The Primate Line

Life in the trees rewards a particular kit of tools, and the animals that would become us spent tens of millions of years acquiring it. The primates as an order are old, reaching back perhaps sixty-five to eighty million years, close to the very impact that cleared the dinosaurs, and from the start they were shaped by arboreal life. To move safely through a canopy you want hands that grip, so the primates evolved grasping fingers, and soft-tipped digits with flat nails instead of claws, and thumbs that could close against them. To judge the leap from one branch to the next without falling to your death you want to see depth, so their eyes migrated round to the front of the face and learned to fuse two overlapping images into a single three-dimensional world. And to manage a life of shifting social bargains inside a troop, tracking who groomed whom and who could be trusted and who bore a grudge, you want processing power, so their brains grew. Grasping hands, forward-facing eyes, and an enlarging brain: the arboreal starter kit, assembled for the strictly practical business of not falling out of a tree, turned out to be the exact set of gifts that a future toolmaker, artist, and stargazer would one day need. Evolution was not aiming at us. It was solving the problem in front of it, and it happened to build the raw materials of a mind.

There is a curious possibility woven into that sharp primate vision, and because it will matter greatly later in this book, we introduce it here and hold it lightly, as a hypothesis under test rather than a settled fact. The anthropologist Lynne Isbell has argued, in a 2006 paper and a 2009 book with the arresting title The Fruit, the Tree, and the Serpent, that one of the deep pressures sharpening primate eyesight, and wiring a specific fast-alarm pathway into the primate brain, was the ancient and lethal need to spot snakes among the branches before they could strike. On this account our exceptional vision was honed not only to find ripe fruit but to catch, in a fraction of a second and below the level of conscious thought, the particular shape of a coiled serpent. Whether the snake did as much of this work as Isbell claims is genuinely debated, and we flag that honestly. But the deep entanglement of primate vision, primal fear, and the serpent is real and measurable, and it will resurface with great force when we reach the strange, near-universal role the snake plays in human myth and religion. For now, simply note that our lineage and the serpent have very old business with one another, older than humanity itself.

Somewhere between six and seven million years ago, in Africa, one branch of the ape family split away onto the road that leads to us. Genetics can date the parting closely, and can measure how near we remain to our cousins: we share something like ninety-eight point seven percent of our DNA with chimpanzees. What set our branch apart, at first, was not a big brain at all. It was a way of walking. We now have a whole gallery of these early upright walkers, and their names mark the milestones: Sahelanthropus, from around seven million years ago in Chad, whose skull hints that it already held its head as an upright walker does; Ardipithecus, "Ardi," from four and a half million years ago, a remarkably complete skeleton whose mosaic body still had a grasping big toe for the trees but a pelvis already modified for walking, and who, tellingly, lived in woodland rather than open savanna, quietly overturning the old tidy story that we stood up to see across the grass; and the famous Australopithecus skeleton called Lucy, from about three point two million years ago, who strode across the African ground on two feet while carrying a brain scarcely larger than a chimpanzee's. If any doubt remained, it was erased by a trail of footprints pressed into wet volcanic ash at Laetoli three and a half million years ago and then preserved, three individuals walking upright across the page of the Earth, their gait, with its heel-strike and pushing toe, indistinguishable from yours. Upright walking came first. The great brain came millions of years later.

We should be honest that bipedalism was not an obvious upgrade, and the fossil record wears its costs openly. It was, in many respects, a botch job of hasty redesign. Standing an ape's horizontal spine on end gave us chronic lower-back pain, vulnerable knees, hernias, and, most cruelly, an obstetric bind that has no clean solution: a pelvis narrow enough for efficient striding must deliver a head that kept getting larger, which made human childbirth uniquely difficult and dangerous, a lifelong tax paid across all of history by every mother of our line. But set against all that was a single overwhelming dividend. Walking upright freed the hands completely and permanently from the work of getting about. And hands that no longer had to touch the ground could carry food and infants across distances, wield and eventually make tools, and, in the long run, gesture and point and shape the empty air into meaning. The awkward, aching, costly gait was one of the best bargains our ancestors ever struck, though the bill still arrives in every human back.

Then, beginning around two and a half million years ago and accelerating thereafter, something drove the brain to balloon. Over that stretch the brain of our lineage roughly tripled, from around four hundred cubic centimeters, the chimpanzee range, to the fourteen hundred or so you are using to read this sentence. It is the most dramatic burst of brain growth in the entire history of life, and there is, honestly, no consensus on what caused it. The candidates are not mutually exclusive, and the truth is probably a braid of all of them: the sheer cognitive load of an intensely social existence, where keeping track of alliances and debts and deceptions is a full-time mental job; the tightening feedback loop between making tools and needing the wits to make better ones; the whiplash climate of a geologically restless Africa, split by its Great Rift Valley, which may have rewarded flexible generalists over narrow specialists; and, not least, fire. The discovery of cooking, championed as a turning point by the primatologist Richard Wrangham, effectively pre-digests food outside the body, unlocking far more energy from every mouthful, and a brain is a monstrously expensive organ to run. Yours is about two percent of your body weight and burns roughly a fifth of your energy at rest. Such an organ could only grow once a rich, reliable, calorie-dense food supply arrived to pay for it, and it appears the bill was met partly by shrinking the gut, another costly organ, as cooked and meaty food made a long digestive tract unnecessary. The brain grew, at a steep metabolic price, and toolmaking marched in lockstep beside it, from the first crudely sharpened Oldowan stones toward ever finer and more deliberate work.

Tool use itself, it turns out, was never uniquely ours. When Jane Goodall watched a chimpanzee strip the leaves from a twig and poke it into a termite mound to fish out a meal, in 1960, she quietly demolished one of the last walls we had built between ourselves and the rest of the animals, the comforting definition of humanity as the sole toolmaker. What was becoming genuinely ours was not tool use but its runaway degree: the spiraling loop in which hand taught brain and brain taught hand, each making the other capable of more.

And if you want a single piece of evidence that we are, past any reasonable doubt, one limb of the ape family and not a separate and special creation, it is written in our chromosomes, and it arrives with the clean elegance of a prediction confirmed. Every great ape, the chimpanzee, the gorilla, the orangutan, carries twenty-four pairs of chromosomes. We carry only twenty-three. If we genuinely share a common ancestor with them, that missing pair is not a refutation but a demand: somewhere on our line, two of the ancestral ape chromosomes must have fused end to end into one. That is a hard, testable prediction, and when biologists went looking, they found it exactly. Human chromosome number two bears the unmistakable scars of precisely such a fusion, the frayed protective end-caps of two ancient chromosomes stranded uselessly in its middle, and the worn-out remains of a second control center where a single chromosome should have only one. The prediction was made from the theory; the genome was then read; and the fusion was sitting right there, just as descent from a shared ape ancestor requires. Our kinship with the rest of life is not a matter of opinion or belief. It is stamped into the architecture of every cell in your body.

The march through our own genus, once it begins, is a gallery of half-familiar faces, and it is worth meeting them, because the tidy image of a single ladder from ape to man is one of the most persistent falsehoods about our origins. Around two and a half million years ago comes Homo habilis, the "handy man," small and still half-arboreal but the maker of the first deliberately struck stone tools, the simple sharp-edged Oldowan flakes. Then, close to two million years ago, arrives the real hero of this stretch, Homo erectus, and we do not honor this ancestor nearly enough. Erectus stood and walked with a body much like ours, mastered fire, crafted the elegant teardrop-shaped Acheulean handaxe to a design so successful it stayed essentially unchanged for over a million years, and became the first human to leave Africa altogether, spreading across Asia as far as Java and China. Erectus endured for something like a million and a half years, which is to say roughly five times as long as our own species has so far existed; by the only measure that counts in deep time, survival, it was a far greater success than we have yet earned the right to call ourselves. From something like the later, larger-brained heidelbergensis, the lineage split, sending one branch toward the Neanderthals and another toward us. And the story refuses to stay tidy even now: in a South African cave, the recently discovered Homo naledi combined a brain no bigger than an australopithecine's with startlingly modern hands and feet, and appears, astonishingly, to have carried its dead deep into a pitch-black cave chamber, a hint of something like ritual in a creature we would never have credited with it from the size of its skull alone. The lesson recurs: mind and meaning do not map neatly onto brain volume, and the road to us was not a ladder but a branching, blundering bush, most of whose limbs are now broken off.

By around three hundred thousand years ago, the pieces had come together in Africa, at sites like Jebel Irhoud in Morocco. A big-brained, upright, tool-making, fire-using branch of the African apes had crossed some hard-to-define threshold into the genus we immodestly named Homo, "man." But it had not crossed it alone, and that is the surprising, almost eerie heart of the next chapter. The world into which our own species was born was not an empty stage awaiting its star. It was already crowded with other kinds of human.