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

by Chip Walter

  These geologic alterations were unfolding because the planet was warming up, thinning its ice caps and making land scarcer and Earth more watery. Ironically, the world was becoming much like the one scientists now speculate global warming is creating. In looking back at our origins, it seems we are catching a glimpse of our future.

  Climate, however, is complex. Weather systems veer and fluctuate. Tectonic plates beneath the Indian Ocean were shifting and sloshing whole seas. As the planet generally warmed, some parts of the world became wetter, and more tropical, while others grew drier. Among these were northeast and north-central Africa, where grasslands were gradually transforming themselves into desert, and rain forests were breaking up into semiwooded savannas. Here, a new kind of primate was evolving, probably several. Primates that were not, purely speaking, any longer creatures of the jungle.

  Scientists peg the emergence of Earth’s first human about seven million years ago largely because around that time the fossil evidence, sparse as it is, points to a primate splitting from the last common ancestor we shared with today’s chimpanzees. There is no precise method for fixing dates of these kinds. Paleoanthropology, with its reliance on the chance discovery of ancient bones and the sediments in which they lie, is replete with perplexity and, as sciences go, is far from exact.

  In fact, the likelihood of any ancient bone even becoming a fossil is so vanishingly small that it’s just short of miraculous any discoveries are made at all. If you hoped, for example, that some part of you might be discovered fully fossilized in the distant future, there would be no chance of its happening unless you dropped dead in a layer of soft sediment that takes an impression of your body, or into some place lacking the oxygen that so enthusiastically decomposes every molecule of us when we bite the dust. A peat bog or shallow, muddy river would be a good place.

  From there you would have to hope that the tectonic shenanigans of the planet, lashings of wind and water and climate, the shifting courses of rivers, or the encroachments of deserts or glaciers wouldn’t toss or shuttle your bones from their resting place to some location less hospitable to your preservation. Assuming they didn’t, then at least some of the solid parts of your remains would have to be replaced molecule by molecule with other dissolved solids that leave behind a stone replica of your formerly carbon-made skeleton. Then finally, if all of this happens just so, you must count on the wind or rain or the instinct of an exceedingly lucky paleoanthropologist to reveal what is left of you to him or to her.

  The chances of your being preserved in this way are, by some estimates, one in a billion. The likelihood of this small part of you then actually being found is so small, it can’t accurately be calculated. Add to this that many of our earliest ancestors met their fate in forests or jungles where decomposition happens rapidly and without leaving a trace, and you can see why the fossil record we rely upon to unlock our origins is not only tiny, but serendipitously skewed. At best we have been left with random clues that provide only the sketchiest images of the deep past. In fact, whole lines of primeval relatives were almost certainly long ago obliterated and now lie beyond discovery.

  We do have tools other than fossils that can help divulge our ancestry. The science of genetics is still fledgling, but it provides ways to explore the past by providing a kind of clock that allows scientists to estimate when certain branches of our family tree made off in different directions. (See the sidebar “Genetic Time Machines,” page 76.) Yet the best genetic evidence is currently so foggy that it places the time we and chimpanzees shared a common ancestor somewhere between four and seven million years ago, rather a loose estimate. So neither the fossil record nor genetic science can provide anything very detailed about the precise time of our emergence.

  Still, we have to start somewhere. It sometimes shocks people to learn that at least twenty-six other human species once lived on earth. It further shocks them that many of them lived side by side. The point is there was not, as we often think, an orderly march of ape–men that led from chimp to you and me.

  One reason science has tentatively settled on seven million years as the birth date of the human species is that the oldest fossil that might reasonably lay claim to being human was found in Chad at various times between July 2001 to March 2002 (he was unearthed piecemeal). His discoverer, a student named Ahounta Djimdoumalbaye, called him Sahelanthropus tchadensis—Sahel man, after the part of Africa south of the Sahara where he was found. Not much remained of this particular primate—a skull, four lower–jaw fragments, and a few teeth, but because the fossils indicated his head was positioned much like ours is, in line with his torso rather than at a forty–five–degree angle like a knuckle–walking gorilla, some paleoanthropologists speculate he (or she) walked upright. They see this as a reason to consider him (or her) an early human. All we know for certain is that tchadensis was either one of the last ancestors humans shared with other great forest apes or was one of the first humans to have evolved. Or tchadensis might be an evolutionary dead end. The best we can say is, the bones left behind were found in sediments that tell us tchadensis walked the earth about seven million years ago, and so that is where we shall begin.1

  When compared with the billions of years it has taken to make a universe or its suns and planets, seven million years may appear minute, but to those of us who aren’t stars, comets, oceans or mountain ranges it remains a very, very long time. We are used to measuring time in hours and days, months and years, perhaps generations when forced to push the envelope. Epochs and eons bend the mind and are as incomprehensible as light–year–measured galactic distances or quantum calculations computed in qubits.

  To help wrap our minds around these numbers, imagine that we could squeeze the seven million years that have passed between the arrival of Sahelanthropus tchadensis and the present into a single year’s calendar, and then plot the arrival—and in some cases the departure—of every known human species from January to December. Let’s call this the Human Evolutionary Calendar or HEC. If we look at it this way, tchadensis arrives January 1. Lucy, the famous upright walking member of a line of savanna apes known as Australopithecus afarensis, who lived about 3.3 million years ago, appears July 15. Neanderthals don’t show up until near Thanksgiving, November 19, and we Homo sapiens sapiens finally reveal ourselves near the winter solstice, December 21, a little more than a week before the end of the year.

  Looking at this timeline, you can’t help but conclude the human species seems to have gotten off to a slow start, at least based on the current sketchy evidence.b Following tchadensis nothing at all happens for more than a million years, then a creature researchers call Orrorin tugenensis (Millennium Man) finally appears just before the spring equinox—on March 8. Like tchadensis, tugenensis didn’t leave much for us to inspect—two jaw fragments and three molars. Later finds turned up a right arm bone and a small piece of thigh—altogether enough information for paleontologists to conclude that Orrorin was almost certainly human, and lived about 5.65 to 6.2 million years ago, mostly in wet grasslands and fairly thick forests that eventually became the Tungen Hills of modern Kenya. Thus the name tugenensis. Whether he walked upright all the time or even part of the time is debated, but if he spent his days between grasslands and jungle, he may have done a bit of both, walking on all fours in the forest and upright now and again in and among the trees and grasslands he called home.

  As we move into spring not one, but three new and indisputably human species arrive. On March 18 two emerge from the mists of time: Ardipithecus ramidus and Ardipithecus kadabba; then on May 20, Australopithecus anamensis. These were all distinct species, yet all three bear a stronger resemblance to today’s chimpanzees than to us, and all three probably walked upright sometimes, and at other times on all fours.

  By summer in the HEC, signs emerge that the human experiment was gathering momentum. Multiple species begin to appear and overlap. Recalling their names is a little like trying to follow the characters in a Russian novel, but bea
r with me. (We can thank the brilliant zoologist Carl Linnaeus for the long and respected tradition of assigning elongated, Latin names to all living things.) In mid–October, Paranthropus robustus (sometimes known as Paranthropus crassidens) arrives. Then on July 4, Kenyanthropus platyops; ten days later, Australopithecus afarensis (Lucy); and then in August, Paranthropus aethiopicus and Australopithecus garhi join the ranks of humans that have walked the planet.

  These creatures, each of whom found their way in and out of time and the plains and forests of Africa, arose and departed subject to the cantankerous whims of evolution. When we compress time this way, it’s easy to forget that some of these species lived for hundreds of thousands of years. All of them were intelligent, with brains that ranged from the size of today’s chimps, 350 cubic centimeters (cc) to as large as 500 cc, still a quarter to a third the size of the brain modern humans carry around, but enormous and enormously complex when compared with those of most other mammals.

  Something strange and intriguing was afoot across Africa’s sprawling lands. Like an Olympian god, the continent’s changing climate was forcing the emergence of multiple kinds of humans, all of them descended from jungle primates similar to those that still live in Africa’s rain forests today (albeit in ever–dwindling numbers). In time the selective pressure that different environments exerted coupled with random genetic changes resulted in new varieties of humans that emerged all over the continent.

  Aethiopicus arose along the banks of Lake Turkana in Kenya and the Omo River basin of Ethiopia. Lucy and her kind roamed as far north as the Gulf of Aden and as far south as the ancient volcanoes of modern Tanzania, while Australopithecus africanus lived thousands of miles south, not far from Johannesburg, South Africa. A later addition to the human family, dubbed Australopithecus sediba, was recently discovered in South Africa as well. The partial skeletons of a young boy, and an adult female, who lived between 1.78 and 1.95 million years ago (between mid and late October), were scraped from the dust.

  Depending on where they lived, all of these species dealt with surroundings that ranged from densely wooded and fairly wet, to dry, open grassland. As Africa’s jungles retreated toward the center of the continent, troops of apes must have been left scattered over hundreds of thousands of square miles to adapt or die. They had no tools, only the randomly provided equipment their genes conferred upon them, all better fit for life in the jungle than the environment they now faced. Where once the rain forest provided them with ready supplies of fruits and berries that delivered plenty of energy and nutrients, they now found themselves dealing with savannas where less food was spread out over larger areas, inhabited by growing numbers of predators each exceedingly focused on making meals of them. Life was, in the immortal words of Thomas Hobbes, “poor, nasty, brutish, and short.” Everything was more dangerous, and staying alive demanded more energy, mobility, toughness, and cunning.

  Wherever they lived and however they survived, all the hominin primates that emerged during the summer in the Human Evolutionary Calendar were players in a grand, African experiment now three million years in the making. The world was testing them, harshly, and the forces of evolution were remorselessly molding them into a new kind of ape.

  While the random forces of evolution endowed each with different genetic attributes that helped them all survive, every one of them seems to have developed one predominant trait: for the first time in the incomprehensibly long story of evolution on earth, species had emerged that walked upright on their hind legs. Because we do this so effortlessly every day, it may escape you how exceptionally strange this mode of transportation was four million years ago among living mammals, or any other animal for that matter. But strange it was. Yet precisely because it was so peculiar, it set in motion a string of evolutionary events that eventually enabled you and me to come into existence.

  We are so steeped in technology, so used to controlling our environment, that we forget that the only way the vast majority of living things can hope to survive in a changing world is to come by the right genetic mutation at just the perfect time, something that happens entirely by accident. Serendipity is the enemy and the ally of every species. It might provide you with the claws needed to bring down prey or the speed required to escape another’s claws. Or it might not, in which case you are doomed to be “selected out,” unfit for your new habitat and relegated to the genetic landfill. For living creatures of all kinds, and that included our ancestors living during the balmy summer months of the HEC, there are no evolutionary shortcuts, no quick technological fixes, no ways to take charge and change the rules of the game with an invention.

  But sometimes you get lucky.

  If you stand back and look at the sprawling landscape of life’s evolution on Earth, it’s easier to pick out big trends, and that can help clarify a mystery or two. For example, when similar creatures find themselves in similar situations, they sometimes develop nearly identical traits, but by entirely separate evolutionary paths. Take seals, dolphins, and whales. All of them were former land mammals, but each developed fins. They didn’t, and couldn’t, inherit these traits from one another because they were distinct species that evolved independently. But because living in water seems to favor creatures that grow fins of some kind, each shares this trait. Scientists call this convergent evolution.

  Something like this seems to have happened with several lines of savanna apes beginning around four million years ago. While they all descended from jungle cousins who walked on all fours, many forsook knuckle–walking. And it makes evolutionary sense that they did. In the jungle, food is never far away: plenty of low–hanging fruit. Wild gorillas, for example, travel only about a third of a mile on average every day, sometimes only a few hundred feet. Everything they could ask for is close by.

  On the savanna, though, life was profoundly different. Beneath the hot equatorial sun, temperatures would often have risen into triple digits (Fahrenheit). Food was scarce and rarely at hand. So while walking upright in the thick underbrush of a tropical rain forest would have done nothing to improve your chances of living a longer life in the jungle—in fact, it might shorten it—perambulating on your hind legs in open grasslands provided several advantages. You gained better visual command of your world, which is useful if you are on the daily menu of ancient jackals, hyenas, and the lion–size, saber–toothed cats called megantereons. Traveling on two feet is also more efficient than scrambling along on four. Studies reveal that knuckle–walking chimpanzees burn up to 35 percent more energy than we humans do as we stroll blithely down the street. Making your way around the broad, hot grasslands of the Pleistocene epoch on your knuckles and hind legs looking for food, watching out for predators and taking care of your young, would have been slow, tiring, and ultimately deadly. That is presumably why upright walking became the preferred form of navigation for all savanna apes no matter what locality they called home. Those that failed to come by this trait were wiped out.

  Precisely how ancient humans such as Lucy, aethiopicus, and Australopithecus africanus eventually pulled off the physical tricks necessary to stand upright remains a mystery, but they did, and one reason they did is thanks to a genetic trait common to all apes—a big toe.2

  For some time zoologists have known that early in gestation the big toe of gorillas, chimps, and bonobos is not bent, thumblike, but is straight, similar to ours. But as they continue their development the big toe departs from the other four so that by the time they are born it has become thumblike, making it easy for their feet to grasp, stand on, or hang from limbs. But what if one of the descendants of a jungle ape found itself living in sparse forests and open savannas? And what if one of those apes was born with a big toe that never became thumblike, but instead remained straight, a freak genetic deformity?

  Deformities, autoimmune diseases, even mental illnesses, are often the result of genetic mutations. Somehow a gene is reshuffled, a hormone misfires, or a genetic switch is delayed. Even DNA makes mistakes. In fact, evo
lution depends upon it. We humans are sometimes born with an extra digit, webbed fingers, shortened legs. But one creature’s deformity can become another’s salvation. Every living thing on Earth is, one way or another, an amalgamation of genetic gaffes.

  Imagine, then, that some primates were born “deformed” with a straighter toe preserved from their time in the womb rather than a more opposable one like all other normal primates. What sort of life could such a creature look forward to? In the jungle, not a promising one. Unable to effortlessly grasp tree branches like his fellow apes, he would struggle mightily to keep up with the troop, die swiftly, and his genetic predilection for straight toes would bite the dust with him.

  But in a partially wooded savanna, where the grasslands were expansive and the forest broken and less dense, an ape with a straight big toe would be lucky indeed. That deformity would enable him to stand and walk upright. Without a straight big toe our current brand of walking would be impossible. With every step we take, our big toes support 30 percent of our weight, and they make the upright running, jumping, and rapid shifts in direction we excel at possible. Other complicated anatomical changes had to have taken place before our style of two–footed walking came into existence, and eons passed before those modifications were completed, but the transformation arguably began with a straight big toe. Such an odd foot would have made any savanna ape better at standing upright and able to walk for longer periods on his hind legs. In time he would have found himself endowed with a birth defect that would eventually prove a lifesaver.