The following is an edited excerpt from Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived, by Chip Walter. Walker & Company, January 29, 2013. Copyright © William J. (Chip) Walter, Jr.

Striding on two legs efficiently—not waddling the way a chimp or gorilla does when it walks upright—requires, among other adaptations, a fundamental rearrangement of pelvic architecture. An upright stride narrows the hips, and for females, narrowing the hips narrows the birth canal, and a slimmer birth canal makes for increasingly snug trips for newborns out of the womb. Despite the many advantages that upright walking delivered, it creates problems when one is simultaneously evolving bigger brains and larger heads, which was precisely what our gracile ancestors were up to. Yet, since both adaptations were working, what could be done? Each was an evolutionary blessing, yet both were on a collision course. Something would have to give.

Lucky for us, the forces of evolution worked out an exceedingly clever solution: gracile humans began to bring their children into the world early. We know this because you and I, being extreme versions of gracile apes, are the living, breathing proof. If you, for example, were to be born as physically mature and as ready to take on the world as a gorilla newborn, you would have to spend not nine months in the womb, but twenty, and that would clearly be unacceptable to your mother. Or, looked at from a gorilla’s point of view, we humans are born eleven months “premature.” We do not reach full term, which makes us fetal apes. Of course if we didn’t make our departure from the womb ahead of schedule, we wouldn’t be born at all because our heads, after nearly two years in the womb, would be far too large too make an exit. We would be, literally, unbearable.

It’s impossible to overstate the colossal impact this turn of events had on our evolution, but it requires some context to fully appreciate what it means. Our habit of being born early is part of a larger, stranger phenomenon that scientists call neoteny, a term that covers a lot of evolutionary sins at the same time it explains so much of what makes us the unique, even bizarre creatures we are.

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Given its dictionary definition, you might think that neoteny is simply a matter of a species holding on to as many youthful traits of an ancestor as long into adulthood as possible (a little like Joan Rivers or Cher). But it’s not that simple. Undeniably, in some ways we are childlike versions of our pongid ancestors, but in others our maturity is accelerated, rather than stunted. For example, while our faces and heads may not change as radically as an ape’s as we enter adulthood, our bodies still continue to grow and change. We don’t retain the three-foot stature of a two-year-old toddler. In fact at an average (worldwide) male height of five feet nine inches, give or take a few centimeters, we are among the largest gracile apes to have ever evolved. Nor is our sexual maturity slowed, though it is delayed compared with other human species (including Neanderthals, as we will see soon). And our brain development is anything but arrested. In fact, just the opposite. As I said, complicated.

The different ways some parts of us seem to accelerate and mature while others bide their time or halt altogether has generated a flock of terms related to neoteny—paedomorphosis, heterochrony, progenesis, hypermorphosis, and recapitulation. The debate is ongoing about what exactly neoteny and the rest of all of these labels truly mean. In the end, however, it comes down to this—each represents an evolution of evolution itself, an exceptional and rare combination of adaptations that changed our ancestors so fundamentally that it led to an ape (us) capable of changing the very planet that brought it into existence.5 Put another way, it changed everything.

Mostly we think of Darwin’s “descent by natural selection” as a chance transformation of newly arrived mutations—usually physical—into an asset rather than a liability, which is then passed along to the next generation. So paws become fins in mammals that have taken to the sea. The spindly arms of certain dinosaurs evolve into the wings of today’s birds. The ballasting bladders of ancient fish become the predecessors of land animals’ lungs. All of that is true. But what neoteny (and paedomorphosis and all the rest) illustrate is that the forces of evolution don’t simply play with physical attributes, they play with time, too, or more accurately they can shift the times when genes are expressed and hormones flow, which not only alters looks but behavior, with fascinating results.

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Put another way, after birth, processes that were once prenatal in our ancestors become postnatal in us. By being born “early,” our youth is amplified and elongated, and it continues to stretch out across our lives into the extended childhood that makes us so different from the other primates that preceded us. We see it in the fossil record. Almost without exception, the dusty bones scientists have unearthed and fitted together reveal that the faces of gracile primates such as habilis, rudolfensis, and ergaster, while still plenty simian, grew step by step to increasingly resemble us. Their snouts were flattening, their foreheads were growing higher and less sloped, their chins stronger. Features that once existed only in fetal forest apes like big toes and heads that rested upright on shoulders now not only existed in youth but also persisted into adulthood.

Exactly how all of this unfolded on the wild and sprawling plains of Africa isn’t clear precisely, but there can be no doubt that it did. We stand as the indisputable proof. All of the evidence emphatically points to our direct, gracile ape ancestors steadily extending their youth. They were inventing childhood. But most important, to us at least, in the inventing they were becoming more adept at avoiding extinction’s sharp and remorseless scythe. And the main reason that was happening was because the childhood that was evolving enabled the development of a remarkably flexible brain. That is where the grand story of our evolution made an extraordinary turn.

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The clustered neurons that together compose the brains of all primates grow at a rate before birth that even the most objective laboratory researcher could only call exuberant, maybe even scary. Within a month of gestation primate brain cells are blooming by the thousands per second. But for most species that growth slows markedly after birth. The brain of a monkey fetus, for example, arrives on its birthday with 70 percent of its cerebral development already behind it, and the remaining 30 percent is finished off in the next six months. A chimpanzee completes all of its brain growth within twelve months of birth. You and I, however, came into the world with a brain that weighed a mere 23 percent of what it would become in adulthood. Over the first three years of your life it tripled in size, continued to grow for three more years until age six, underwent massive rewiring again in adolescence, and finally completed most, but not all, of its development by the time you reached your second de cade (assuming that as you read this you have reached your second decade).

Being born so “young,” you might conclude our brains arrive comparatively underdeveloped at birth, but that is not the case. Despite our early arrival we still come into the world bigheaded, even compared with our more mature cousin primates. At birth the brains of apes constitute 9 percent of their total body weight, hefty by the standards of most mammals. We, however, weigh in at a strapping 12 percent, which makes our brain 1.33 times larger than an infant ape’s, relatively speaking, despite our abbreviated sojourn in the womb. In other words even arriving in our early, fetal state, with less than a quarter of our brain development under our belts, we are still born with remarkably large brains.

Keep in mind that this approach to brain development is so extraordinarily strange and rare that it is unique in nature. And dangerous. If an engineer were planning the optimum size of a brain at birth, it would clearly be illogical to bring newborns into the world this cerebrally incomplete. Too fragile, and too likely to fail. Far more practical to do all the work in the safety of a mother’s body. But evolution doesn’t plan. It simply modifies randomly and moves forward. And in this case, remember, remaining in the womb full term was out of the question. For us it was be born early, or don’t be born.

As much as we might like to know the answer, exactly when it became necessary for our ancestors to exit the birth canal “younger” is frankly impossible to say. Since we Homo sapiens are the only human species to still be walking the planet since Africa’s retreating jungles orphaned the rain-forest apes that preceded us, and since the skeletal remains of those who came before us are rare and difficult to decipher, we simply haven’t yet gathered enough clues to know precisely when an early birth became unavoidable. There are, however, a few theories.

Some scientists believe earlier births would have begun when the adult brain of some predecessor or another reached 850 cc. Anthropologist Robert D. Martin calls this the “cerebral Rubicon,” a line that once crossed would have required that some sort of longer, human-style childhood become part of that creature’s life. If that’s true, that narrows the candidates to those human species living between 1.8 and 2 million years ago—species like Homo rudolfensis or Homo ergaster. Until recently scientists felt Homo habilis (Handyman) was the best candidate, but new evidence has caused some realignment of the human family tree. For de cades the common wisdom had it that we descended from Homo habilis by way of Homo erectus, which in turn evolved into what paleoanthropologists call “anatomically modern humans” (AMH), our kind. But new fossil finds now indicate that erectus and habilis were East African contemporaries for nearly a half million years, making it rather difficult to have descended from one another. Furthermore, ergaster and rudolfensis, which were often tossed in with Homo erectus, are now more often considered to be their own separate species.’

This means that in the ever-shifting drama (and nomenclature) of human evolution, Handyman now represents an evolutionary dead end and Homo erectus may turn out to be not one species, but many, with only one particular representative leading directly to us, if that.

Whatever the case, around this time, when humans began to grow adult brains about three quarters of the size that ours are today, the offspring of upright walking humans may have been forced to arrive prematurely as the fit between head and pelvis grew increasingly tight. Who, the question then becomes, were the people from whom we directly descended, and where can we suppose they lived?

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If you check a map of Africa today, you will notice the slender imprint of this lake we now call Turkana (formerly known as Lake Rudolf). It is still vast, a long, liquid gem that lies on the breast of East Africa, most of it in northern Kenya with just its upper nose nudging the highlands of southern Ethiopia. Today Lake Turkana fails to be as hospitable as it was earlier in its life. The rivers that once drained it are gone, so evaporation is the only exit for Turkana’s waters. That has turned it a splendid jade color and made it the world’s largest alkaline lake. These days the land that surrounds it is mostly dry, harsh, and remote. However, 1.8 million years ago it was an exceedingly fine place to set up housekeeping.

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The lake, the streams and the rivers that fed it, and the variability of the weather made the area a kind of smorgasbord of biomes—grasslands, desert, verdant shorelines, clusters of forest and thick scrub. The bones of the extinct beasts that lie by the millions in the layers of volcanic ash beyond the shores of Lake Turkana today attest to its ancient popularity.

The existence of a habitat this lush and hospitable wasn’t lost on our ancestors any more than it was on the elephants, tigers, and antelope that roamed its valleys. In fact it was so well liked that Homo ergaster, Homo habilis, and Homo rudolfensis were all ranging among its eastern and northern shores 1.8 million years ago, sharing the benefitsof the basin with their robust cousin Paranthropus boisei. As many as a million years earlier, Paranthropus aethiopicus came and went along the northwestern fringe of the lake, and half a million years before that the flat-faced one, Kenyanthropus platyops, braved Turkana’s winds and watched its volcanoes rumble and spew.

Despite de cades of sweltering work, paleoanthropologists have yet to categorically determine which of these humans who trod the shores of Turkana led directly to us, but it is possible to make an informed guess, at least based on the limited evidence scientists have to work with. We already know Homo habilis is out of the question, an evolutionary dead end unrelated to Homo erectus. Homo rudolfensis is also unlikely because he bears such a strong resemblance to Paranthropus boisei and his robust ancestors. He may have been a bridge species of some sort. Boisei himself would seem not to qualify given that he wasn’t gracile (we are) and possessed the smallest brain of the group, the largest jaws, and the most apelike features.

That leaves Homo ergaster, “the worker” (ergaster derives from the Greek word meaning “workman”), formerly considered Homo ergaster an example of Homo erectus. Truthfully, ergaster wouldn’t seem to be apromising candidate for a direct ancestor either, except for one remarkable fossil find that has been, after some heated debate, assigned to the ergaster line. In the scientific literature he is known as Turkana (or sometimes Nariokotome) Boy because Kamoya Kimeu, a paleoanthropologist who was working at the time with Richard Leakey, came across him on the western shore of Lake Turkana.

His discovery first stunned his fellow anthropologists and then the world with the completeness of what he had found. In a scientific field where scraping up a tooth or a jaw fragment, or a wrecked piece of tibia, can be cause for wild jubilation, Kimeu and his colleagues uncovered not only a skull, but a rib cage, a complete backbone, pelvis, and legs, right down to the ankles. There, in the brittle detritus of the Dark Continent, lay the nearly complete remains of a boy who had lived 1.5 million years ago and died in the swamps of the lake somewhere between the ages of seven and fifteen. It was nothing short of remarkable.

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Viewed from either end of the spectrum, none of the clues about his age have made much sense to the teams of scientists who have labored over them. Each was out of sync with the other. Some life events were happening too soon, some too late, none strictly adhering to the growth schedules of either modern humans or forest apes. Still, the skeleton’s desynchronized features strongly suggested that the relatives of this denizen of Lake Turkana were almost certainly being born “younger,” elongating their childhoods and postponing their adolescence. Apes may be adolescents at age seven and humans at age eleven, but this creature fell somewhere in between.

If the Rubicon theory is correct, and an adult brain of 850 cc marked the time when newborns begin to struggle to successfully make their journey through the birth canal, ergaster children were likely already coming into the world earlier than the rain- forest primates that preceded them five million years earlier. On the other hand, Turkana Boy was not being born as “young” as we are. His large brain, as large as any other in the human world at that time, and his slim hips, optimized for upright walking and running, reinforce the evidence. He must have been born “premature” or he wouldn’t have been born at all. But if he was being born earlier, how much earlier?

Suppose the brain of a fully grown Turkana Boy was 60 percent the size of our brain today. (We have to suppose because we have no adult ergaster cranium to consult.) And let’s assume ergaster children would have come into the world after fourteen months of gestation, approximately 30 percent sooner than a chimp. This isn’t as drastically different as the eleven-month disparity between other primates and us modern humans, but it would have represented the beginning of a significant human childhood, and it would have begun to upend the daily lives of our ancestors in almost every way.

Why? First, there would have been more death in a world where, unfortunately, death was no stranger. Many “early borns” would have died after birth, unable, unlike today’s chimps and gorillas, to quickly fend for themselves. Because gorilla and chimp newborns are more physically mature than human newborns, they often help pull themselves out of the birth canal and quickly crawl into their mother’s arms or up onto her back. It’s unlikely that ergaster infants were capable of this. Of all primates, human newborns are by far the most helpless. When we arrive, we are utterly incapable of walking or crawling. We can’t see well or even hold our heads up. Without immediate and almost constant care, we would certainly die within a day or two. Though these “preemies” were not likely as defenseless at birth as we are, they would have been far less physically developed than their jungle or even early savanna predecessors.

But even if the newborns didn’t die in childbirth, their mothers might have, their narrow pelvises unable to handle what scientists call the expanding “encephalization quotients” of their babies. To compensate, ergaster newborns may have begun to turn in the birth canal so that they were born face-up, a revolutionary event in human birth. Unlike other primates, our upright posture makes it necessary for babies to rotate like a screw so they emerge face-up. If they came out with their faces looking at their mother’s rump as chimps and gorilla infants do, their backs would snap during birth.

The job of bringing a child into the world would not only have become more complicated, but imagine life for the mothers of these offspring, assuming both survived the ordeal of birth. They were already living a precarious existence in a menacing world—open grasslands or at best thick brush with occasional clusters of forest. Predators such as striped hyenas and the scythe-toothed Homotherium had appetites and needs, too. There was no such thing as a campfire to keep predators at bay. Fire had yet to be mastered. When night fell, it was black and total with nothing more than the puny illumination provided by the long spine of the Milky Way, a fickle moon, or an occasional wildfire in the distance sparked suddenly and inexplicably by lightning or an ill-tempered volcano. And the big cats of the savanna like to hunt when the sun has set.

Not only were these new human infants more helpless than ever, but their neurons were proliferating outside the womb at the same white-hot rate they once did inside. Rapidly growing brains demand serious nutrition. Studies show that children five and younger use 40 to 85 percent of their standing metabolic rate to maintain their brains. Adults, by comparison, use 16 to 25 percent.9 Even for ergaster children, a lack of food in the first few years of life would often have led to premature death. Nariokotome Boy might have been undernourished himself. His ancient teeth reveal he was suffering from an abscess. His immune system may not have been strong enough to defeat the infection, and lacking antibiotics, scientists theorize blood poisoning abbreviated his life. He was probably not the first among his kind to die this way.

In every way, early borns would have made life on the savanna more difficult, more dangerous, and more unpredictable for their parents and other members of the troop. So why should evolution opt for larger brains and earlier births? And how did it manage to make a success of it?

Difficult question to answer. Looking back on the scarce orts of information science has so far gathered together, premature birth doesn’t make an ounce of evolutionary sense. Not on the surface. Darwinian adaptations succeed for one reason—they help ensure the continuation of the species. That means if your kind misplaces the habit of living long enough to have sex successfully, extinction will swiftly follow. Since this is the ultimate fate of 99.9 percent of all life on earth, it is difficult to fathom how the mountains of challenge that early-arriving newborns heaped on the backs of their gracile ape parents could possibly help them successfully struggle to stay even a single step ahead of the grim reaper.

It certainly wouldn’t seem to make much sense to lengthen the time between birth and sex. Keeping that time as brief as possible has immense advantages after all. It’s a powerful way to maximize the number of newborns either by having large numbers of them at once or by having them often, or both. Dogs, for example often enter the world in bundles of five or six at a time, are weaned by six weeks, and ready to mate as early as six months. They aren’t puppies long, and once they are done breast-feeding, they are soon prepared to fend for themselves. For mice the process is even more compressed. The result is that mothers bear more children with every birth, do it more often, and those off spring are quickly ready to mate and repeat the cycle. All of this accelerates the proliferation of the species and improves its chances of survival.

We humans, however, wait an average of nineteen years before bearing our first child. Why? If shortening the time between being born and bearing as many offspring as often as possible works so well for other mammals, for what reasons would evolution twist itself backward with Africa’s struggling troops of savanna apes? Why bring increasingly defenseless infants into the world? Why expose their parents to greater danger to feed and protect them? Why insert this extra, unprecedented cycle of growth, this thing we call childhood, into a life—a time when we rely utterly on other adults to take care of us? And what advantage is there in taking nearly two decades to bring the first of the next generation into the fold? …

We do know this: around a million years ago or so—early November in the Human Evolutionary Calendar—the robust primates had met their end, and so had many gracile species, but a handful continued and even flourished. Already some had departed Africa and had begun fanning out east to Asia and the far Pacific. The cerebral Rubicon had been crossed and there was no going back.

This meant that evolution’s forces had opted, in the case of our direct ancestors, for bigger and better brains rather than more sex and more off spring as a survival strategy. And, against all odds, it was working—a profound evolutionary shift. Over time, in the crucible of the hot African savanna, far away in time from the Eden of rain forests, an exchange was made—reproductive agility for mental agility. If bringing a child into the world “younger” was what it took, fine. If expending more time and energy on being a parent was necessary to ensure that a creature with a bigger, sharper brain would survive, then so be it. If evolving an entirely new phase of life that created the planet’s first children was required, then it had to happen. The imponderable forces of evolution had made a bet that delivered not greater speed or ferocity, not greater endurance or strength, but greater intelligence, or put in flat Darwinian terms, greater adaptability. Because that is what larger, more complex brains deliver—a cerebral suppleness that makes it possible to adjust to circumstances on the fly, a reliance not so much on genes as on cleverness. ] It is strange to think that events could well have gone another way. Earth might today be a planet of seven continents and seven seas and not a single city. A place where bison and elephants and tigers roam unheeded and unharmed, and troops of bright, robust primates live throughout Africa, maybe even as far away as Europe and Asia, with not a single car or skyscraper or spaceship to be found. Not even fire or clothing. Who can say? But as it happened, childhood evolved, and despite some very long odds, our species found its way into the present.