The First Humans

Here is a fact that has been strangely edited out of the story most of us grew up with, and it is worth restoring to its rightful place at the center, because it is both true and quietly astonishing: for almost the entire span of the human story, there was never just one kind of human. There were several kinds, at the same time, sharing the same planet, sometimes the same valleys. Our present condition, a single human species alone on Earth, is not the normal state of affairs. It is a recent and unusual outcome, and possibly a lonely accident.

When our own species, Homo sapiens, first appears in Africa a few hundred thousand years ago, it steps onto a world already populated by other humans. In Europe and western Asia lived the Neanderthals, and we should be finished, by now, with the tired cartoon of the shambling, brutish caveman. The Neanderthals were formidable and fully human people: they had brains as large as ours, and on average a little larger; they made fine tools, controlled fire, hunted big and dangerous game in coordinated groups, cared for their injured and elderly over years, and buried their dead. Deep inside a cave at Bruniquel in France, someone built careful ring-shaped structures out of broken stalagmites, in total darkness, needing fire to see, roughly a hundred and seventy thousand years ago, long before our own species reached Europe. Neanderthals did that. Across the vast expanse of Asia lived a second population, the Denisovans, and their discovery is one of the strangest triumphs of modern science, because we met them, in effect, as a genome before we ever recognized them as a face. In 2008, in Denisova Cave in the Siberian Altai, a team recovered a single fragment of a girl's finger bone, and when Svante Pääbo's laboratory read the DNA inside it, in 2010, it belonged to no known human. An entire population of humanity was identified from a scrap the size of a pea. And on the Indonesian island of Flores lived perhaps the most haunting of all our lost relatives, Homo floresiensis, unearthed in 2003, a fully upright, tool-using, fire-using human that stood barely more than a meter tall with a brain the size of a chimpanzee's, a real and startlingly recent people whom the researchers and then the whole world could not help but nickname, and this is not entirely a joke, the hobbits.

We did not merely coexist with these other humans. We met them, and the meeting is written inside your own body. When at last the Neanderthal genome was reconstructed from bones tens of thousands of years old, a feat so difficult that it took decades to perfect and earned Pääbo the Nobel Prize in 2022, it was set beside our own and it delivered a revelation that is, quite literally, personal. If your ancestry lies anywhere outside Africa, you carry, in your cells right now, somewhere between one and a half and two and a half percent Neanderthal DNA, and many people carry a further trace from the Denisovans. The other humans did not simply die out and leave us the stage. Where we met them, we mingled with them, made children across the line of species, and those children's descendants are reading this sentence. The evidence of the encounter can be almost shockingly intimate: one man who died in Romania around forty thousand years ago carried so much Neanderthal DNA, in such long unbroken stretches, that he had a Neanderthal ancestor just four to six generations back, a great-great-grandparent of another kind of human. And some of what we inherited still works in us: Neanderthal versions of certain immune genes that helped our ancestors meet the new diseases of Eurasia, and, from the Denisovans, a gene that to this day lets Tibetans live and thrive at altitudes that would sicken the rest of us. That same Siberian cave held, across its long occupation, the remains of all three peoples, Neanderthal, Denisovan, and modern human, and, most poignantly of all, a single bone from a girl of about thirteen who turned out to be a first-generation hybrid, her mother a Neanderthal and her father a Denisovan. Two vanished human species, and we hold in our hands the child of their union.

The road our own kind traveled was not smooth, and at least once it nearly ended. Around seventy-four thousand years ago the supervolcano beneath Toba, in Sumatra, detonated in one of the largest eruptions of the last several million years, throwing up so much ash that it blanketed much of southern Asia and, by some accounts, dimmed the sky for years. A number of geneticists once read in our DNA the mark of a severe bottleneck around this time, a moment when the entire breeding population of our species may have collapsed to as few as a few thousand individuals, all of humanity funneled through the eye of a needle. We should flag honestly that the severity of the Toba bottleneck is now seriously debated, and some archaeological sites show people carrying on through the eruption better than the grimmest version allows. But the larger truth stands regardless of Toba's exact toll: our ancestors passed through hard and narrow gates more than once, and there were long stretches when the whole future of humanity could have fit inside a few scattered valleys and been extinguished by a run of bad luck.

From one of those African populations, sometime after about seventy thousand years ago, bands of Homo sapiens began to walk out of the continent of our birth and into the entire rest of the world, and it is worth pausing on how total that conquest became. Reading the DNA of everyone alive today, geneticists can trace all of our maternal lines back to a single woman in Africa, nicknamed Mitochondrial Eve, and all of our paternal lines to a Y-chromosome Adam. It is worth being precise about what that does and does not mean, because it is so often mangled into a myth: they were not the only people of their day, and they did not live at the same time or meet, and they were certainly not a first couple. They are simply the most recent individuals from whom an unbroken chain of daughters, in Eve's case, or of sons, in Adam's, reaches every living person. From that African root the branches spread with astonishing speed and daring. Humans crossed open sea to reach Australia at least fifty thousand years ago, a feat that required real boats and real courage and stands as the earliest evidence of seafaring anywhere on Earth. They pushed north into the teeth of the ice, and when the ice ages locked up enough of the ocean to lay bare a land bridge between Siberia and Alaska, they crossed it, filtered down the unglaciated coasts, and within a few thousand years had run the entire length of the Americas to the cold tip of Patagonia. By the end of it there was scarcely a habitable corner of the planet without a human footprint pressed into it.

And behind that spreading footprint, again and again, the great beasts fell. Within a window of the arrival of humans in place after place, the largest animals of each land, the mammoths and mastodons and giant ground sloths, the enormous marsupials of Australia, the moa of New Zealand, the giant lemurs of Madagascar, went extinct. How much of this was our spear and how much a shifting climate is genuinely and sometimes fiercely argued, and we will not pretend to have balanced that ancient ledger of guilt. But the pattern is uncomfortable and hard to wave away, and it is the first faint sounding of a theme this book will be forced to confront squarely at its end: that our species has a very long track record of remaking, and unmaking, the living world it walks into, usually without meaning to and often without noticing.

Then, across the last tens of thousands of years, something happened that we still do not fully understand, and it leaves the story of this Part resting on a note of loneliness. The other humans went away. The Neanderthals faded from Europe around forty thousand years ago. The Denisovans dissolved into their own long silence. The little people of Flores held on for a while on their island and then were gone. Whether we outcompeted them for game and shelter, absorbed them through interbreeding until their lineages simply merged into ours, carried some disease they could not survive, or merely happened to be the ones still standing when the climate lurched again, we cannot yet say for certain, and the honest answer is probably some different mixture in each case. But the outcome is not in doubt, and it is a strange and heavy inheritance. Of all the kinds of human that once walked the Earth together, only one remains. We are it. We are the last humans, the sole survivors of a family that was once many and various, and we have spent almost the whole of our solitary reign so convinced of our uniqueness that we forgot, until the DNA reminded us, that we ever had kin at all.

One species, then, alone and everywhere, with an oversized and hungry brain and a pair of hands freed to make anything they could imagine. And somewhere inside that brain, in the last chapters of this long four-billion-year ascent, a new kind of fire had caught, one that no amount of careful skull-measuring can fully explain. Our ancestors had begun to make marks that stood for things that were not present. They had begun to bury their dead with ornaments and provisions, as if for a journey. They had begun to carve small figures of creatures and beings that existed nowhere in the visible world but only inside their own heads. They had begun, in short, to think in symbols, to hold the absent and the imagined in mind and to share it with one another. That is where Part IV begins, and with it the human story proper: not the story of the body any longer, but the story of the mind's new power to dream a world and then, slowly, to build it.