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

by Chip Walter

  At the tip of South Africa where the Indian and Atlantic Oceans meet lie shores of basalt rock that look out on an expanse of cold and turbulent water that doesn’t see another shoreline until it meets the ice cliffs of the Antarctic more than a thousand bracing, windblown miles away. If ever there was a place you could call the end of the earth, this is it.

  Seventy thousand years ago, a few hundred human beings lived here; anatomically modern humans or AMH, as anthropologists like to call them. They were like us in every way it seems, except for the technologies they used to survive. They were bereft of cell phones and SUVs but looked like us and carried around the same evolutionary and psychological baggage we do. In those days, they were also the last remaining members of our species, a tiny enclave of humanity twisting precariously at the end of an evolutionary thread, rubbing elbows with extinction.

  One hundred and twenty thousand years earlier this species, one that would later name itself Homo sapiens, had come into existence, a new branch of the human family, split off from an earlier primate that had arisen on the Horn of Africa, where so many other varieties of humans had emerged.

  This particular tribe, the one that lived along Africa’s southern shore, were gracile, built for running, and clever hunters. Because of their high foreheads, prominent chins, and brains weighing more than three pounds, triple the size of those of the first upright walking primates from which they had descended, they looked far less apelike than their predecessors, though you could certainly see the family resemblance. They were inventive, too. Not only did they use fire, they controlled it, cooking food with it and applying it like a tool to harden and shape an impressive assortment of other cleverly fashioned gadgets—knives and axes more advanced than any used before. They may have been at the ends of the earth, but this was the Silicon Valley of its time, a hotbed of innovation. They had also developed an extremely powerful way to communicate—words.

  Fortunately for these last survivors, the land was Eden–like. Not tropical, but temperate and sustaining. What it lacked in the big game that walked the northern savannas, it made up for with lush stores of fruit, nuts, and beans, and an inexhaustible supply of protein–rich seafood. Life must have looked very good. After all, deprived of CNN and the Weather Channel, they had no way of knowing they were the last representatives of their species, nor that much of the world and the continent beyond their small slice of paradise had been under climatological assault for thousands of years. A harsh and unrelenting ice age had already wiped out others like them farther north. Europe, Asia, North America, and the Mediterranean had been buried for millennia beneath uncounted miles of snow, howling winds, and frozen seas. Oceans of water were now locked in enormous ice sheets, leaving seas more than 225 feet shallower, and the rest of Africa chilled and bone–dry. This was the apocalypse.

  It was possible that they were not entirely alone. Tiny pockets of other modern humans may have survived the ice epoch in the north and west of Africa, but no one can say with certainty.

  No, this was probably it. Just a few hundred people dug in, the current crop of an extended family who had colonized the area as many as fourteen thousand years earlier. One catastrophic event, a plague, a typhoon, or a freeze, and that would have been the end of Homo sapiens. And none of the seven billion of us who exist today would ever have been the wiser; in fact we would not have “been” at all. We came that close to being snuffed out.

  That, at least, is how paleoanthropologist Curtis Marean sees it. It’s a sobering thought, the idea that we were closer to extinction than today’s mountain gorillas, and not much better off than India’s dwindling prides of tigers.1

  Plenty of scientists dispute Marean’s scenario. It wouldn’t be paleoanthropology if they didn’t. Our past is a messy business, and today’s efforts to understand how we came into existence, based largely on the ossified leavings found in the world’s dust and rock, has been something like a blind man’s trying to describe the details of a football stadium by feeling his way through it. If we didn’t have ourselves around to inspect, we would know more about Homo habilis and Neanderthals than we know about Homo sapiens. You would think that our being among the most recently arrived branch of the human family we would be knee deep in the evidence of our own existence, but that’s not the case. Outside of Africa, fossils of early Homo sapiens are nearly nonexistent. Thankfully, we have been learning to read the path of our evolution in our DNA (see sidebar “Genetic Time Machines,” page 76), and that, together with some meager findings in the fossil record, has illuminated the story of our emergence at least a bit. The story goes something like this.

  Between 160,000 and 200,000 years ago the first anatomically modern humans emerged, probably near Ethiopia. (But there is anything but universal agreement on this.) Among these was a woman, now called the matrilineal “Eve,” the “mother” of the human race, though that term is a little misleading. Eve wasn’t herself the first modern human, and unlike the biblical Eve, she wasn’t the only woman alive two hundred thousand years ago. She was, however, the sole woman alive then that still has descendants today. Other modern human women lived during her time and before it, but she is the one to whom every living human today is related. So it’s more accurate to say she is our “most recent common ancestor,” at least when looking at mitochondrial DNA as a marker.

  Genetic Time Machines

  When it comes to DNA, the only certainty is change. It’s restless. As DNA alters, so do genes, and when genes mutate and unwittingly express new traits, their accumulated mistakes eventually result in entirely new species—by some estimates, thirty billion separate forms of life over the past 3.8 billion years. Despite the messy nature of genetic mutations, they create markers whose rates of change are startlingly predictable. These signposts enable scientists to calculate, with reasonable, but far from perfect, accuracy where in the evolutionary picture your particular branch of the family tree diverged from other branches.

  Two primary kinds of DNA allow scientists to pull off this neat trick. One is the DNA of organelles that live within each of our cells, called mitochondria. Groups of mitochondria exist within each of the fifty trillion cells that make you and me possible. In an evolutionary partnership agreed to some two billion years ago, some single–celled bacteria took up residence in other single cells, but refused to give up their DNA in the bargain.d The relationship has remained unbroken ever since. Today, in exchange for the protection and nutrition they receive living within other cells, mitochondria create the chemical energy needed to power nearly every plant and animal on earth, including us.

  The second kind of DNA is the nuclear variety, the sort that belongs directly to you and me and within whose cells those mitochondrial guests live.

  It is now possible to take a fossil of our ancestors, closely scan the DNA trapped within (usually mitochondrial because there is more of that than the nuclear variety), and, if the information is robust enough, compare it with samples of our DNA and see how different the two are. Then by comparing the markers—the average rate of mutations over time—we can estimate how deep in the past the two genomes were once identical and when they went their separate ways. This is a little bit like standing on the limb of a tree and pacing off the distance between the branch you are standing on and the one from which it sprouted. Each pace provides an indication of how long ago you and other tree limbs separated.

  The ancestor that all humans share going back to Sahelanthropus tchadensis (see The Human Evolutionary Calendar, page 7) would be represented by the tree’s trunk. Each divergence, each limb, represents a new human species—Homo habilis, Ardipithecus ramidus, Homo rudolfensis, and all the rest. Some lead to new branches others, some don’t. You can also imagine the mutations themselves as the landscape through which a kind of time machine can travel with the genetic markers as mileposts that indicate how far back or forward in time you have journeyed.

  Whichever metaphor you chose, this is how scientists can compare our DNA with a
Neanderthal’s and conclude that we parted ways from a common ancestor—Homo heidelbergensis—200,000 to 250,000 years ago. Or how they have come to discover that Neanderthals and Denisovans both shared a bed with ancestors of ours whose offspring eventually made their way to Europe, Asia, and New Guinea, even though, especially in the case of the Denisovans, we have almost no fossils to inspect.

  A variation on this same technique (more often this time looking at nuclear DNA) makes it possible for scientists to track down the patterns and timing of our own global wanderings—when one group remained in central Africa, for example, but another headed north. When some members of that tribe made west into Europe and others branched off to Asia and the east. This is because our DNA has mutated as we have traveled the world, though not enough in the past 190,000 years to have sprouted an entirely new species. These mutations indicate where we lived, and when.

  Thanks to the genetic records all creatures carry within them, and thanks to the ability of computers to compare them, we are developing a clearer, if not pristine, picture of how much we have in common with our fellow humans, when we parted ways with them, and how we have, ourselves, managed to make our way from a couple of pockets in Africa to nearly every spit of land earth has to offer.

  If Marean’s theory is correct, the first “moderns” that arose in the Ethiopian plateaus must have spread out west and south during a population explosion shortly before the punishing ice age, known today by the memorable meteorological term Marine Isotope Stage 6 (MIS6), began to take its devastating toll. This climatic shift sabotaged life everywhere, as we will see, and may have been further boosted by the largest known volcanic eruption in the history of earth on Sumatra, Indonesia, which blasted ash into the stratosphere, causing a “volcanic winter” that rapidly accelerated the cooling of earth. (See “Killer Explosion?” sidebar, page 80.)

  Other genetic studies indicate that sometime between one hundred thousand and eighty thousand years ago, three lines of Homo sapiens made off in separate directions from East Africa. One headed south and became the ancestors of today’s Central African Pygmies as well as the Khoisan (Capoid) peoples of South Africa. A second genetic group migrated to West Africa, but also departed the continent by way of the Arabian Peninsula. Many West Africans are descended from this branch, and so are many African–Americans and South Americans who, millennia later, were transported across the Atlantic in slave ships. The third branch remained on the Horn of Africa, but others of them branched northwest and north. From these migrants descended the people who today live along the Nile Valley, the Sahara, and the Niger River, which flows through, of all places, Timbuktu in Mali into the Gulf of Guinea. Some of these people also found their way out of Africa. Ten percent of today’s Middle Easterners have the blood of this third group running in their veins.2

  Given these apparently enthusiastic migrations, you might think that as a species we were finally off and running, but there was that wintry climate that was setting in. By seventy thousand years ago it was in full, frigid swing and had begun to systematically rub out life everywhere on the planet. (We are living right now in what scientists call a slim “interglacial” period of this ice epoch, a bit of information that is itself chilling.)

  Genetic studies confirm that during this time Homo sapiens underwent what scientists call a “bottleneck event.” That is to say, we had been worn down to something like ten thousand total adult members, a troop or tribe here or there, scraping out a living, probably along ocean shorelines and receding lake beds.

  Ice ages rarely result in cold weather in Africa. Instead they parch the land, turn rivers into dry wadis, evaporate lakes, and wipe out the sustenance each provides. During some of these periods, the Nile itself was reduced to swamp and muck. Even today the continent is filled with ancient lake beds scarred by desiccated mud cracks that testify to exactly how arid the landscape had become. Whichever humans survived the first waves of these droughts, they had tools, but little else, and when water disappeared, so did the other animals, nuts, tubers, and fruit that supported them. Being at the top of the food chain did them little good once the chain itself was demolished.3

  Dramatic as the scenario is, it’s unlikely that the small tribe at Marean’s Pinnacle Point represented the very last bastion of Earth’s Homo sapiens. More likely they were among dozens of tribes that the changing climate squeezed into small pockets throughout the continent. Each being winnowed down until they must have wondered daily how much longer they might make it. By this time early forms of trade had undoubtedly developed, but the increasing isolation would have made it more difficult to stay in touch, share resources, or help one another out.

  Eventually, however, the climate relented. For three million years—an appalling length of time to us, yet less than one thousandth of the planet’s life—Earth had been undergoing some of the most erratic climate fluctuations it had ever seen, shifting from cold to warm and dry and wet every few thousand years. To make matters worse, for three hundred thousand years Earth’s orbit around the sun had been elongating. That led to even deeper and more frequent climatic swings.4

  But finally, fifty thousand years ago, this particular climatic pendulum began to swing in the opposite direction, and just as the ice had once relentlessly crept from the polar caps to endanger the species at lower latitudes, it now casually reversed itself, and Africa grew warmer and wetter. The sparse pockets of the human family, like Marean’s survivors living at the tip of Africa, and elsewhere here and there, again found themselves blossoming and fanning out. Isolated tribes, separated by heat and desert and their own reduced numbers, began to flow back into one another, setting the stage for a remarkable migration that changed the world.

  Killer Explosion?

  Seventy millennia in the past, long before the pharaohs of Egypt ruled the Nile, even three hundred centuries before the cave painters of Lascaux began doing their remarkable work, the most powerful volcanic explosion to rock the planet in two million years shattered the island of Sumatra, Indonesia, in an area now known as Lake Toba, and nearly wiped out every Homo sapiens on Earth. Or at least it may have. The explosion of rock, ash, and hot magma was so violent it’s difficult to find the words to characterize its power. Scientists have coined multisyllabic terms like megacolossal and supereruptive. It was twice as powerful as the largest eruption in recorded human history, which took place in 1815 at Mount Tambora in Indonesia. Historians called the twelve months that followed it the “year without a summer” because the globe–circling debris from the eruption so severely cooled the planet.

  It is precisely this kind of climatic effect that makes the Toba explosion so interesting. The evidence indicates that it spewed between twelve hundred and eighteen hundred cubic miles of the planet into the sky. Some scientists believe that together with an ice age that was already in the making, Toba may have accelerated cooling and drying worldwide, and driven global temperatures down as much as 27° F. This, in turn, dropped mountain snow and tree lines by nine thousand feet, plunged the planet into a six–to–ten–year volcanic winter and possibly an additional one thousand–year cooling episode.

  As you might imagine this would have made life for the human species that were alive at the time even tougher than it already was, especially if they were living west and downwind of the eruption. The immediate effect would have been to drop uncounted cubic tons of choking volcanic ash on everything for thousands of miles around. Studies show that an ash layer a half foot thick draped all of south Asia, and quickly blanketed the Indian Ocean, and the Arabian and South China seas as well.

  A layer of ash this thick would have decimated plant and small animal life on land and sea for years, catastrophically rattling the food chain and every creature that relied on it for survival throughout Asia and into Africa. Recent fossil and genetic studies suggest that the populations of gorillas, chimpanzees, orangutans, and even cheetahs and tigers dropped to near extinction levels.

  Neanderthals in Europe and
west Asia were apparently spared the direct effects of the volcanic fallout. Homo erectus living at the time in east Asia and (possibly Australia), and Homo floresiensis, the “hobbits” who lived close by seemed to have escaped because they were upwind of the debris. They may all, however, have suffered at the frigid hands of the explosion’s longer–term effects.

  The humans who seemed to have been hit hardest by the remarkable eruption were our ancestors, pockets of Homo sapiens scattered throughout Africa. As the debris spread, some scientists believe the eruption’s cooling effects nearly wiped us out, a genetic coup de grâce that would have made this book, and you and I, entirely impossible.

  It’s unlikely that Toba by itself can explain the sudden whittling of our ancestors around this time in prehistory, but it certainly didn’t help. Except in one surprising way. By isolating Homo sapiens settlements and placing even more survival pressure on them, it may have led to hardier and more adaptable men and women. There could be something to that hypothesis because while other primate species seem to have slowly rebounded, Homo sapiens not only bounced back, its population exploded and began to move quickly into Asia, Europe, and the remainder of the planet.