Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived

by Chip Walter

  Life of every kind thrived along Turkana’s shores in the early days of the Pleistocene epoch, despite the occasional ferocity of the weather and the ominous belching of its volcanoes.8 Crocodiles bathed in its warm waters; Deinotherium, an ancient version of the elephant, and both black and white rhinoceroses grazed among the grasslands. Hyenas yelped and hooted, scavenging what they could and hunting flamingos that fed in the shallows, while the grandcousins of lions, tigers, and panthers harvested dinner from herds of an early, three–toed horse called Hipparion. 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 (left), Homo habilis, and Homo rudolfensis were all ranging among its eastern and northern shores 1.8 million years ago, sharing the benefits of 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.

  Homo ergaster

  Despite decades 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 an example of Homo erectus. Truthfully, ergaster wouldn’t seem to be a promising 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.

  You may have noticed the wide range of the boy’s possible age. There’s a reason for that. Despite being among the most studied fossils in the annals of paleoanthropology, scientists cannot seem to universally agree on the age of their owner, a mystery that brings us back to the issue of long and lengthening childhoods. The boy’s age is elusive because we have only two living examples of primates that we can use as benchmarks to determine his age when he died—forest apes and us. But Turkana Boy is neither. With an adult brain that would likely have been about 880 cc, he falls almost midway between the two extremes. Take away half the mass of his brain, and it would be about the size of a chimpanzee’s. Add the same amount and he would be within the range of most modern humans.

  When scientists first inspected the boy’s fossilized teeth, they immediately realized he was, in fact, a boy because several of them had not yet entirely arrived. In his lower jaw a few permanent incisors, canines, and molars had formed, but not all of them were fully grown. In his upper jaw he still had his baby, or milk, canines and no third molar. If a dentist were looking into a mouth like that today, she would conclude she was dealing with an eleven–year–old. But if the mouth belonged to a chimpanzee, seven would be a better guess.

  Teeth represent one type of clue scientists use to help estimate the age of a skeleton (or more precisely, the skeleton’s former owner) when he died. Another is growth plates. Long bones like those in our arms and legs don’t fuse permanently with the joints attached to them until they are fully grown. The state of growth plates reliably predicts age. Turkana Boy’s long leg bones were still growing and had not yet fused, particularly at the hips, although one of his shoulder and elbow joints was fusing. Given the state of his growth plates, researchers concluded the boy could have been as young as eleven or as old as fifteen the day he met his untimely end, if he was human. Or a mere seven if he was a chimpanzee.

  A final feature that helps determine age is height. Nariokotome Boy’s thighbone is seventeen inches long, which would have made him roughly five feet three inches tall, about the size of an average fifteen–year–old Homo sapiens, or a full–grown chimpanzee. Compared with other fossil primates, australopithecines or even his Turkana contemporaries like Homo habilis and rudolfensis, for example, Nariokotome was tall, and depending on his exact age, he might have grown considerably taller, had he survived. So how old was the “working” boy?

  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 faceup, 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 faceup. 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 sythe–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 offspring 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?10

  In his landmark book Ontogeny and Phylogeny, Stephen Jay Gould spends considerable time discussing two types of environments that drive different varieties of evolutionary selection. One he calls r selection, which takes place in environments that provide plenty of space and food and little competition. A kind of animal Valhalla. The other is called K selection, environments where space and resources are scarce and competition is nasty and formidable.

  R selection (Gould points out many studies that back this up) encourages species to have plenty of offspring as quickly as possible (think rabbits, ants, or bacteria) to take advantage of the lavish resources at hand. But K–style environments require species to slow down, create fewer offspring, and take more time doing it because it reduces stress on the environment and the competition among those trying to survive in it. By random chance, evolution begins to favor the creation of fewer competitors within a species who will only die off from lack of resources.11 By reducing death and lengthening life, in particular early life, K selection also provides species extra time to develop in ways that make them more adaptable. In our case, as Gould put it, K selection made us “an order of mammals distinguished by their propensity for repeated single births, intense parental care, long life spans, late maturation, and a high degree of socialization.” Today you and I stand as the poster children for K–strategy evolution. Yet, while the simple fact that we are walking around today provides conspicuous proof that K strategies can succeed, it still fails to explain why they succeed.12

  It is possible that it didn’t, at least not all the time. Multiple species arguably walked down this Darwinian road and were snuffed out. Several—about whom we may never know a thing—were surely done in over time by the unrelenting pressures of protecting their helpless infants, braving their environment to get them more food, or becoming dinner themselves for some salivating savanna cat. Is this what wiped out Australopithecus garhi? Does this explain the demise of Homo habilis or rudolfensis? So far the sparse, silent, and petrified clues that the fossil record has left us aren’t parting with those secrets. They are stingy that way.

  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 offspring 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 th
at 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 existence.

  Chapter Three

  Learning Machines

  It is easier to build strong children than to repair broken men.

  —Frederick Douglass

  Boy, n.: a noise with dirt on it.

  —Anonymous

  Give me the child until he is seven, and I will give you the man.

  —Jesuit aphorism

  Youth, the french writer François Duc de la Rochefoucauld once observed, “is a perpetual intoxication; it is a fever of the mind.” Ralph Waldo Emerson was more blunt: “A child is a curly, dimpled lunatic.” We have all witnessed a toddler or two in action (usually and most memorably our own), and it is a sight to see. The average two–year–old is thirty inches tall and twenty-eight pounds of pure, cerebral appetite determined without plan or guile to snatch up absolutely everything knowable from the world. She, or he, is, indisputably, the most ravenous, and most successful, learning machine yet devised in the universe.