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

by Chip Walter

  Moving in this odd, vertical way didn’t mean that ancient humans entirely forsook swinging in trees or walking on their knuckles. Lucy, who provided paleoanthropologists with one of the most complete skeletons of an early ancestor, appears to have been a hybrid, clearly capable of walking on her knuckles if she felt like it, and outfitted with shoulders and arms that were nicely adapted to swinging through and climbing trees. Yet, the architecture of her pelvis, the tilt of her head, and the shape of her foot tell us that upright walking was her preferred way of getting around; so preferred that her footsteps in wet mud or sand would have looked almost exactly like yours or mine.

  Australopithecus afarensis

  Walking upright was one evolutionary trait our predecessors shared as they marched resolutely to the present, but not the only one. Another was in play that would also have enormous ramifications: their brains were growing larger. Not immensely larger, but the difference is measurable. While a chimp’s brain is about 350 cc, these grassland primates’ brains ran from 450 cc to 500 cc, a 25 to 40 percent increase.

  The big question is why. The traditional scientific answer to this question is that a bigger brain is a better brain, so evolution’s forces tended to favor smarter animals. That is true enough, but it still doesn’t explain the mechanism that was causing the growth. What was forcing the issue? Why were larger brains evolving at all? Strange as it may seem, starvation might be the answer.

  When an animal is having a chronically difficult time filling its belly, something intriguing happens in its body at a molecular level. Aging slows down, and cells don’t die as quickly as they do when food is available. Contrary to what you might think, a cell’s health in this situation doesn’t deteriorate. It improves. The body, sensing deprivation, seems to call all hands on deck, husbands its energy, and prepares for the worst. In a sense each cell grows tougher and more wary. This is thanks largely to a class of proteins called sirtuins, which some scientists suspect reduce the rate of cell growth.3

  Numerous studies show that reducing the normal diets of creatures as different as fruit flies, mice, rats, and dogs by 35 to 40 percent will increase life span as much as 30 percent. (Scientists can’t ethically perform these sorts of experiments on humans, but all indications are that the same holds true for us.) When food is scarce, fertility also drops and animals mate less frequently, an additional way of slowing down the cycle of life. While the deprivation makes life horrible for the creatures enduring it, from an evolutionary point of view it carries with it the quality of pure genius. Nutritional penury not only extends the life of an animal, but fewer offspring improves the chances of the whole species remaining in the evolutionary sweepstakes. Fewer offspring also places less stress on already overburdened food resources. The whole process of living decides, it seems, to bide its time until the storm passes. Cell growth on every level slows except for one key and remarkable exception: brain–cell growth increases.

  There the cells last longer, and they begin to make new versions of themselves faster, or at least the neurotrophins generated by the hypothalamus, which are the precursors of new brain cells, do. Not only that, other experiments show that food deprivation increases an appetite–stimulating peptide called ghrelin, which enables synapses to transform themselves by some molecular magic into cortical neurons. You could say the body and the brain strike a bargain. To compensate for the aggressive growth of new neurons, the rest of the anatomy fasts, stretching scarce nutritional resources that it then redirects to the brain. Or put another way, the body slows down aging and accelerates intelligence. This means that 3.5 million years ago, by the time Lucy and her contemporaries were desperately scavenging at the margins of an unpredictable land, the chronic deprivation they were facing was accelerating the growth of their brains.4

  So our ancestors had two assets going for them. Upright walking made them more mobile and efficient, able to cover more ground and better equipped to evade the predators that were evolving along with them. Their larger brains meanwhile made them more capable of adapting to dangerous situations on the fly, more adept scavengers, and better at collaborating successfully with one another. All good in these strange and dangerous environments. That they survived despite their desperate circumstances proves that the combination of the two adaptations was succeeding. But there was now a new challenge: the two trends were on a collision course and bound, in time, to make it impossible to survive. Something had to give.

  Chapter Two

  The Invention of

  Childhood (Or Why It Hurts to Have a Baby)

  My mother groaned, my father wept,

  into the dangerous world I leapt.

  —William Blake

  The human birth canal inlet is larger transversely than it is anteroposteriorly

  (front to back) because bipedal efficiency favors a shorter anteroposterior

  distance between a line that passes through both hip joints and the

  sacrum … This size relationship, along with a twisted birth canal shape,

  makes human parturition mechanically difficult.

  —Robert G. Franciscus

  “When Did the Modern Pattern of Childbirth Arise?,”

  Proceedings of the National Academy of Sciences

  Two and a half million years ago, around the end of August in the Human Evolutionary Calendar, primates like Kenyanthropus platyops, Australopithecus afarensis, and Australopithecus africanus begin to disappear from the fossil record. They may not actually have disappeared, but the evidence of them does. Either way, their evolutionary run was apparently nearing an end. However, with their disappearance a new, rich wave of human species began to crest and break on Africa’s broad and windy plains. In the space of one million years, nine new varieties of humans emerged. Stepping back and looking at the aggregated remains scientists have labored to pick out of the hills, valleys, and ancient lake beds of Africa like so many needles from an incomprehensibly huge haystack, these species give you the impression that the savanna apes that had been struggling so long to survive were finally getting the hang of living in their new environment, fanning out in more directions, deepening the peculiar evolutionary experiment we call humanity.

  And it was an experiment; make no mistake, because not all branches of the human family were evolving along the same lines. More precisely, species were striding down two distinctly different roads—one that included smaller, slimmer, so–called gracile apes, and another that embraced bigger, thicker humans with large jaws and teeth, known in the world of paleoanthropology as robust apes. Each approach had its advantages. But in the long run, only one would succeed.

  The members of the robust branch of the human family first showed their simian faces in late August among the flooded grassland along the Omo River in southern Ethiopia and the western shores of Lake Turkana in northern Kenya. Scientists call this specimen Paranthropus aethiopicus, and he is perplexing because he combines so many contradictory characteristics. His bone structure seems to say that he more often than not walked on all fours among the elephants, saber–toothed cats, and hyenas with which he passed his days. Yet he lived in wet, open grasslands munching on tubers and roots with his big, flat teeth and ample jaws, rather than in wooded areas where you might think knuckle–walking would make more sense. Despite his chimplike anatomy and relatively small brain (no more than 450 cc in adulthood), he may have been the first to pull off the astounding trick of fashioning the first stone tools, preceding even the famous feats of “Handyman” (Homo habilis), who followed him and is generally considered the inventor of the first Neolithic technology. (Scientists are debating anew who should get credit for this remarkable advance.)

  Whatever aethiopicus accomplished, more like him were to follow. Later in the calendar year—the middle of October—two other Paranthropus species, boisei and robustus (also known as crassidens in the ever–changing argot of paleoanthropology), arrived, also generously jawed, large headed, and big of tooth, like aethiopicus.

&
nbsp; Paranthropus humans represent an evolutionary “strategy” that modified the behaviors of jungle apes, but didn’t leap dangerously far from them. Of the two routes down which evolution was walking earth’s humans, this was the safer, more conservative one. Like their predecessors in the rain forests, troops of robust apes roamed from location to location, gathering what food they could find in the thinning forests, bush, and grasslands where they lived. Because of the sorts of foods they ate, Paranthropus possessed heads that sported thick, sagittal crests like the ones you see on the silver–backed gorillas at your local zoo, though they were more chimp–size than gorilla–size. The crests are a stout, jagged line of bone that runs from the top of the forehead to the back of the neck like the metal rim of a medieval helmet. Anchored to these were thick ropes of muscle that ran to their massive jaws and dense necks so that the broad, square rows of teeth in their mouths could crush the cement–hard shells of the nuts they consumed, pulverize bark and seedy berries, crunch the exoskeletons of large insects, or masticate the bones of an unfortunate small animal they might have been lucky enough to snatch up.

  Beneath these crests the brains of boisei and crassidens had expanded roughly a third during the four million years that had passed since the first human emerged from Africa’s rain forests. They were undoubtedly resourceful and even more socially bonded than the apes from which they had descended, mostly thanks to the menace that surrounded them. Danger breeds reliance and cooperation. Day–to–day living would have been unimaginably harsh: a life of slow migration, eating to gather the strength to move forward and moving forward to gather more food to eat. Despite its hardships, however, this was by no means an unsuccessful evolutionary path. By current accounts, boisei roamed the plains of Africa for a million years, foraging the foods at hand and getting along, if not famously, then well enough. If we measure success by how long species survive, we Homo sapiens, amount to little more than rookies still wet behind the ears. We have been in the game of life a scant two hundred thousand years. Boisei held sway on the Horn of Africa five times as long before exiting the gene pool. If we become this lucky, we will someday be dating our letters July 12, 802013.

  The other path plotted by the combinations of genes, environment and random chance was the one taken by members of a branch of the human family paleoanthropologists like to call gracile. This includes Australopithecus garhi, a creature who, along with aethiopicus, made his debut on the Horn of Africa about 2.5 to 3 million years ago. There is some slim evidence that garhi may also have fashioned simple stone tools, but as in the case of aethiopicus that’s a controversial and unresolved theory. At best, garhi may have used crude stone hammers to break open bones to get at the marrow inside, or sharp flint to scrape and hack meat away from a bone left behind by larger predators. But even these uses of rock mark a colossal technological advance.

  About 1.9 million years ago another gracile human, dubbed Homo rudolfensis, appeared along the shores of Lake Rudolph, now known as Lake Turkana, a long body of water that runs in the shape of an index finger from southern Ethiopia into the western heart of Kenya. Homo habilis and Homo ergaster soon followed, both slim and light–boned, both also passing their time in East Africa.

  In 1991 scientists scrounging among rocks near Dmanisi, Georgia, west of the Caspian Sea, unearthed the remains of still another species of gracile human from this epoch—Homo georgicus. While he remained simian in his looks, his face was flatter, a step closer to ours. Like Homo habilis, georgicus was a lean toolmaker, but with a considerably more advanced case of wanderlust. He lived in a river valley more than twenty-five hundred miles north of the grasslands where Homo habilis passed his days. He may be an indicator that other species, so far unknown, also strode beyond the borderlands of the Dark Continent, settling who knows where, still awaiting discovery.

  Although all of these species came upon the world clustered, like a posse, information about the majority of them is sketchy. Drawing any conclusions about them is a little like drawing conclusions about a long–lost family relative who headed off to the merchant marine or the French Foreign Legion. The best we have in most cases is a few battered bones that offer scant insights into the creatures’ lifestyles or appearance. Georgicus, for example, has seen fit to provide us with three skulls—one with jaws attached, one with a solitary jawbone, and one missing its jaws altogether. Nor did they leave anything much in the way of teeth, let alone whole limbs or vertebrae. Homo rudolfensis has bequeathed a similarly ungenerous array of jaws and skulls, and a scattering of other fragments that may not belong to the species. What we know of ergaster (the Workman) is based on a bundle of six or so skulls, jawbones, and a few teeth, several of which don’t much resemble one another, creating some lively academic brawls about exactly which species is which.

  The stinginess of these creatures makes them mysterious, even among our ancestors, humans who have steadfastly held the cards of their pasts close to their primeval vests. Of all these slender primates, however, one has been a little less secretive—Homo habilis, otherwise famously known as Handyman, long thought to be our direct ancestor and the first toolmaking primate. We have been able to infer a little more about the life of habilis only because we have been lucky enough to have stumbled across more parts of his body than his other contemporaries—several skulls, a hand bone complete with fingers, and multiple leg and foot bones that can’t conclusively be connected with the skulls, but at least provide some clues about the creature’s size and gait. Together the evidence tells us that habilis, though slight in stature, walked upright all the time and possessed considerably larger brains than the first ancient humans, as spacious as 950 cc, depending on which skull you inspect. The shapes of their heads and jaws indicate that unlike their robust cousins, they didn’t care much for nuts, bark, and berries, but had developed an appetite for meat, and the protein it provided, which may account for their larger brains. (See sidebar “Big Guts vs. Big Brains” p. 21.) Nor did they sport great sagittal crests, or huge, square teeth made for grinding. Their teeth were better at tearing. Chances are they hunted small game in packs, not unlike the way chimpanzees sometimes do. And they helped themselves to savanna carrion and whatever other more adept and deadly predators left behind in the way of their prey’s remains.

  Big Guts vs. Big Brains

  Cows, as we all learned in grade school, have four stomachs. They do because it requires a lot of work to extract enough nutrients from grass to transform it into beef and milk. The same was true of our early savanna–roaming ancestors, at least some of them. Subsisting on a diet of nuts, roots, thistles, berries, and other plants required long intestines and strong stomachs if they hoped to squeeze enough nutrients from them to stay alive.

  As the climate changed in Africa and the savannas became broader and drier, the old jungle ways of gathering low–hanging fruit from nearby trees and not moving very far from day to day simply didn’t work. Fruit and foliage became increasingly rare, and three humans had to cover more distance to gather it, which required still more energy. Ultimately that was not a sustainable survival strategy.

  But if you could get your hands on some meat! Then you were instantly rewarded with much more nutritional bang for your hunting–and–gathering buck. That is precisely what the robust lines of savanna humans did. But this choice paid an additional, unexpected dividend. A diet of meat of any kind (even dining on termites and small rodents) made larger brains possible, and less cowlike intestinal tracts necessary. This is something paleoanthropologist Leslie Aiello dubbed the Expensive Tissue Hypothesis when she first came up with the idea in the early 1990s. What this meant, and what fossil finds reveal, is that as our ancestors began to consume more meat, their bodies could redirect the energy those complex intestinal tracts demanded to the business of constructing larger brains. It was a close question two million years ago which approach might work best. Both experiments were tried, and for hundreds of thousands of years both worked. Ultimately, though, larger br
ains turned out to be a more effective survival tool than longer intestines, something the fossil record bears out. While australopithecines and the robust members of the human family were relatively small brained, often not much more cerebrally endowed than a chimpanzee, Homo ergaster’s brain size ballooned to 900 cc or so. After a run of more than one million years, the last of the robust humans finally made their exit 1.2 million years ago.

  This evolutionary path had other ramifications as well. We aren’t as strong as our primate cousins—chimps, gorillas, orangutans—for example. We seem to have exchanged brawn for brains. Richard Wrangham has argued that mastering fire and cooking made meat and other foods of all kinds easier to digest, increasing the protein we could consume and reducing the need for longer intestinal tracts even further. In time bigger brains delivered better weapons, and more strategic ways of hunting. And that likely led to bigger game, more meat, more protein, more cerebral horsepower. The result? Over the past two million years, the brains of the gracile line of humans nearly doubled their size.

  The scattered fossils of both of these sides of the human family tell us that evolution was putting a series of unstated questions on the table 1.5 million years ago: Which approach is best? Gracile or robust? A steady diet of tubers, nuts, and berries? Or a sparse, starvation diet of scavenged carrion along with whatever else could be scraped from nature’s table. A serviceable brain with a cast–iron stomach, or a great brain with a simpler, less sturdy digestive system?

  If you were a betting primate, you couldn’t be blamed for putting your money on the robust branch of the family. At the time, they looked to be winning the battle. They were strong and durable and had adapted the jungle ways of their antecedents to the savanna exceedingly well, meandering through flooded grasslands and clusters of forests, sometimes upright, sometimes on all fours, consuming, if not jungle fruit like their more gorilla–like ancestors, then the next–closest foods that give the term high fiber a whole new meaning.