Sapiens++

Before Us

Anyone who grew up with an analogue television knows the sound: the hiss between channels and the dancing grey snow that went with it. Most of that snow was mundane, electrical noise from the set, from the Sun, from a neighbour's motorbike. But a small fraction of it, by some estimates around one flicker in a hundred, was something else. It was radiation that had been travelling since before any star existed, arriving in a living room after more than thirteen billion years to be ignored by someone looking for the football. That faint, uniform glow is the oldest light that can be observed, and it is the main reason cosmology is a science rather than a branch of speculation. It is evidence of a universe that was once hot, dense and nearly featureless, with no one in it. The question of this chapter is what can be said about such a place, and what cannot.

Begin with what the glow tells us. In 1964 two radio astronomers in New Jersey found a persistent hiss in their horn antenna that would not go away whatever they pointed it at, and after evicting the pigeons they concluded it was coming from the sky in every direction equally. Physicists down the road had been preparing to look for exactly that. If the universe had expanded from a hot dense state, there ought to be a leftover bath of radiation, cooled by the expansion to a few degrees above absolute zero. It is now measured at 2.725 kelvin, with a spectrum matching a perfect thermal source more closely than anything made in a laboratory.

That radiation was released when the universe was roughly 380,000 years old. Before then it was a plasma too hot for atoms to hold together, and light scattered off free electrons the way a torch beam scatters in fog. When it cooled enough for electrons to settle onto nuclei, the fog cleared, quickly by cosmic standards, and the light set off across a suddenly transparent space. We see it from every direction because it comes from a shell around us: the places whose light has had just enough time to arrive. Look further and you would be looking earlier, but there is nothing to see; the fog is in the way.

The expansion itself was found first. In the 1920s astronomers noticed that the light of distant galaxies is stretched towards the red, and that the further the galaxy, the greater the stretch. Run that film backwards and everything converges. Measured carefully, from the glow, from the galaxies and from exploding stars used as distance markers, the convergence happened about 13.8 billion years ago. The glow is not perfectly smooth. Satellites have mapped ripples in it of about one part in a hundred thousand, slightly warmer and cooler patches that are the seeds of everything that followed: galaxies, stars, planets, you. The size and pattern of those ripples let cosmologists weigh the universe, and the weighing finds that ordinary atoms make up about five per cent of it. The rest is two things we have named but do not understand: dark matter, which pulls, and dark energy, which pushes.

That is the established core, and it is worth being precise about its shape. The Big Bang is not an explosion at a point in a pre-existing space; it is space itself stretching, everywhere at once, with no centre and no edge that anyone has found. The 13.8 billion years is the age of the expansion, not a measured moment of creation. Trace the equations back far enough and they break; that breakage is called a singularity, and it is a sign the theory has run out, not a description of an event.

What this does is give the ingredients of a body a timeline instead of a backdrop. The universe is not a stage that was always there with the lights on. It had a first morning, or at least a first measurable one, and for most of its history nothing in it was alive. Compress 13.8 billion years into a single year and the glow is released in the first quarter of an hour of 1 January; the Sun and Earth form at the start of September; the first humans appear in the final quarter of an hour of 31 December; everything in this book from farming onwards happens in the last half-minute. The ordinary matter that will become a hand exists from the start, as protons and electrons in a plasma, and then waits. Nothing is looking. There are no colours, because there are no eyes. The history was real without being witnessed. That is a strange thing to hold, and most human traditions have found it easier to put someone at the beginning to do the watching. The evidence does not need them. It does not refute them either; it is silent on the matter, which is a different thing.

The honest edges should be marked. Cosmology can investigate the observable universe, the region whose light has reached us, now some tens of billions of light years across because space has stretched while the light travelled. Beyond that it cannot see, and it does not know whether the whole is finite or infinite. It cannot say what, if anything, preceded the hot dense state. "Why is there something rather than nothing?" is a fine question, but no measurement has been proposed that would answer it, and a physicist who tells you the universe came from nothing has quietly redefined nothing to mean a vacuum with laws, which is a great deal of something. The same goes for purpose: the glow does not read as a message. The leading account of the first fraction of a second, a burst of extraordinarily rapid expansion called inflation, explains the smoothness and the ripples neatly, but its distinctive predictions are hard to test and some cosmologists regard it as a placeholder. There is also a live quarrel inside the measurements. The expansion rate inferred from the early glow and the rate measured from nearby exploding stars disagree by several per cent, more than their stated errors allow. Either a measurement is subtly wrong or the model is missing a piece. That is how a healthy field is supposed to feel.

In the Human++ atlas, this chapter is the dark stage before the figure appears: the particle field inside the silhouette still switched off, the space around it faintly lit by a glow that is everywhere and nowhere. It is the first and widest form of dependence. A body relies on a universe that had the right history, and nothing about that history was negotiable or chosen. The rest of this part fills the dark stage with matter. To do that, the first question is small and physical rather than grand: when the plasma cooled, what exactly was in it, and why is one of those ingredients still the most numerous atom in your hand?

Evidence: Established: the cosmic microwave background at 2.725 K with a near-perfect thermal spectrum; its release at roughly 380,000 years; cosmic expansion and an age of about 13.8 billion years; temperature ripples of about one part in 100,000; a matter–energy budget of roughly 5% ordinary matter, 27% dark matter, 68% dark energy. Debated or uncertain: inflation as the account of the first instant; the "Hubble tension" between early- and late-universe expansion rates; the nature of dark matter and dark energy; the fraction of analogue TV static attributable to the background (popularly quoted near 1%, not a precise measurement). Outside evidence: what preceded the hot dense state, whether the universe is finite, and any question of purpose.

The First Ingredients

A child lets go of a helium balloon at a fair, and it rises, dwindles and is gone, and somewhere a parent explains that helium is lighter than air. That is true, and it is the least interesting thing about it. The gas in the balloon was not drawn from the atmosphere, which holds almost none; it was pumped from underground, where it collects in pockets of natural gas as uranium and thorium in the rock slowly decay and spit out helium nuclei. Earth's original helium, the share it was born with, is long gone. The planet's gravity is too weak to hold anything so light, and it leaks upwards and away. The balloon's helium will do the same. Within a few years it will be at the top of the atmosphere, and then in space, where it joins the overwhelming majority of its kind: helium made not in rocks or stars but in the first minutes of the universe. Two elements come from that time, helium and hydrogen. One of them is the most numerous atom in your body, and it has no other origin story at all.

The window was short. In the first second the universe was a soup of particles hot enough that protons and neutrons flickered into each other. As it cooled, that conversion slowed and stopped, leaving roughly one neutron for every six protons. A free neutron is unstable; left alone it decays in about fifteen minutes. So there was a race. For nuclei to form, protons and neutrons had to stick together, and the first step, a proton and a neutron making deuterium, was constantly undone by the flood of high-energy light still filling space. Only when the temperature fell below about a billion degrees, a few minutes in, could deuterium survive long enough to take the next step. By then decay had thinned the neutrons to about one in seven.

Once deuterium held, the rest went fast. Deuterium plus deuterium, or deuterium plus a proton, led quickly to helium-3 and then to helium-4, which is exceptionally stable and acted as a trap: nearly every neutron that still existed ended up inside one. Do the arithmetic with one neutron in seven and you get about a quarter of the mass of ordinary matter as helium and about three quarters as hydrogen, with traces of leftover deuterium, helium-3 and lithium-7. That is almost exactly what astronomers measure in the most pristine gas clouds they can find.

Then the process stalled, for two reasons. There is no stable nucleus with five or eight particles in it, so helium could not simply add a proton or merge with another helium to climb the ladder. Getting past those gaps needs three helium nuclei to meet almost at once, which requires densities and temperatures that exist only inside stars. And the universe was expanding and cooling too quickly to supply them. Within about twenty minutes the furnace was off. For something like a hundred million years, until the first stars, the entire periodic table consisted of hydrogen, helium and a trace of lithium.

Notice what this means for hydrogen. Stars do not make it; they burn it. Every hydrogen nucleus that exists today, with negligible exceptions, is a proton from that first second. The hydrogen in your hand's water, in its proteins, in the fat under its skin, was not forged in any star. It was there before stars, and it has been waiting for roughly 13.8 billion years to be part of something.

Being made mostly of the earliest material changes the shape of your biography, if you count atoms rather than weigh them. By number, around six in every ten atoms in a human body are hydrogen, because water is two hydrogens to one oxygen and the body is mostly water. By mass, hydrogen is only about a tenth, because each atom is so light. So the body's most common atom and its heaviest contributors come from different eras entirely, separated by hundreds of millions of years and by the lives and deaths of stars. The slogan that we are stardust, which a later chapter weighs more carefully, is true of the carbon, oxygen and iron and simply false of the hydrogen.

The early universe did something else as well: it fixed the proportions of fuel. Because the first minutes left three quarters of all ordinary matter as hydrogen, every later star began with a full tank. Had the neutron been a little more stable, or the expansion a little slower, more of the hydrogen would have been locked into helium at the start, and the stars that followed would have burned differently and died sooner. That is not evidence the universe was arranged for us. It is a statement that we are the kind of thing that happens in this one.

How do we know any of it? Partly from the glow of the last chapter, whose ripples reveal the density of ordinary matter, which in turn predicts the exact amount of leftover deuterium. Astronomers measure deuterium in gas clouds billions of light years away, lit from behind by distant quasars, and the number agrees with the prediction to within a few per cent. A theory about the universe's third minute, tested against light that has crossed most of its history and matching to that precision, is as close as cosmology comes to a receipt.

One entry on the receipt does not add up. The same calculation predicts how much lithium-7 should have been made, and the oldest stars in our galaxy show about a third of it. Astronomers have argued about this for decades. Perhaps those stars have slowly destroyed their lithium by mixing it down into hotter layers, which is plausible but hard to prove. Perhaps a nuclear reaction rate is mismeasured. Perhaps some unknown particle altered the early chemistry, which would be far more exciting and is correspondingly less likely. The lithium problem is small in a periodic table of ninety-odd natural elements, but it is a useful reminder that "established" in science means tested and not yet broken, not closed.

A second caution concerns the word beginning. The first minutes are not the first instant. Before the protons and neutrons there was a hotter state of quarks; before that, the story is reconstructed from particle physics and extrapolation rather than from anything observed. Nucleosynthesis is the earliest process for which there is a quantitative fossil. Earlier claims are physics, often good physics, but they are not yet evidence.

In the Human++ atlas, this is the moment the particle field inside the silhouette first lights up, and it lights up almost entirely in one colour: the hydrogen that fills the body by count, before any heavier element has been made. Select a single hydrogen atom in the palm and the origin panel shows no star and no explosion, just a cooling plasma and a clock reading three minutes. That is the body's oldest dependence, older than light itself. But a universe of thinly spread hydrogen and helium is not yet a universe that can make anything. Something had to pull it together, and the something was the weakest force there is.

Evidence: Established: Big Bang nucleosynthesis in the first minutes producing about 75% hydrogen and 25% helium by mass with traces of deuterium, helium-3 and lithium-7; the neutron-to-proton ratio and the deuterium bottleneck; the absence of stable mass-5 and mass-8 nuclei; measured primordial deuterium agreeing with predictions from the cosmic microwave background; terrestrial helium being radiogenic and escaping to space; hydrogen as roughly 60% of body atoms by number and about 10% by mass. Debated or uncertain: the primordial lithium problem (observed about one third of predicted); the exact timing of the first stars (quoted as "something like a hundred million years"); physics earlier than nucleosynthesis, which is extrapolated rather than directly observed.

Gravity Builds a Stage

Print a map of the nearby universe, one dot per galaxy, and pin it to a wall. From across the room it does not look like scattered dust. It looks like a sponge, or the inside of a bone: bright knots joined by long filaments, with great dark voids between them where almost nothing lives. Our galaxy is one dot on one filament. Astronomers have charted millions of galaxies this way and the pattern holds at every scale they can reach. That is the stage on which everything in this book takes place, and it was built by the weakest force in nature. Gravity is so feeble that a fridge magnet beats the pull of the entire Earth on a paperclip. But gravity has two advantages the other forces lack. It only ever attracts, and it never switches off. Give it a nearly smooth universe and a few billion years, and it will produce a sponge.

The nearly smooth universe is the one the glow described: matter spread almost evenly, with ripples of a part in a hundred thousand. Gravity feeds on differences. A region slightly denser than its surroundings pulls a little harder, gathers a little more, and so pulls harder still, while the regions it drains grow emptier. Over hundreds of millions of years the faint ripples steepen into the sponge: matter streaming out of the voids along sheets, sheets draining into filaments, filaments feeding the knots where clusters of galaxies form. Structure grew from the bottom up, small clumps first, merging into larger ones, which is why old galaxies carry the remains of many smaller ones they swallowed.

Two things complicate the simple picture, and both matter. The first is that most of the pulling was not done by ordinary matter. The ripples in the glow are too faint, by themselves, to have grown into galaxies in the time available; light was pushing the ordinary gas apart almost as fast as gravity pulled it together. But dark matter, whatever it is, does not feel light. It began collapsing early, forming invisible halos, and the ordinary gas fell into wells that were already dug. Every galaxy, including ours, sits inside a halo of material that outweighs its stars and gas several times over. No one has yet caught a dark matter particle; its existence is inferred from how things move and how light bends.

The second complication is that gathering is not enough. A cloud of gas squeezed by gravity heats up, and hot gas pushes back; left to itself it would simply bounce. To collapse further the cloud has to lose heat, which means radiating it away as light. Atoms do this well once they have electrons to jiggle, but in the early universe, with nothing but hydrogen and helium, the only efficient coolant at low temperatures was molecular hydrogen, which forms grudgingly and radiates weakly. So the first clouds that managed to collapse were enormous and hot, and the objects that condensed inside them were correspondingly large. Cooling, not gravity, set the size of the first stars.

The sequence, then: ripples, invisible halos, gas falling in and cooling, fragmentation into clouds, clouds into dense cores. By around a hundred million years after the beginning, by current estimates, the first of those cores reached the temperature at which the next chapter can start.

What gravity did for the eventual body is less obvious than what stars did, but it comes first. A universe of evenly spread gas, however rich, makes nothing. There is no fire without concentration, no chemistry without places where things meet. Gravity created places: here dense, there empty; here hot, there cold. Every later process in this book runs on differences of that kind. A star is a difference between its core and the void around it. A planet is a difference between rock and space. A living cell, a few chapters on, will be a difference maintained between inside and outside. Gravity is the first maker of insides and outsides, and it needed no instructions.

It also set a budget. The amount of ordinary matter gathered into our galaxy, and the pace at which it was gathered, decided how many generations of stars could form before the Sun, and so how much carbon and iron the Sun's cloud would inherit. Galaxies that made stars too greedily and too early blew their gas away and sit quiet now; dwarf galaxies that never gathered much are poor in heavy elements to this day. The Milky Way happened to be a galaxy of the right mass and temperament to have run several rounds of stars by the time one cloud in one spiral arm collapsed into a Sun. "Right" there means right for producing us, not right in any absolute sense. From a dwarf galaxy's point of view, it is doing fine.

The temptation at this point is a story in which the dials were set for our benefit. Had gravity been a little stronger, stars would burn out fast; a little weaker, and perhaps no stars at all. The observation is true and the inference from it is weak. We can only find ourselves in a universe whose settings permit finding, so the fact that they do is not a surprise requiring a planner; it is a selection effect. Whether there are other universes with other settings is an open question with no observational handle, which places it, like absolute beginnings, outside evidence.

There are more productive disputes. The standard model of cold dark matter predicts, at small scales, more dwarf satellite galaxies around the Milky Way than are easily found, and denser cores in those dwarfs than are measured; proposed fixes range from better accounting of ordinary gas to different kinds of dark matter. And since 2022 the new infrared space telescope has found galaxies in the first few hundred million years that look brighter and more massive than some simulations expected. Whether this breaks the model or merely means early star formation was more efficient than assumed is being argued now. The sponge is not in doubt. The recipe for its finest threads still is.

The atlas treats this chapter as the pull-back shot: the camera leaves the particle field inside the silhouette, rises past the figure, and keeps rising until the stage itself becomes one bright knot in a web of them. Then it returns. The point is orientation, not vertigo. The body is a local concentration in a universe whose default is emptiness, and its material came to it through a long series of gatherings it had no part in. The first of those gatherings is about to reach a temperature no cloud had reached before, and a dark universe is about to get its first light.

Evidence: Established: growth of cosmic structure from the small density ripples seen in the cosmic microwave background; the filament-and-void "cosmic web" mapped in galaxy surveys; the need for dark matter to explain structure growth, galaxy rotation and gravitational lensing; hierarchical (bottom-up) assembly; poor cooling of metal-free gas via molecular hydrogen and its effect on the first collapsing clouds. Debated or uncertain: the nature of dark matter (never directly detected); small-scale tensions such as the "missing satellites" and "core–cusp" problems; the apparent maturity of some very early galaxies seen by JWST since 2022; the exact epoch of the first collapsing cores ("around a hundred million years"). Outside evidence: fine-tuning arguments and multiverse hypotheses.

The First Stars

Stand in winter sunlight and turn the back of your hand to it. The warmth arrives after an eight-minute crossing from the Sun's surface, which is quick. The crossing before that was not. The energy on your skin was released in the Sun's core, and light born there cannot travel in a straight line; it is absorbed and re-emitted countless times, staggering outwards through dense plasma a fraction of a centimetre at a time. Estimates of how long that random walk takes range from tens of thousands of years to well over a hundred thousand. So the warmth on your hand is, in a reasonable sense, older than agriculture. It is also second-hand. The Sun is doing now what a much earlier generation of stars did first, in a universe that had never seen a star, and this chapter is about those first ones: what happened inside them, what came out, and why they are not the factories a tidy story would prefer.

A star is a stand-off. Gravity squeezes a ball of gas; the gas heats; the heat pushes back. For most of a star's life the two forces balance, and the balance is held by a furnace at the centre that burns just fast enough to replace the heat leaking from the surface. The furnace is nuclear fusion. In the core, at around fifteen million degrees in the Sun's case, hydrogen nuclei move fast enough to approach one another closely despite their electrical repulsion, and quantum mechanics lets them occasionally tunnel through the last of it. Through a short chain of steps, four protons become one helium nucleus. The helium weighs about seven tenths of one per cent less than the four protons did. The missing mass has become energy. The Sun converts around four million tonnes of matter into light every second and has been doing so for four and a half billion years without noticeably dimming.

The first stars did the same with worse ingredients. They formed, by current estimates, between a hundred and two hundred million years after the beginning, from gas that was hydrogen and helium and nothing else. As the previous chapter noted, such gas cools badly, so the clouds that collapsed were massive, and the stars they made were probably giants: tens of solar masses, perhaps hundreds, hot enough to glow blue-white and pour out ultraviolet. Big stars burn fast. A star of a hundred solar masses lives a few million years, a thousandth of the Sun's span, before its core runs out of hydrogen.

Then the stand-off resumes at a higher stake. With the furnace out, gravity squeezes the core further; it heats until helium itself can fuse, and here, for the first time anywhere, three helium nuclei meet to make carbon, and carbon takes a fourth to make oxygen. In the most massive stars the squeezing continues through stages, carbon, neon, oxygen, silicon, each shorter than the last, each leaving a heavier ash, until the core is iron. Iron is where it ends. Fusing iron costs energy rather than releasing it, so the furnace has nothing left to burn, and the star has, within about a day, a very bad time. A later chapter follows that ending. For now, the point is what the burning did to the universe outside. Ultraviolet light from the first stars stripped electrons from the hydrogen around them, and over several hundred million years those bubbles of ionised gas grew and merged until the whole universe was transparent again, in a new way. The dark age ended, and the first heavy elements, blown out when those stars died, mixed into the surrounding gas.

The change that matters most is one the first stars made for the second ones. Even a small dusting of carbon and oxygen lets a gas cloud cool far more efficiently than pure hydrogen can, and a cloud that cools well can fragment into smaller pieces. So once the first generation had seeded the gas, the stars that formed from it could be smaller, longer-lived and more numerous. Something like the Sun, a modest star that burns steadily for ten billion years, is possible only in enriched gas. The first stars rewrote the rules of star-making, and every later star, and every planet around a later star, lives under the amended rules.

The second change is light. Before the first stars the universe was a warm, darkening fog, lit only by the fading glow of its own beginning. After them there were points of light, and shadows, and places brighter than others. This matters more than it sounds. Every process that will later drive chemistry, climate and eventually life on a planet is driven by a flow of energy from a hot source to a cold sink, and fusion in stars is the source for nearly all of it. The warmth on the back of your hand is the far end of a supply chain that began in those first furnaces. Nothing in the chain was arranged for your hand. It is simply what hot hydrogen does.

No one has seen a first star. Everything above is inference: from simulations of collapsing primordial gas; from the chemistry of the oldest stars in our galaxy, which are poor in heavy elements and carry patterns that look like the ash of very massive predecessors; and from faint early galaxies whose light hints at hot, metal-free populations. That evidence is good enough to be confident the first stars were massive and short-lived, and not good enough to settle how massive. Early simulations suggested lone monsters of several hundred solar masses; more recent ones find the collapsing discs tend to fragment, producing clusters and pairs with a wider spread, perhaps including some stars small enough to be alive today. If one is ever found, this chapter will need a new paragraph.

There is also a word to resist. A star is not a factory, in the sense of a place arranged to produce something. It makes carbon because carbon is what helium does under pressure, with no more intent than a kettle has in making steam. The elements of a body came out of stars as exhaust and wreckage. Calling them products puts a purpose into the furnace that the physics does not contain.

The atlas opens this chapter from the particle field inside the silhouette with a single schematic star: not the Sun but a hot, blue, early one with no planets, labelled as a reconstruction because no photograph of it exists or ever will. Following the star back from the hand is an exercise in dependence without obligation. The body needs fusion to have happened; fusion owes the body nothing. What the first stars made in bulk was helium, carbon and oxygen. Which elements, exactly, a hand requires, and where each of them was made, is the question the next chapter puts to the hand itself.

Evidence: Established: stellar fusion as a balance of gravity and pressure; the proton–proton chain converting about 0.7% of mass to energy; the Sun's roughly four million tonnes per second; helium burning to carbon and oxygen and the advanced burning stages ending at iron; cosmic reionisation by early ultraviolet sources, largely complete by about one billion years; enriched gas cooling and fragmenting more readily than primordial gas. Debated or uncertain: the mass distribution of the first stars (tens to hundreds of solar masses; recent simulations favour fragmentation); whether any low-mass first-generation stars survive; the exact formation epoch ("between a hundred and two hundred million years"); the photon escape time from the solar core (estimates span roughly 10,000 to 170,000 years). No first-generation star has been directly and unambiguously observed.

The Elements of a Body

Hold your hand up to a lamp, palm towards you, and look at the thin web of skin between thumb and forefinger. Light comes through it faintly red. That red is iron, carried in haemoglobin, in blood that is mostly water, which is mostly hydrogen by count of atoms and mostly oxygen by weight. The skin itself is protein and fat: long chains built on carbon, hinged with nitrogen, salted with sulphur and phosphorus. Nothing in the hand is exotic; every element in it appears on the wall chart of an ordinary school chemistry room. What is strange is not the ingredients but their provenance. The atoms in your hand did not come from one place or one time. Some are almost as old as the universe. Some were made inside stars that died before the Sun existed. A few were probably forged when two dead stars collided. Your hand is an inventory of very old events, most of them violent, none of them concerned with you.

Take the inventory by mass, roughly. Oxygen makes up around two thirds of a human body, carbon just under a fifth, hydrogen about a tenth, nitrogen around three per cent. A dozen others, calcium and phosphorus foremost, make up most of the remainder; iron, zinc, iodine and the trace elements would fit in a teaspoon. By number of atoms the order flips: hydrogen is by far the most numerous, because it is light and because the body is largely water.

Those two lists have different biographies. Nearly all the hydrogen was made in the first minutes after the Big Bang, when the universe had cooled enough for protons to exist and was still briefly hot enough to fuse some of them into helium and a trace of lithium. Then the universe had expanded too far for anything heavier. For something like a hundred million years, by current estimates, that was the entire periodic table.

Carbon required stars. Inside a star that has burned most of the hydrogen in its core, temperatures rise until helium nuclei can fuse. Two of them make a nucleus of beryllium so unstable that it falls apart in a fraction of a femtosecond; the trick is that a third helium nucleus occasionally arrives before it does, and the result is carbon. This triple collision is so improbable that it works only because the carbon nucleus happens to have an energy level that resonates with the incoming pieces, a coincidence physicists predicted before anyone measured it. Add another helium nucleus to carbon and you have oxygen. Massive stars continue the ladder, neon, magnesium, silicon, up to iron, which releases no energy when fused and so brings the furnace to a halt. Nitrogen comes mostly by another route, a cycle in which carbon acts as a catalyst for hydrogen fusion and leaves nitrogen behind. Calcium and phosphorus are largely products of massive stars and the explosions that end them.

Anything heavier than iron cannot be built by fusion at a profit. It is built by neutron capture, either slowly over thousands of years in the outer layers of ageing stars, or violently in seconds inside supernovae and in the merger of two neutron stars. In 2017, astronomers watched the afterglow of one such merger and read in its spectrum the signatures of freshly made heavy elements. Some of the iodine in your thyroid, and the gold in a ring, plausibly came from events like that.

How do we know? Mostly from light. Every element absorbs and emits light at its own fixed wavelengths, so a star's spectrum is a fingerprint of its composition, and the oldest stars carry less of the heavy elements than younger ones, as a slow build-up over generations would predict. Laboratories measure the nuclear reactions directly. Meteorites preserve mineral grains older than the Solar System whose isotopes match particular kinds of star.

What this inventory changes is the scale of your own history. Before there were stars, a hand was impossible, not because life was unlikely but because there was nothing to make it from. Carbon's talent for forming four bonds, and for linking into chains and rings that are stable but not too stable, is what makes complicated chemistry possible at all; without several generations of stars synthesising it and scattering it, there would be no proteins, no DNA, no fat beneath the skin. Oxygen was needed for water, phosphorus for the backbone of genes and for the small molecule that carries energy inside every cell, iron for the blood that shows red against the lamp.

There is a subtler change, too. The elements did not simply exist; they were distributed. A star that makes carbon and keeps it contributes nothing to a future planet. The carbon had to be blown off in stellar winds or hurled out in explosions, mixed through clouds of gas, and gathered again into a new star and its disc of rubble. The Sun is at least a second-generation star and probably a later one; the Earth is built from debris that had been through a furnace more than once. Your hand is therefore not merely made of old atoms. It is made of recycled ones, and it is still recycling them. Over years, most of your atoms are swapped for others as cells die and food is eaten. You are a pattern that borrows material, not a fixed lump of it.

The temptation here is a slogan. "We are stardust" is beautiful and roughly ninety per cent right by mass, which is a high score for a slogan, but it hides two things. First, hydrogen, your most numerous atom, is not stardust at all; it predates every star. Second, the phrase slides easily into a story in which the cosmos was labouring towards us, in which stars were factories with a product in mind. Nothing in the evidence supports that. The processes that made your carbon made the carbon in a limestone cliff, a diamond and a smear of soot. The universe did not aim.

There are honest gaps as well. Astronomers still argue about how much of the heaviest elements came from neutron star mergers and how much from rare kinds of supernova, because the timing of when those elements appear in old stars does not fit mergers alone. And the body-by-mass figures are averages; children, older adults, heavier and lighter people differ in water, fat and bone. The index is useful. It is not a receipt.

In the Human++ atlas this is the particle field that fills the silhouette: select an atom in the palm, follow it out to a schematic star, come back. The journey is not there to flatter the hand but to settle its first dependence. Before any tool, any institution, any machine, the body already relied on capability that lived entirely outside it, in furnaces it will never see. Nothing in that history was chosen. What is new, and is the rest of this book, is that choosing has begun. First, though, a question of delivery: carbon locked inside a living star is no use to anyone, so how did the furnaces give their products back?

Evidence: Established: Big Bang nucleosynthesis of hydrogen, helium and trace lithium; carbon via the triple-alpha process and the predicted carbon resonance; oxygen through iron in massive stars; neutron-capture origin of elements heavier than iron; spectroscopic signatures of heavy elements in the 2017 neutron-star merger afterglow; approximate body composition by mass and by atom count. Debated or uncertain: the relative contribution of neutron-star mergers versus rare supernova types to the heaviest elements, and the timing problem in old stars; precise per-element source fractions for the Solar neighbourhood; the exact age at which the first stars formed (quoted here as "something like a hundred million years").

Death That Distributes

In the summer of 1054, court astronomers in Song dynasty China recorded a "guest star" near one horn of the constellation we call Taurus. It was bright enough to be seen in daylight for about three weeks and stayed visible at night for the better part of two years before fading. Point a modest telescope at the same spot today and you find a smudge: a tangle of expanding filaments some six thousand light years away, still flying apart at around 1,500 kilometres a second, with a spinning neutron star at its centre flashing thirty times a second. The Crab Nebula is what is left of a star. It is also a delivery. Everything that star made in its last million years of burning, and some things it made only in its final second, is now spread across a region several light years wide and thinning into the gas between the stars. Give it a few hundred million years and some of it will be in a planet.

Stars return material to space by several routes, and the body's elements take all of them.

The gentlest is wind. Every star sheds some of its outer layers continuously, and ageing stars shed a great deal. A star like the Sun, once its core hydrogen is spent, swells into a red giant and in its final phase pulses and puffs away its outer half over a few tens of thousands of years, leaving the hot core exposed as a white dwarf. The expelled shell glows for a while as what astronomers unhelpfully call a planetary nebula. It carries carbon dredged up from the star's interior, nitrogen, and a dusting of heavier elements built slowly, one neutron at a time, during the star's old age. A large share of the universe's carbon may have reached space this way, from unspectacular stars going quietly.

The violent route belongs to stars more than about eight times the Sun's mass. Having burned their way to an iron core, they have nothing more to burn, and once that core exceeds about 1.4 solar masses it can no longer hold itself up. It collapses in under a second to a ball of neutrons some twenty kilometres across, or further into a black hole. The outer star, suddenly unsupported, falls inward, meets the rebounding core, and is blown out by a shock and a vast flood of neutrinos. In 1987 detectors on Earth caught a couple of dozen neutrinos from a supernova in a neighbouring galaxy, hours before its light arrived, which is how we know the picture is roughly right. The ejected layers carry the oxygen, neon, magnesium, silicon, sulphur and calcium made in the star's onion-skin burning, along with iron and nickel cooked in the explosion itself.

A third route needs no massive star. A white dwarf in a close pair can gain mass from a companion, or merge with another white dwarf, and if it tips over the same limit it detonates completely, leaving nothing behind. These thermonuclear supernovae make iron in quantity; by most estimates more than half of the iron in our galaxy came from them. The rarest route is the collision of two neutron stars, which the previous chapter described: the source of much of the heaviest material.

What leaves a star does not go straight into a planet. It ploughs into the interstellar gas, is stirred by other explosions and the slow turning of the galaxy, and mixes over hundreds of millions of years. By the time the Sun's cloud collapsed, about nine billion years into the story, heavy elements made up between one and two per cent of its mass. That small percentage is the Earth.

The consequence for a body is that it is built from the output of deaths. Much of its carbon it likely owes to the gentle route, red giants shedding their skins. Its oxygen, calcium and much of its phosphorus to the explosions of massive stars. A good part of its iron to white dwarfs detonating. Its iodine to colliding neutron stars. No single star made a person's ingredients; it took a galaxy's worth, dying in sequence over billions of years, and the mixing in between.

This is inheritance, but it is also recycling, and the distinction matters for the book's thesis. Nothing in a body was created for it. The matter was used before, in a star's core, as fuel or ash, and after it leaves the body it will be used again. The cosmic story is not one of new material appearing but of old material being handed on in altered form, by processes that do not know they are handing anything to anyone.

There is a nearer reminder. In crusts of iron and manganese on the deep ocean floor, which grow by a few millimetres per million years, scientists have found a rare radioactive form of iron that natural processes on Earth do not make. It is concentrated in layers laid down between about two and three million years ago, and again earlier. The best explanation is that one or more supernovae exploded within a few hundred light years of the Sun around then and dusted the planet with debris. Early human ancestors were walking about while it settled out of the sky.

The death language is doing a lot of work. A star does not die in any sense that matters to a star; it exhausts a fuel and changes state. "Sacrifice" is worse. The romance of stars giving themselves so that we might live is pleasant and precisely backwards: the elements were scattered because scattering is what unsupported gas does, and most of them went nowhere near a planet.

The mechanics are also less settled than the narrative. For decades, computer simulations of core-collapse supernovae failed to explode: the shock stalled, and the models needed the neutrinos to revive it in ways that were hard to get right. Modern three-dimensional simulations usually do explode, but which stars explode and which quietly collapse into black holes, and how much of each element escapes, remain active research. The white dwarf route has its own argument, over whether most such explosions come from a dwarf stealing gas from an ordinary companion or from two dwarfs merging, which changes how fast the galaxy was salted with iron. And the share of the heaviest elements from neutron star collisions versus rare kinds of supernova is still contested. The distribution happened. The itemised invoice is being audited.

In the atlas, this chapter sits in the particle field between two frames, the star of the previous chapters and the disc of the next. Select the field and each element in the palm shows its route out of a star: wind, blast or collision. The body's dependence here is on events violent enough to tear stars apart, and on the long, patient mixing that followed. There was never a plan for the material to land anywhere in particular. Yet some of it, drifting in a cold cloud in one arm of one spiral galaxy, was about to be gathered a second time, into a disc, with a new star at the centre.

Evidence: Established: SN 1054 as recorded in Chinese annals and its remnant the Crab Nebula with a central pulsar; mass loss through stellar winds and planetary nebulae carrying carbon, nitrogen and slow-neutron-capture elements; core collapse of massive stars above roughly 1.4 solar masses of iron core, neutrino emission confirmed by SN 1987A; thermonuclear white-dwarf supernovae as a major iron source; solar heavy-element fraction of roughly 1–2% by mass; live iron-60 in deep-sea crusts from nearby supernovae a few million years ago. Debated or uncertain: the detailed explosion mechanism and which progenitors explode; single- versus double-degenerate origins of thermonuclear supernovae; the precise iron share from each route ("more than half" is an estimate); the relative contribution of neutron-star mergers and rare supernovae to the heaviest elements.

A Sun, a Disc, a World

A slice of a certain kind of meteorite, polished and held up to the light, looks like a dark fruitcake: a grey matrix studded with small round grains and, here and there, irregular white flecks a few millimetres across. The white flecks are the oldest solid objects anyone has dated. They are rich in calcium and aluminium, condensed from hot gas, and their radioactive clocks read 4,567 million years, give or take less than one. That number is, by convention, the birthday of the Solar System. The meteorite carrying them drifted in space, essentially unchanged, for the whole of that time, until it fell on a farm or a desert and someone picked it up. Earth has nothing that old at its surface; the planet has been melting, folding and re-eating its rocks for too long. To read the beginning of your world you have to hold a piece of what it was made from before it was a world.

It began with a cloud in trouble. Somewhere in a spiral arm of the Milky Way, a cold, dark cloud of gas and dust, enriched by the deaths of the previous chapter, had a dense core that gravity could no longer be argued out of. Possibly a nearby supernova gave it a shove; certain short-lived radioactive isotopes preserved in the meteorites suggest fresh stellar debris was present, though whether it triggered the collapse is debated. The core fell inward. But a cloud is never perfectly still, and any slight rotation is amplified as material falls in, the way a skater spins faster pulling in her arms. Material could fall freely along the spin axis but not across it, so the collapsing core flattened into a spinning disc with a growing lump at the centre. The lump became the Sun. The disc became everything else.

The disc was overwhelmingly gas, with a sprinkle of dust: about one part in a hundred. Near the young Sun it was hot enough to vaporise rock; further out, beyond a boundary astronomers call the snow line, water froze onto grains and there was suddenly several times more solid material to work with. That is why the Solar System is sorted: small rocky worlds close in, giants with deep atmospheres further out, ice beyond. Dust grains collided and stuck, growing to pebbles. Pebbles concentrated in eddies of the gas and collapsed under their own gravity into bodies a kilometre or more across, though exactly how that step happened is one of the field's livelier arguments. Those bodies collided and merged into planetary embryos the size of the Moon or Mars. The gas itself did not last: within a few million years the Sun's radiation and wind had blown it away, so the giant planets, which needed gas, had to grow fast. The rocky planets were in less of a hurry. Earth was assembled over tens of millions of years from embryos smashing together, the last and largest collision probably the one that threw off the debris that became the Moon.

Since 2014 radio telescopes high in the Chilean desert have photographed other discs around other young stars, and they show concentric bright rings and dark gaps, very likely carved by planets forming inside them. The picture above is not only reconstruction from our own rubble. It is being watched happening elsewhere.

For the atoms that would become a body, the disc was a sorting machine. The material of the previous chapters arrived as a near-uniform mixture; the disc separated it by temperature and distance. Volatile things, hydrogen, helium, water, carbon compounds, were driven outward or lost; rock-forming elements stayed close in. Earth therefore began drier and poorer in carbon than the cloud it came from, which will matter when the question of water arrives. The iron that will carry oxygen in blood was gathered here into a planet about to melt and sink most of it to the centre, keeping only a fraction near the surface.

The disc also made "solid" a historical condition rather than a permanent one. A planet feels like the opposite of an event. It is in fact the end state of a long series of them: condensation, sticking, collision, melting, re-collision. The ground under a person is debris that stopped moving, for now. The Moon in the night sky is, in the leading account, the splash from the last great impact, which is why its rocks are chemically so like Earth's outer layers. Collisions did not merely scar the young planets. They built them.

Exoplanet surveys add a corrective. More than five thousand planets have now been found around other stars, and most systems do not resemble ours: giants in tight, scorching orbits, worlds between Earth and Neptune in size with no local equivalent, discs that produced nothing like a rocky planet at a comfortable distance from its star. Our arrangement is one outcome among many. It is not the template.

The disputes here concern the order of events rather than whether they happened. The giant planets probably did not form where they sit now; models in which Jupiter wandered inward and back out, or in which the outer planets rearranged themselves hundreds of millions of years later, explain features of the asteroid belt and the outer system that a static picture does not, but the models disagree with one another. The Moon-forming impact, though the leading explanation, has a nagging problem: simulations tend to build the Moon mostly from the impactor, yet the Moon's isotopes are almost indistinguishable from Earth's, which has prompted proposals of a faster-spinning Earth, a different kind of collision, or more thorough mixing afterwards. The leap from pebbles to kilometre-sized bodies remains partly theoretical. And the interval between the first solids and a finished Earth is argued in tens of millions of years. None of this threatens the outline. It is the normal state of a field with real data and more than one way to read it.

In the atlas, the particle field inside the silhouette contracts here into a single schematic disc, the Sun at its centre and one small dot at the position of the third planet, with the standard reminder that nothing is to scale. The body's dependence at this stage is on a sorting it did not choose: on being made from the inner, rocky, water-poor fraction of a cloud, around a star of modest temperament, in a system that happened to settle rather than fling itself apart. What the sorting produced was a hot ball of mixed rubble. It was not yet a place. The next chapter watches it become one.

Evidence: Established: calcium–aluminium-rich inclusions dated to about 4,567 million years as the oldest Solar System solids; collapse of a rotating cloud core into a star and a flattened disc; dust-to-gas ratio of roughly 1:100; the snow line and the rocky/giant/icy sorting; gas-disc lifetimes of a few million years; Earth's assembly over tens of millions of years through embryo collisions; ALMA images of ringed discs since 2014; more than 5,000 confirmed exoplanets with diverse architectures. Debated or uncertain: a supernova trigger for the cloud's collapse; the mechanism of planetesimal formation from pebbles; giant-planet migration scenarios; the details of the Moon-forming impact and the isotopic similarity problem; the exact duration of Earth's accretion.

The Restless Earth

The oldest piece of Earth you could hold would be almost invisible on your palm. It is a crystal of zircon less than half a millimetre long, weathered out of a sandstone ridge in Western Australia, and its uranium clock reads a little over 4.4 billion years. The planet was barely a hundred and sixty million years old when it formed. Everything around that crystal has since been destroyed: the rock it crystallised in, the continent that rock belonged to, the water that may have lapped against it. The zircon survived because zircon is nearly indestructible, and it carries a surprise. Its oxygen isotopes suggest it grew from a melt that had been in contact with liquid water at the surface. The standard picture of the early Earth as a glowing hell of lava and incoming rock may be partly wrong. Within a geologically short time of forming, the planet seems to have had a crust, water on it, and a surface already being eroded and remade. It has not stopped since.

Earth is restless because it is hot inside and cannot easily get rid of the heat. Some of that heat is left over from assembly: the energy of all those collisions, above all the impact that made the Moon, which probably melted most of the planet. More came from sinking. Molten iron is denser than molten rock, and within the first few tens of millions of years, according to the radioactive clocks in meteorites and Earth rocks, the iron drained to the centre, releasing a vast amount of energy as it fell. That gave the planet its structure: a metal core, now roughly the size of Mars, a thick rocky mantle, and a thin crust. The rest of the heat is being made continuously by the decay of uranium, thorium and a radioactive form of potassium scattered through the rock. Roughly half of the heat now leaking from the surface is that radioactive supply; the other half is the original warmth, still escaping.

Rock conducts heat badly, so the mantle moves instead. Over millions of years solid rock creeps like very stiff putty, rising where it is hot, spreading, cooling and sinking. At the surface this shows up as plate tectonics: a shell of rigid plates pushed apart at mid-ocean ridges where new crust forms, and dragged back into the mantle at trenches. Continents, too light to sink, ride on top and are shuffled about, splitting and colliding, raising mountains that rain and ice then grind back down to sediment. The outer core, liquid iron stirred by the same heat, generates the magnetic field. Volcanoes vent gases that had been locked in the rock: water vapour, carbon dioxide, nitrogen, with no free oxygen at all. That outgassed breath, with whatever was delivered from space, became the first atmosphere and the first ocean.

One of these cycles deserves its own mention because it kept the planet livable. Carbon dioxide from volcanoes warms the surface. Rain dissolves it, and the slightly acidic water slowly eats rock, carrying carbon to the sea, where it is buried as carbonate. The warmer the planet, the faster the weathering, the faster carbon is removed, the cooler it gets. Plate tectonics then drags the carbonate back down to be cooked and vented again. This slow thermostat, acting over hundreds of thousands of years, is the best current explanation for why Earth stayed temperate while the young Sun was only about seventy per cent as bright as it is now.

For a body, the consequence is that its material is not merely cosmic but geological, and still on loan. The calcium in bone and the phosphorus in every strand of DNA were locked in rock. They reach a person only because weathering, volcanoes and roots prise them loose, rivers carry them, and food concentrates them; the body is downstream of mountains. Phosphorus in particular has no gaseous form and reaches the living world almost entirely through the slow erosion of rock, which makes it a bottleneck for life on land and at sea alike. Iodine, iron, zinc, magnesium: all mined from the crust by processes nobody operates.

The restlessness also decided what kind of surface was available. A planet that stopped moving would, over time, wear its continents flat and lose its thermostat; carbon would pile up in the air, or be drawn out of it and never returned. Venus, nearly Earth's twin in size, appears to have a stagnant lid and a surface hot enough to melt lead. Mars, smaller and quicker to cool, has been geologically quiet for most of its history and has lost most of its air. Earth's continued churning is the reason there is still a stage at all, and still ground that is young, broken and chemically various enough to react. The body depends on the planet not only for what it is made of but for the planet continuing to move.

When this machinery started is one of the most argued questions in Earth science. Some read the ancient zircons and the chemistry of early continents as evidence of plate tectonics within the first half billion years; others find the clearest signatures only after about three billion years ago and suspect the early Earth ran a different regime, a thick, sluggish lid pierced by plumes, with modern-style tectonics arriving late. Whether tectonics is necessary for a habitable planet, rather than merely helpful on this one, is a further question with a sample size of one. The "late heavy bombardment", a proposed spike in large impacts around 3.9 billion years ago that was once standard, is now doubted; the dated lunar samples that suggested it may all come from a single basin. Even the magnetic field's role is debated. It certainly deflects the solar wind, but whether it protected the atmosphere from being stripped is less clear than textbooks suggest, since Venus has no such field and a great deal more air. The planet is restless. The choreography of its first billion years is a working hypothesis, held with varying confidence depending on whom you ask.

In the atlas, this is the stage floor beneath the figure: select it and it opens as a cutaway globe with the particle field's calcium, phosphorus and iron traced to crust, mantle and core. The dependence on show is current, not merely ancestral. A body is a brief arrangement of material on a planet that is constantly rearranging itself, and the arrangement continues only because the planet keeps lending. Of all its circulations, one has done more for chemistry than any other, and it arrived, or was kept, against the odds. The next chapter follows it: water, and what it does and does not promise.

Evidence: Established: Jack Hills zircons dated to about 4.4 billion years with oxygen-isotope evidence of surface water; early core formation within tens of millions of years (Hf–W chronology); roughly half of present surface heat flow from radioactive decay; mantle convection and plate tectonics as the present regime; the outer-core geodynamo; volcanic outgassing of an oxygen-free early atmosphere; the carbonate–silicate feedback and the faint young Sun (about 70% of present brightness); phosphorus supplied to life almost entirely by rock weathering. Debated or uncertain: when plate tectonics began (estimates span roughly 4 to below 3 billion years ago) and what regime preceded it; whether tectonics is required for habitability; the reality of a late heavy bombardment; how much the magnetic field protects the atmosphere; the exact radiogenic share of heat flow ("roughly half").

Water and Possibility

After rain, on a field of black volcanic rock, the water does not spread out. It beads. Each drop pulls itself into a dome, holds its shape against gravity, trembles when the wind touches it and does not break. Tilt the rock and the drop rolls, keeping itself together, and leaves the stone darker where it has been, because a little of the rock has gone into it. In that small behaviour is most of what makes water strange. Its molecules cling to one another and to almost anything else; they pull salts apart and carry them off; they take a great deal of heat to warm and give it back slowly; and, almost alone among substances, water is lighter as a solid than as a liquid, so ice floats and lakes freeze from the top. A drop on basalt is also a fair picture of the early Earth: hot, dark rock and water finding its way across it. The question is what water made possible there, and what, despite the slogans, it did not promise.

Start with the molecule. Two hydrogen atoms sit on one oxygen at an angle, and the oxygen hogs the shared electrons, so each molecule has a slightly negative end and a slightly positive one. The positive hydrogen of one molecule is drawn to the negative oxygen of the next: a hydrogen bond, weak and short-lived, breaking and reforming trillions of times a second. That shifting network is why water is liquid at temperatures where molecules of its size should be a gas, why it absorbs heat so readily, why it dissolves charged and polar substances so well, and why the open lattice of ice is less dense than the jostling liquid. For chemistry the key property is the first: water lets things meet. Dissolved in it, molecules move, collide and react in ways they cannot while locked in rock or scattered in air.

Where Earth's water came from is less settled. The disc near the young Sun was too hot for ice, so the inner planets should have formed dry. One source is the outer system: water-rich asteroids and perhaps comets, flung inward as the giant planets shifted. The fingerprint is the ratio of heavy to ordinary hydrogen, which in Earth's oceans matches a class of water-bearing meteorites well and most measured comets poorly. Another is that the rocky building material itself held more hydrogen in its minerals than once thought, and the planet baked out much of its own ocean. The current reading is some of both, with the balance still argued.

Whatever its source, the water was here early. The ancient zircons hint at surface water before 4.3 billion years ago; by 3.8 billion years ago there are pillow lavas, which form only when lava erupts under water, and sediments that settled through it. And it stayed. That is the remarkable part. Venus, almost Earth's size and only a little closer to the Sun, appears to have lost an ocean's worth of water to a runaway greenhouse that boiled it into the air, where sunlight split it and the hydrogen escaped to space; the heavy-hydrogen ratio in its remaining atmosphere, about a hundred times Earth's, is the fingerprint of that loss. Mars, smaller and further out, once had rivers, lakes and perhaps a sea, and rovers have driven across their dried beds; it lost most of its atmosphere and the surface water with it. Earth kept its water through a combination of distance, size, the carbon thermostat of the last chapter, and probably luck.

Water changed what the gathered material could do. On a dry planet, the elements of the previous chapters would sit as minerals, reacting at the slow pace of solids. In water they circulate: dissolved, concentrated by evaporation, split and recombined by heat, sunlight and the electrical gradients across mineral surfaces. Water carries the products of weathering to the sea and the products of the sea back to rock. It moderates temperature, so a shoreline stays within a range in which complex molecules neither freeze solid nor fall apart. And it provides a medium in which something could eventually have an inside and an outside, a boundary with different chemistry on each side, which the next part of the book treats as the first thing life needed.

A body remembers this. An adult is roughly sixty per cent water by mass, more as a child, and every reaction in every cell happens in it. The hydrogen in that water is primordial, from the first minutes; the oxygen is from stars; the molecules themselves were assembled somewhere in the cold outer disc or the early mantle and have been cycling through oceans, air, rock and living things for four billion years. A person is, among other things, a temporary pond.

The habitable zone, the band of distances from a star at which a planet's surface can hold liquid water, has become the standard first filter for worlds worth a second look. Thousands of planets have now been found, and dozens sit in or near that band. The filter is sensible. It is also a statement about water, not about life.

That distinction is the chapter's warning. "Follow the water" has guided the search for life beyond Earth for good reasons, but water is a permission, not a promise. Mars was habitable, by the water criterion, for hundreds of millions of years, and no life has been found there. Moons of Jupiter and Saturn hold oceans beneath their ice; whether anything lives in them is unknown. On Earth, the one case where water and life coincide, we cannot yet say whether life arose quickly because water makes it easy or because a long shot happened to come in. A sample of one supports no statistics.

Water is also, chemically, a problem as much as a solution. The long chains life depends on, proteins and nucleic acids, are joined by bonds that water slowly pulls apart. Building them in a wet world means working against the solvent, which is one reason some origin-of-life researchers favour settings that dry out and rewet, such as pools around hot springs, over the open ocean. Whether other liquids could serve elsewhere, ammonia, say, or the methane lakes of Saturn's moon Titan, is an open and so far purely theoretical question. The droplet on the rock is necessary for everything that follows in this book. It is not sufficient, and nothing about it guarantees the next chapter.

In the atlas this chapter lights the particle field inside the silhouette blue: the body's water traced to a cold disc, a bombarded young planet, and an ocean that was kept while a neighbouring world lost its own. It is a dependence on a planetary accident that has so far held for four billion years, and the body's own water budget, a few days' supply at any time, is a reminder that the holding continues daily. With rock, heat, atmosphere and water in place, the stage of this part is nearly complete. The final chapter stands on its shoreline and takes stock of what, exactly, was there and what was not.

Evidence: Established: water's polarity, hydrogen bonding and resulting properties (solvent power, heat capacity, ice less dense than liquid); Earth's ocean deuterium-to-hydrogen ratio matching carbonaceous chondrites better than most comets; evidence of surface water by about 3.8 billion years ago and hints before 4.3; Venus's elevated D/H ratio as a signature of water loss; past rivers and lakes on Mars; adult body water around 60% by mass; the habitable-zone concept and thousands of confirmed exoplanets; hydrolysis as a challenge for prebiotic polymer formation. Debated or uncertain: the mix of delivered versus inherited water on Earth; whether the earliest zircon evidence records oceans; how much Venus once had and when it was lost; whether wet–dry cycles at hot springs or deep-sea vents were the cradle; whether liquids other than water could support life anywhere.

A Planet Ready to Ask

Imagine standing on a shoreline about four billion years ago, with breathing apparatus, because the air would kill you. The Sun is dimmer, and the sky, with no ozone layer, lets through ultraviolet that would burn skin in minutes. The Moon is nearer and larger, and the tides it raises are correspondingly bigger, racing in and out across black rock through days that are, by most models, well short of today's twenty-four hours. Behind you a volcanic island steams. At your feet is a pool left by the last tide, warm, slightly acidic, rimmed with mineral crusts where earlier pools dried. Nothing is alive in it. Nothing is alive anywhere. But a chemist with a sampling kit would find the pool busy: dissolved salts and metals, gases bubbling up from below, and a thin film of carbon compounds, some arrived from space and some made here. Everything this part of the book has assembled is present. The question is what, if anything, it adds up to.

Take stock in three columns.

Energy first. Sunlight falls on the pool, less than today but plenty. Heat rises from below: the young planet's interior, still shedding the warmth of its assembly, drives hot springs on land and vents on the sea floor. Lightning cracks through a thick, carbon-dioxide-rich atmosphere. Impacts, still frequent, deliver bursts of heat and shock. And in certain places there is a subtler supply: where seawater soaks into fresh volcanic rock and reacts with it, hydrogen gas is released, and hydrogen meeting carbon dioxide is a store of chemical energy waiting for a path to be spent along. Energy is not scarce. What is scarce is a way of using it.

Chemistry second. In 1953 a graduate student sealed water and a mix of simple gases in a glass apparatus, ran electric sparks through it for a week, and found amino acids, the components of proteins, in the brown residue. The gases he chose are probably not the early atmosphere, but the lesson has survived every revision: the small molecules of life form readily from simple ingredients given energy. Meteorites confirm it from the other direction. A stone that fell on Australia in 1969 contains dozens of amino acids and, as later analyses showed, all five of the bases that spell out genetic information, made in space with no help from anything alive. Building blocks were cheap. They were falling from the sky.

Environments third, and here the pool matters as a type. Chemistry needs more than ingredients and energy. It needs concentration, so that molecules meet often; cycles, so that what is built in one condition can be tested in another; and surfaces, since mineral faces can hold molecules in useful positions. A tidal or hot-spring pool offers all three. It fills, concentrates as it evaporates, dries to a film in which small molecules can be joined into chains, and is wetted again. Far below, a vent in the dark offers a different set: steady warmth, steep gradients between fluid and seawater, porous mineral walls riddled with compartments. Two research traditions have grown up around these two settings, and the argument between them is one of the most productive in the field.

Between the building blocks and the first thing that could be called alive, however, lies a gap no experiment has yet closed. The pool contains parts. It does not contain a system that maintains itself, copies itself and changes.

What had changed, by this shoreline, was the planet's readiness, not its intentions. Every chapter of this part has added a necessary condition. A universe with a history. Hydrogen from the first minutes. Gravity's concentrations. Fusion and its ash. Deaths that scattered the ash. A disc that sorted it. A planet that kept moving. Water that stayed. None of those steps was taken in order to produce the next, and several nearly did not happen. The result, by around 4.3 or 4.4 billion years ago, was a surface on which complex chemistry was possible: rock, heat, water, a carbon-rich atmosphere, a steady flow of energy from hot to cold.

And then, surprisingly quickly, there were things living in it. The firmest evidence is in Western Australia: layered mounds of sediment about 3.4 billion years old, with the shapes and textures that microbial mats produce today, and microfossils in rocks of similar age. Chemical traces in Greenland push the date back towards 3.7 or 3.8 billion years, less certainly. Taken together they say that life was present within a few hundred million years of the planet becoming habitable. On the cosmic calendar of the first chapter, that is a few days. Whether that speed means life is easy, or that Earth was lucky early, cannot be told from one planet. What can be said is that the ingredients did not sit idle for long.

Honesty about the dates is required, and so is honesty about the gap. Claims of life older than 3.8 billion years rest on isotopic signatures or microscopic shapes that non-living processes can also produce, and each has been challenged; a reported carbon trace in a 4.1-billion-year-old zircon is intriguing and far from accepted. The 3.4-billion-year mounds are widely trusted. Between those bounds, the start date of life on Earth is uncertain by several hundred million years.

The larger gap is conceptual. No laboratory has turned a pool of building blocks into a self-maintaining, self-copying chemical system, and the proposals for how it happened on Earth, at vents or in pools, with genes first or metabolism first, remain hypotheses with partial experimental support. Moving the origin off-planet, as some suggest, does not help; it changes the address of the question. Nor does readiness mean inevitability. The title of this chapter is deliberately modest. The planet was ready in the sense that a kitchen with ingredients is ready, which is to say it was ready to produce nothing at all unless something happened. The something is unknown. That is not a failure of the story. It is where the story becomes a different kind of story.

The atlas closes this part by holding the particle field inside the silhouette at full brightness, every element in the hand traced to its origin, and then dimming it, because from here the body is no longer explained by its ingredients. The hand has been accounted for as matter and not at all as a hand. All of its dependence so far has been on things that could not have chosen otherwise: stars, clouds, impacts, tides. The next part moves the atlas's focus from the particle field to a small magnified window in the torso, a single cell, and asks the question the pool could not answer: how can matter maintain, reproduce and change itself?

Evidence: Established: an oxygen-free, carbon-dioxide-rich early atmosphere with no ozone layer; a closer Moon, larger tides and shorter days in the early Archean (magnitudes model-dependent); hydrogen release from seawater–rock reactions; the 1953 spark-discharge synthesis of amino acids; amino acids and all five nucleobases in the Murchison meteorite; stromatolites and microfossils around 3.4–3.5 billion years old; a habitable surface by roughly 4.3–4.4 billion years ago. Debated or uncertain: the exact early atmospheric composition; evidence of life older than about 3.8 billion years (Greenland isotopes, 4.1-billion-year zircon carbon, putative vent microfossils); hot-spring pools versus alkaline vents, genes-first versus metabolism-first; whether a rapid origin implies that life is common. No laboratory has yet produced a self-maintaining, replicating chemical system from prebiotic ingredients.