Read Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived Online
Authors: Chip Walter
Tags: #Science, #Non-Fiction, #History
Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived (14 page)
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.
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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.
Mitochondrial genetic studies tell us that around this time one small group of modern humans living in Ethiopia or the Sudan, armed with an assortment of high–tech tools—bone and ivory hand axes, long spears, and fire–hardened stone knives mostly—headed northeast, then over the Red Sea into Yemen on the Arabian Peninsula.
Undoubtedly, being human and curious, several waves of our direct ancestors ventured from their mother continent into the Mideast during this time. Modern human migration wasn’t likely one solitary foray northward. During cold oscillations, seas everywhere would have grown shallower, including the Red Sea and the Gulf of Aden.
Right where these meet at a place with the dramatic name the Gate of Grief (Bab el Mandeb), the continents of Asia and Africa nearly kiss. Even today the distance between them is slender: no more than twenty miles of seawater divides the huge continents. But during frigid climatic swings the Red Sea sometimes dropped more than 210 feet, narrowing the straits by several more miles, nearly attaching Africa and Asia like two continent–size Siamese twins.
Though there is no evidence the seaway ever completely dried out, climatologists believe that a chain of small islands sometimes emerged between the immense landmasses. The shorelines of those islands would have made excellent places to fish and eat before moving northeast to the next island. In time, the traveling tribes inevitably made their way, perhaps in small boats or on rafts, island by island, to the underbelly of Asia.
