzania, footprints pressed into damp volcanic ash some 3.6 million years ago show the same striding gait.

The genus Homo appeared around 2.8 million years ago, and with it came stone tools, bigger brains and longer journeys. Homo erectus, tall and long-legged, was the first known traveller: by about 1.8 million years ago its kind had reached Dmanisi in Georgia, and later Java and China. Other human species followed, including the stocky, cold-adapted Neanderthals of Europe and western Asia and the mysterious Denisovans, known mostly from their DNA.

Our own species, Homo sapiens, emerged in Africa. The oldest fossils yet found, at Jebel Irhoud in Morocco, are about 300,000 years old. Small groups ventured into the Middle East more than once, but the great dispersal that peopled the rest of the planet began roughly 60,000 to 50,000 years ago. Modern humans reached Australia by at least 50,000 years ago, which meant crossing open sea; Europe by about 45,000 years ago; and the Americas, across a land bridge where the Bering Strait now lies, by at least 15,000 years ago and possibly earlier.

Did you know? Most people whose ancestry lies outside Africa carry around 1 to 2 per cent Neanderthal DNA, a genetic souvenir of encounters some 50,000 years ago.

Those encounters show that the story is not a tidy ladder but a tangled bush. Several human species shared the Earth at once, met, and occasionally had children together. By about 40,000 years ago the Neanderthals had vanished, and the Denisovans faded away too, leaving Homo sapiens as the last human standing. Because our species spent most of its history in Africa, African populations today hold more genetic variety than the rest of the world combined.

Every map, city and language that follows in this book was made by the descendants of those few thousand wanderers.

The First Farmers

For around 95 per cent of its history, Homo sapiens lived by hunting, fishing and gathering. Then, from roughly 12,000 years ago, people in several parts of the world began to do something radical: instead of searching for food, they started to grow it. The change, known as the Neolithic or Agricultural Revolution, unfolded over thousands of years, but it reshaped human life more than anything before it.

It began in the Fertile Crescent, an arc of hills and river valleys curving from the Levant through south-eastern Turkey to the Zagros Mountains of Iran. There, around 9500 BCE, people began cultivating wild wheat and barley, alongside lentils, peas, chickpeas and flax. By replanting seeds from plants whose ears did not shatter and scatter, farmers unknowingly bred crops that waited to be harvested. Within about a thousand years sheep and goats had been domesticated, and cattle were being bred from the huge wild aurochs.

The Fertile Crescent was not alone. Farming was invented independently in half a dozen or more places: rice along the Yangtze and millet in northern China, taro and bananas in the highlands of New Guinea, squash and then maize in Mexico, potatoes in the Andes, and sorghum in Africa's Sahel. Maize is a striking feat of breeding, because its wild ancestor, teosinte, has cobs smaller than a little finger and seeds locked in stony cases.

Did you know? Göbekli Tepe in Turkey, a complex of carved stone pillars up to about 5 metres tall, was raised around 9500 BCE by people who had not yet begun farming.

Food that could be stored meant people could stay put. Villages became towns. Jericho had a stone wall and tower by about 8000 BCE, and Çatalhöyük in Anatolia, from around 7100 BCE, packed several thousand people into mud-brick houses so tightly that residents climbed in through holes in their roofs. Pottery, weaving, granaries and private property followed, and with surpluses came specialists, chiefs and, eventually, kings.

The bargain had a darker side. Early farmers often worked longer hours than foragers, ate a narrower diet and grew shorter; their skeletons show more tooth decay and more signs of hunger. Living close to animals and to one another bred new diseases, and stored wealth invited raids. Yet farming fed far more people from each square kilometre, and populations grew relentlessly. A world of perhaps a few million foragers became, within ten thousand years, a world of hundreds of millions.

Every loaf, rice bowl and tortilla eaten today is an inheritance from those first fields.

Mesopotamia

The name Mesopotamia is Greek for "between the rivers", and the rivers were the Tigris and the Euphrates, which flow through what is now Iraq. Rain was scarce on the southern plain, but the rivers carried snowmelt from the mountains of Turkey, and farmers who dug canals to spread that water could raise astonishing harvests of barley. Surplus grain fed people who did not farm, and from around 4000 BCE the world's first true cities grew here.

The greatest was Uruk. By about 3000 BCE it may have held 40,000 people or more behind walls some 9 kilometres long, making it probably the largest city on Earth. Its temples managed land, workers, herds and stores, and keeping track of all that produced the region's most important invention. Around 3300 to 3200 BCE scribes began pressing marks into damp clay tablets with a cut reed. At first these were pictures of sheep, grain and jars, but over the centuries the signs became wedge-shaped strokes: cuneiform, from the Latin for "wedge". Writing started as bookkeeping and grew into letters, laws, hymns and the Epic of Gilgamesh.

Sumer was a patchwork of rival city-states, among them Ur, Lagash and Kish, each with its own patron god. Around 2334 BCE Sargon of Akkad conquered them and created what is often called the first empire. Later, King Ur-Nammu raised the great ziggurat of Ur, a stepped mountain of mud brick crowned by a shrine to the moon god Nanna. Around 1754 BCE Hammurabi of Babylon had 282 laws carved on a basalt stele more than two metres tall, now in the Louvre. Its penalties depended on social rank: an eye for an eye applied between equals, but injuring a commoner usually cost only a fine.

Did you know? The 60 minutes in an hour and the 360 degrees in a circle descend from the Mesopotamian habit of counting in base 60.

The Mesopotamians gave the world far more than that. They were among the first to use the wheel, around 3500 BCE, and they charted the stars, solved problems we would now call algebra and drew up contracts, receipts and loans. Their success carried a hidden cost: centuries of irrigation left salt in the soil, and in the south wheat gave way to hardier barley as yields fell.

Empires came and went, Assyrians, Babylonians and Persians among them, until Alexander the Great took Babylon in 331 BCE. Cuneiform faded out by the first century CE and lay unread for nearly 1,800 years, until scholars cracked it in the 1850s.

History, in the most literal sense, begins here: with the first written records.

Ancient Egypt

Ancient Egypt was, as the Greek historian Herodotus put it, the gift of the river. The Nile, about 6,650 kilometres long, runs north through a desert where it almost never rains. Each summer, monsoon rains over the Ethiopian Highlands swelled the Blue Nile, and from around July the river spilled over its banks in Egypt, leaving behind a layer of rich black silt. Egyptians called their country Kemet, the black land, and the desert beyond it Deshret, the red land. The boundary between life and barrenness could be a single step.

The flood set the calendar. The year had three seasons of four months each: Akhet, the inundation; Peret, when the waters fell and crops were sown; and Shemu, the harvest and low water. The reappearance of the star Sirius at dawn, just before the flood, marked the new year. Officials read the river's height on nilometers, stone stairways and shafts marked with measures, because the height of the flood predicted the harvest, and the harvest set the taxes.

Around 3100 BCE, Upper and Lower Egypt were united under one king, traditionally named Narmer. For almost 3,000 years, through some 30 dynasties grouped into the Old, Middle and New Kingdoms, the pharaoh ruled as a god-king who upheld ma'at, the order of the universe. The Old Kingdom built the pyramids. The Great Pyramid of Khufu at Giza, finished around 2560 BCE, rose to about 146 metres and remained the tallest human-made structure on Earth for nearly 4,000 years. Contrary to the popular image, it was not built by slaves but by organised crews of labourers, who were housed, fed on bread and beer, and buried in tombs near the site.

Did you know? Cleopatra VII lived closer in time to the first Moon landing than to the building of the Great Pyramid.

Daily life was richer than the tombs suggest. Farmers grew emmer wheat, barley and flax; scribes wrote with reed pens on papyrus, a paper made from the plant that grew along the riverbanks; and hieroglyphs, with some 700 signs in regular use, covered temple walls. Long after the script was forgotten, the Rosetta Stone, carved in 196 BCE with the same decree in hieroglyphs, demotic and Greek, allowed Jean-François Champollion to decipher it in 1822.

New Kingdom rulers such as Hatshepsut, Akhenaten, Tutankhamun and Ramesses II built temples at Karnak and Luxor and tombs in the Valley of the Kings. Egypt was later ruled by Nubians, Persians and Greeks, and when Cleopatra died in 30 BCE it became a province of Rome.

For three thousand years, one river sustained one of history's longest-lived civilisations.

Ancient Greece

Ancient Greece was never a single country. It was a scatter of more than a thousand city-states, each called a polis, spread across rocky mainland valleys, Aegean islands and colonies from Spain to the Black Sea. They shared a language, gods and epic poems such as Homer's Iliad, yet they fought one another constantly. Out of this quarrelsome world came ideas that still shape how people govern, think and play.

The most famous experiment took place in Athens. Around 508 BCE the reformer Cleisthenes reorganised the citizens into ten tribes and gave power to the demos, the people: hence democracy, "people power". Every adult male citizen could speak and vote in the Assembly, which met on a hillside called the Pnyx about forty times a year to decide on war, peace and laws. A Council of 500, fifty men from each tribe, prepared its business. Most officials, and the large juries of the law courts, were chosen by lot, because Athenians believed elections favoured the rich and famous. Only the ten generals were elected. Each year citizens could also vote to exile an over-mighty politician for ten years by scratching his name on a fragment of pottery, an ostrakon: the origin of the word ostracism.

It was a democracy with a narrow electorate. Women, enslaved people and resident foreigners had no vote, so only a minority of the people living in Athens took part. Yet it was remarkably direct: citizens did not elect representatives to decide for them, they decided for themselves.

Did you know? Athenians picked jurors with a kleroterion, a stone slab slotted for citizens' name tokens, which used black and white balls dropped through a tube to select whole rows at random.

Fifth-century Athens was also the stage for an explosion of thought. Socrates questioned everything until he was sentenced to death in 399 BCE; his pupil Plato founded the Academy; and Plato's pupil Aristotle wrote on logic, biology, politics and poetry, then tutored the young Alexander the Great. Other Greek thinkers, from Thales to Democritus, who imagined matter made of tiny atoms, tried to explain nature without the gods. Historians, playwrights and architects left the Parthenon, tragedy, comedy and the very word "history".

The Olympic Games, traditionally first held in 776 BCE at Olympia in honour of Zeus, brought rival cities together every four years under a sacred truce. The earliest event was a sprint of one stadion, about 192 metres, and athletes competed naked. The games ran for more than a thousand years before ending around 393 CE, and were revived in Athens in 1896.

Small, argumentative and often at war, the Greek cities still managed to invent much of the toolkit of Western civilisation.

Ancient Rome

According to legend, Rome was founded in 753 BCE by Romulus, a twin raised by a she-wolf. In reality it began as a cluster of hilltop villages beside the River Tiber, ruled by kings until 509 BCE, when the Romans drove out the last of them and founded a republic. Power was divided so that no one man could hold it for long: two consuls elected each year, a Senate of aristocrats, and assemblies of citizens. The Latin res publica, "the public thing", gave the world a word.

The Republic grew by war and alliance. By 264 BCE it controlled most of Italy, and in three Punic Wars against Carthage, the great sea power of North Africa, it survived Hannibal's invasion across the Alps with elephants in 218 BCE and finally destroyed Carthage in 146 BCE. Conquest brought slaves, plunder and provinces, but also ambitious generals with armies loyal to them rather than to Rome. In 49 BCE Julius Caesar crossed the Rubicon with his legions, won a civil war and was made dictator; senators stabbed him to death on the Ides of March, 44 BCE.

Caesar was never an emperor. That title belongs to his great-nephew and heir Octavian, who defeated Mark Antony and Cleopatra and in 27 BCE took the name Augustus. He kept the outward forms of the Republic while holding real power himself, and began two centuries of relative calm known as the Pax Romana. Under Trajan in 117 CE the empire reached its greatest extent, about 5 million square kilometres stretching from northern Britain to the Persian Gulf, with perhaps 60 million people. The city of Rome itself held around a million.

Did you know? The Pantheon's dome in Rome, completed around 125 CE, is still the largest unreinforced concrete dome in the world.

What held the empire together was engineering, law and Latin. Some 80,000 kilometres of paved roads carried armies, officials and merchants; aqueducts delivered water to fountains and public baths; and Roman concrete, made with volcanic ash, could even set under the sea. Roman law shaped the legal codes of much of Europe, and Latin became the parent of French, Spanish, Italian, Portuguese and Romanian.

The empire proved too large to rule from one place. Diocletian divided its government between east and west in the 280s, and Constantine, the first Christian emperor, founded a new capital at Constantinople in 330. In 476 a Germanic general deposed the last western emperor, a teenager named Romulus Augustulus. The eastern half, which historians call the Byzantine Empire, lasted until the Ottoman Turks took Constantinople in 1453.

Rome fell, but its roads, laws, alphabet and calendar never quite stopped running.

Ancient China

Chinese civilisation grew up along two great rivers, the Yellow River in the north and the Yangtze in the south, and it is one of the few in the world with an unbroken thread of language and writing reaching back more than 3,000 years. Its history is traditionally told as a procession of dynasties, ruling families that rose, governed and fell.

The first dynasty confirmed by archaeology is the Shang, around 1600 to 1046 BCE. Its kings put questions to their ancestors by having diviners heat ox shoulder blades and turtle shells until they cracked, then carving the question and the answer onto the bone. These oracle bones, from around 1250 BCE onwards, carry the earliest known Chinese writing, and many of their characters are recognisable ancestors of ones used today. The Zhou, who overthrew the Shang, justified their conquest with the Mandate of Heaven: heaven granted the right to rule to a just dynasty and withdrew it from a wicked one. Floods, famine and rebellion could be read as signs that the mandate had been lost.

As Zhou power crumbled, China split into rival states. In this age of turmoil Confucius (551 to 479 BCE) taught that order flowed from virtue, respect within the family and rulers who set a good example. Then, in 221 BCE, the king of Qin conquered his rivals and declared himself Qin Shi Huang, the First Emperor. He standardised writing, coins, weights and even the width of cart axles, and he was buried near an army of some 8,000 life-size terracotta soldiers, found by farmers digging a well in 1974.

The Han dynasty (206 BCE to 220 CE) gave its name to China's largest ethnic group. It ran the state through educated officials, opened trade with Central Asia along what became the Silk Roads, and saw papermaking improved by the court official Cai Lun around 105 CE. Later dynasties, among them the Tang, Song and Ming, brought woodblock printing, gunpowder and the magnetic compass.

Did you know? The Great Wall cannot be seen from the Moon with the naked eye; it is very long but only a few metres wide, and much the same colour as the land around it.

The Great Wall is really many walls. Qin Shi Huang linked older ramparts of rammed earth, the Han extended them far to the west, and the brick-and-stone wall most visitors see today was built by the Ming between the 14th and 17th centuries. A Chinese survey published in 2012 measured all known walls and their branches at about 21,000 kilometres. Watchtowers stood within sight of one another, so that smoke by day and fire by night could carry a warning along the frontier far faster than a horse.

Dynasties came and went, yet the idea of a single China, written in a single script, outlasted them all.

Ancient India

In the 1920s, archaeologists digging in the Indus valley of what is now Pakistan uncovered something nobody expected: entire planned cities of baked brick, older than anything then known in South Asia. The Indus, or Harappan, civilisation flourished from about 2600 to 1900 BCE, and its more than a thousand known sites spread over an area larger than ancient Egypt and Mesopotamia combined.

Its two best-known cities, Harappa and Mohenjo-daro, were laid out with remarkable discipline. Main streets ran roughly north to south and east to west, dividing the lower town into blocks. Houses were built of bricks whose proportions were the same, 1:2:4, from one end of the civilisation to the other. Many homes had their own wells, bathing areas and toilets, and waste water flowed into covered brick drains beneath the streets, with openings for cleaning. On the raised citadel at Mohenjo-daro lay the Great Bath, a pool about 12 metres long, made watertight with fitted brick and bitumen and probably used for ritual washing.

Indus merchants used standardised stone weights and traded as far as Mesopotamia, where their goods arrived by sea. They marked their wares with small square seals carved with animals, among them a mythical one-horned beast, and short lines of signs. That Indus script remains undeciphered, partly because no long texts and no bilingual inscription have ever been found. Strikingly, archaeologists have found no grand royal tombs and few signs of warfare. Around 1900 BCE, as rivers shifted and the monsoon weakened, the great cities were gradually abandoned.

Did you know? Mohenjo-daro, a city of perhaps 40,000 people, had drains beneath its streets more than four thousand years before many European cities built sewers.

Centuries later, the Ganges plain became India's new heartland. The Vedas, the oldest Hindu scriptures, were composed, and around the fifth century BCE Siddhartha Gautama, the Buddha, taught there. Around 321 BCE Chandragupta Maurya seized power and founded the Mauryan Empire, which at its height covered most of the subcontinent and was ruled from the great city of Pataliputra, near modern Patna.

His grandson Ashoka conquered Kalinga on the east coast in about 261 BCE, in a war that by his own account killed or uprooted hundreds of thousands of people. Horrified, he turned to Buddhism and to rule by dhamma, moral law. His edicts, carved on rocks and polished stone pillars across the empire, urged tolerance, kindness to animals and respect for all faiths. The lion capital from his pillar at Sarnath is India's national emblem today, and his 24-spoked wheel sits at the centre of the Indian flag.

From covered drains to carved edicts, ancient India left early evidence of cities planned for public health and of a ruler who bound himself to a moral code.

The Ancient Americas

When Europeans arrived in 1492, the Americas were home to tens of millions of people and to civilisations that had grown entirely independently of those in Africa, Europe and Asia. They had domesticated maize, beans, squash and potatoes, built cities and pyramids, and developed writing, mathematics and astronomy, all without iron tools or wheeled carts, and with no large animals to ride or to pull a plough.

The Maya of southern Mexico and Central America built city-states in the tropical forest over some two thousand years. At their Classic peak, around 250 to 900 CE, cities such as Tikal, Calakmul, Palenque and Copán raised limestone pyramids and palaces, and kings recorded their reigns on carved stone monuments called stelae. Maya script, mixing word signs and syllables, is the most fully developed writing system of the ancient Americas, and most of it can now be read. Maya astronomers tracked Venus and predicted eclipses, and their Long Count calendar used a symbol for zero. In 2018, airborne laser scanning revealed tens of thousands of unknown structures hidden beneath the forest of northern Guatemala.

Did you know? The Maya never disappeared: around six million people speak Mayan languages today, and the Long Count's turnover in 2012 marked a new cycle, not the end of the world.

The Aztecs, or Mexica, founded Tenochtitlan in 1325 on an island in Lake Texcoco. They fed the city with chinampas, raised fields built up from lake mud, and linked it to the shore with causeways. By 1519 it may have held around 200,000 people, more than almost any city in Europe, and the Templo Mayor at its centre carried twin shrines to the war god Huitzilopochtli and the rain god Tlaloc. The Aztecs ruled through tribute and fear, and when Hernán Cortés arrived, tens of thousands of resentful subject peoples joined him. Tenochtitlan fell in 1521, ravaged above all by smallpox.

In the Andes, the Inca built the largest empire in the Americas, which they called Tawantinsuyu, "the four parts together". From about 1438, under Pachacuti, they expanded from their capital Cusco along some 4,000 kilometres of mountains, ruling perhaps 10 million people. A road network of around 40,000 kilometres, with rope suspension bridges and relay runners, held it together. Without writing as we know it, Inca officials kept accounts on the quipu, bundles of knotted, coloured cords. Machu Picchu, built around 1450 as a royal estate, escaped Spanish notice altogether. Weakened by civil war and epidemic, the empire crumbled after Francisco Pizarro seized the emperor Atahualpa at Cajamarca in 1532.

Conquest ended these empires, but not their peoples, whose languages, foods and knowledge are woven through the modern world.

The Silk Roads

The Silk Roads were never a single road, and silk was only one of many things that travelled along them. Even the name is modern: the German geographer Ferdinand von Richthofen coined Seidenstraße in 1877. What it describes is a shifting web of caravan tracks, river crossings and sea lanes that, for some 1,500 years, linked China with India, Persia, Arabia, Africa and Europe.

The overland routes took shape in the second century BCE. In 138 BCE the Han emperor Wudi sent an envoy, Zhang Qian, westward to find allies against the nomadic Xiongnu. He was captured and held for about ten years, but he came home with reports of rich kingdoms and of the superb horses of the Ferghana valley. The Han pushed their power west along the Hexi Corridor to the oasis of Dunhuang, where the routes split to skirt the deadly Taklamakan Desert to the north and south, rejoining at Kashgar before climbing over the Pamir mountains.

Beyond lay Central Asia, where Samarkand and Bukhara grew rich. For centuries the great middlemen were the Sogdians, merchants whose language became a common tongue of the routes. Caravans of two-humped Bactrian camels, which can carry around 200 kilograms and go for days without water, moved from oasis to oasis and rested at walled inns called caravanserais. Very few people travelled the whole 6,000 kilometres or more from Chang'an to the Mediterranean; goods were passed from hand to hand, and their price rose with every exchange.

Did you know? Early in the first century CE the Roman Senate forbade men to wear silk, which moralists condemned as decadent and as a drain on the empire's gold.

Westward went silk, lacquer, paper and, later, porcelain. Eastward came horses, glass, silver, wool, grapes and wine. Even more important was what could not be weighed. Buddhism spread from India into China along the routes, leaving cave temples such as the Mogao Caves at Dunhuang. Islam, Christianity and Manichaeism travelled too, as did mathematics, music, medicine and the art of papermaking, which reached the Islamic world by the eighth century. So did germs: the Black Death, which killed perhaps a third of Europe's people in the late 1340s, probably moved west along the trade routes.

The roads flourished most under strong empires: the Han and Rome, Tang China and the Abbasid caliphate and, in the 13th century, the Mongols, whose vast realm let Marco Polo travel to China and back. As ships learned to cross oceans, the sea routes gradually took over much of the trade.

Long before anyone spoke of globalisation, the Silk Roads had already stitched the Old World together.

The Medieval World

The Middle Ages run roughly from the fall of the Western Roman Empire in 476 to around 1500. Later writers called it a dark age, a gloomy gap between the glories of Rome and the Renaissance. The people who lived through it saw things differently. In those thousand years Europe built its cathedrals, founded its first universities at Bologna, Paris and Oxford, and invented the mechanical clock.

Society rested on land. Under feudalism, a king granted estates to great lords in return for loyalty and soldiers. Lords passed land down to knights, who owed military service, and at the bottom of the pile worked the peasants. Many were serfs, tied to their lord's manor, who farmed his fields as well as their own strips and could not leave without permission. It was less a neat pyramid than a tangle of promises, but the principle held: land flowed down, service flowed up.

Power needed walls. After the Norman Conquest of England in 1066, William the Conqueror's followers threw up quick motte-and-bailey castles, a timber tower on an earth mound above a fenced yard. Within a century many had become stone keeps. By the late 1200s, Edward I's castles in Wales, such as Beaumaris and Harlech, used concentric rings of walls, so an attacker who breached one met another.

The Church was the other great power. Monasteries following the Rule of St Benedict, written around 530, organised the day into prayer, work and study. In their scriptoria monks copied texts by hand onto parchment, preserving Latin learning; a single large Bible could use the skins of hundreds of animals. Monasteries also ran hospitals, schools, farms and breweries.

Did you know? Educated medieval people knew the Earth was round. The idea that they believed in a flat Earth is a myth popularised in the 1800s.

Then came catastrophe. The Black Death reached Europe in 1347, probably aboard ships from the Black Sea. The bacterium Yersinia pestis, spread by fleas, killed somewhere between a third and a half of Europe's people within about four years. Fields went untended and labour grew scarce, so surviving workers demanded higher wages. When rulers tried to hold wages down, anger boiled over, as in England's Peasants' Revolt of 1381. The old bonds of serfdom slowly loosened. The plague that emptied the villages also helped to end the feudal world.

The Islamic Golden Age

In 762 the Abbasid caliph al-Mansur founded a new capital on the River Tigris. Baghdad was laid out as a great circle, and within a few decades it was one of the largest cities on Earth, a crossroads of trade between China, India, Persia and the Mediterranean. From around the 8th to the 13th century, scholars across the Islamic world made it a centre of learning unmatched anywhere.

Its symbol was the House of Wisdom, the Bayt al-Hikma, a royal library that flourished under the caliphs Harun al-Rashid and his son al-Ma'mun, who ruled from 813 to 833. It powered a vast translation movement. Works by Aristotle, Euclid, Ptolemy and the physician Galen were turned into Arabic, often by way of Syriac, alongside Persian histories and Indian mathematics. The finest translator, Hunayn ibn Ishaq, was a Christian; scholars of many faiths worked side by side.

A new material made it possible. Paper-making reached the Islamic world from China in the 8th century, and Baghdad had a paper mill by the 790s. Paper was far cheaper than parchment, so books multiplied, and booksellers' markets lined the streets.

Translation soon turned into invention. Around 820, al-Khwarizmi wrote a book on solving equations whose title gave us the word algebra, from al-jabr, "restoring". His own name, in Latin form, became the word algorithm. He also helped spread the Indian decimal numerals, with their zero, that Europe later called Arabic numerals. In Cairo, Ibn al-Haytham (c. 965–1040) tested ideas about light with careful experiments and showed that we see because light enters the eye, not because the eye sends out rays. In Persia, Ibn Sina, known in Europe as Avicenna, completed his Canon of Medicine in 1025; it was taught in European universities for some five hundred years.

Did you know? Many bright stars still carry Arabic names, among them Aldebaran, Altair, Deneb and Betelgeuse.

Learning travelled west through Muslim Spain. Córdoba's library was famed across Europe, and after Christian forces took Toledo in 1085, scholars there translated Arabic books into Latin, feeding the universities of the Middle Ages and, later, the Renaissance.

The golden age had no single end, but one date stands out. In 1258 Mongol armies sacked Baghdad and destroyed its libraries; chroniclers wrote that the Tigris ran black with ink. By then its ideas had already crossed the world.

The Renaissance

Renaissance means "rebirth". The idea came from Italian scholars of the 1300s and 1400s who believed that the learning of ancient Greece and Rome had been lost and must be revived. The poet Petrarch hunted through monastery libraries for forgotten Latin texts, and his followers, the humanists, studied grammar, history, poetry and ethics: subjects about human life rather than only about God.

The movement took root in Florence, a rich city of bankers and wool merchants. The Medici family, bankers to the Pope, spent lavishly on art and scholarship, and rival guilds competed to decorate churches. Around 1415 the architect Filippo Brunelleschi demonstrated linear perspective, a geometric way of making a flat picture look deep, with lines converging on a vanishing point. He then crowned Florence Cathedral with a dome built without the usual wooden scaffolding underneath, completed in 1436. It remains the largest brick dome in the world.

Artists became celebrities. Leonardo da Vinci (1452–1519) painted the Mona Lisa, begun around 1503, and filled notebooks with studies of anatomy, flight and water, written in mirror script. Michelangelo carved his marble David, over five metres tall, between 1501 and 1504, then painted the Sistine Chapel ceiling from 1508 to 1512. Popular legend says he did it lying on his back; in fact he stood on a scaffold and painted upwards, craning his neck. Raphael's School of Athens gathered the great philosophers of antiquity into one imagined hall.

Did you know? Michelangelo's David was carved from a tall block of marble that other sculptors had started on and then abandoned decades earlier.

A machine spread these ideas faster than any artist could. Around 1450 in Mainz, Johannes Gutenberg developed printing with movable metal type, and his Bible appeared around 1455. By 1500 presses were working in more than two hundred European towns, and millions of books had been printed. Scholars could now compare identical copies, and new ideas could no longer be easily contained.

The Renaissance moved north too. Jan van Eyck in Flanders perfected oil painting, Albrecht Dürer in Germany turned prints into fine art, and the scholar Erasmus edited classical and Christian texts for a growing reading public. The word Renaissance for the whole period was popularised only in the 19th century, but people at the time sensed something new. The age taught Europe to look at the world with fresh eyes and draw it in depth.