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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.
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.
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.
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?
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.
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.
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?