by Chip Walter
Their stomachs had to be large and their intestines long to digest these foods, and their eating would itself have required liberal funds of energy. In some ways they were consuming so they would have the energy to consume. According to one theory, this explains why their brains did not grow as large and as fast as their gracile cousins’. Hardworking stomachs can’t afford to redirect energy to cerebral growth. But their arrested development may have saved them and been the secret to the success of their million–year run.
Gracile apes on the other hand looked less likely to succeed. They were smarter—given their increased brain size they had to be—but their diet was unpredictable. They used less energy because they walked upright all the time, but they had to make do with whatever else their smaller, less sturdy stomachs could handle. The robust approach was stable. The gracile approach was risky.
Sometimes, however, risk pays off. A high–stakes wager placed on Paranthropus would have paid nothing, yet against all odds, several underdog gracile species remained in the hunt. Good news for us because it was from one of these lines that you and I descended. Still, trouble loomed. Just as it appeared gracile apes were succeeding, finally brainy and efficient enough to outfox the rough treatment their savanna environment was dishing out, the self–same adaptations that were saving them—an upright gait and bigger brains—were also aligning to become the agents of their doom.1
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.
The dictionary defines neoteny as “the retention of juvenile features in the adult animal.” The term comes from two Greek words, neos, meaning “new” (in the sense of “juvenile”), and teinein, meaning to “extend.” In our case it meant that our ancestors, rather remarkably, passed along to us a way to stretch youth farther into life. The question is, why, and how, did it happen?
When faced with resolute obstacles, evolution—always in the service of survival—has a marvelous way of selecting astonishingly diverse solutions cooked up entirely by random chance. This is how the planet has found itself with the unearthly–looking aye–aye of Madagascar, Borneo’s clownish proboscis monkey, the squashed and unappetizing blobfish of Tasmania, and the rapier–nosed narwhals of the arctic seas. It also helps explain the bizarre mating rituals of porcupines, and male anglerfish, not to mention the torturous eating habits of ichneumon wasps. Each of these creatures is a living testament to the marvelous, if accidental, creativity natural selection conjures, again and again. But as remarkable as these evolutionary banks and turns have been, neoteny can count itself as one of the strangest, and we Homo sapiens are by far the most dramatic and extreme example.2
The term neoteny was coined by Julius Kollmann—a groundbreaking German embryologist and a contemporary of Charles Darwin’s. Kollman had nothing like human beings in mind when he created the term. He conceived it to describe the retention of larval features in the Mexican axolotl (Ambystoma mexicanum), and other species of salamanders like the mud puppy (Necturus maculosus) and the olm (Proteus), all of which refuse in their lives to fully grow up and out of their larval stage, even in their adulthood. They mature normally and sexually, but all within the body of their youth. This would be a little bit like a two–year–old boy behaving in every way like a fully grown, sexually mature twenty-five–year–old. In humans, neoteny isn’t quite that pronounced (probably a good thing), but it is nevertheless remarkable, and remarkably odd, if you are willing to circle around and look at it fresh.
The idea of neoteny predates even Darwin and was explored as far back as 1836, when Étienne Geoffroy Saint–Hilaire, a French scientific prodigy and compatriot of Napoléon’s, first pointed out how astonishing it was that the young orangutans that had recently arrived from Asia at the Paris zoo resembled “the childlike and gracious features of man.”
In the twentieth century a handful of other scientists and evolutionary thinkers adopted Kollmann’s term and Geoffroy’s sentiments when they began applying the idea of neoteny to humans, observing that infant apes bore a striking resemblance to adult humans especially in the shapes of their faces and heads. Naturally this raised a few questions: Was this simply a coincidence? Why would we resemble baby apes? And did this have anything to do with our own evolution?
A professor of anatomy in Amsterdam named Louis Bolk became nearly obsessed with those questions. Between 1915 and 1929 he penned six detailed scientific papers and one entire pamphlet on the subject with the ambitious title Das Problem der Menschwerdung (“On the Problem of Anthropogenesis”). He argued that a surprisingly high number of human physical traits “have all one feature in common, they are fetal conditions [seen in apes] that have become permanent [in adult humans].”3
In one paper Bolk even enumerated twenty-five specific fetal or juvenile features that disappear in apes as they grow to adulthood, but persist in humans right up to death. The flatter faces and high foreheads that we and infant chimps share, for example. Our lack of body hair compared with chimpanzees and gorillas (fetal apes have little body hair). The form of our ears, the absence of large brow ridges over our eyes, a skull that sits facing forward on our necks, a straight rather than thumblike big toe, and the large size of our heads compared with the rest of our bodies. The list is long and Bolk’s observations were absolutely accurate.c You can find every one of these traits in fetal, infant, or toddling apes, and all modern human adults. No less than evolutionary biologist Stephen Jay Gould agreed with Bolk in his own landmark book, Ontogeny and Phylogeny (though he didn’t agree with the elder scientist’s reasons for coming to those conclusions, which were tainted with racism and convoluted views of evolution). Gould called our peculiar brand of neoteny one of the most important twists in all the turns that human evolution has taken.4
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 stil
l 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.
Evolution manages this by not affecting solely what traits it reveals, but when it reveals them. It moves abilities, physical features, and behaviors forward or backward, or stops them altogether by altering the expression of genes that affect developmental hormones. It plays with time like a boulevard–game master plays a shill game with walnut shells and peas. So in us, our big toe remains straight throughout our lives rather than crooking thumblike before birth as it does for chimps and gorillas. We remain relatively hairless, like fetal apes.6 Our jaws stay square and our foreheads flat throughout our lives rather than sloping backward as we leave our early years behind. And instead of decelerating brain growth after birth like orangutans, chimps, and gorillas, the genes that control the amount and interconnections of neurons act as though we are still in the womb and continue to fervently multiply.
Put another way, after birth, processes that were once prenatal in our ancestors become post natal 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 decade (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.7 Anthropologist Robert D. Martin calls this the “cerebral Rubicon,” a line that once crossed would have required that some sort of longer, humanstyle 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 decades 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, b
ut 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?
Some history might be in order.
Thirty–five million years ago the northeast corner of Africa was being carried on the back of a tectonic plate determined to make its way eastward toward Asia, while the rest of the continent was steadfastly refusing to go along. One consequence of this dogged parting of the ways was the emergence of the Arabian Sea and peninsula (with, coincidentally, all its oil beneath it). Another was the formation of a long and immense lake in East Africa made possible by the three substantial rivers that drained into it from the surrounding mountains. Two million years ago, the evidence of this great African rift, and the lake it created, was still all around. The ruptured land had left behind dozens of volcanoes, smoldering ominously and erupting unpredictably. One even rose defiantly out of the great lake itself, a disdainful sentinel that stood unfazed by the storms that howled when the seasons changed or the dust devils that spun along its flanks in the hot summer months.
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.