Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived (2 page)

Our universe has been around roughly fifteen billion years; our sun, perhaps six; Earth has made four billion circuits around its mother star, and the life upon it has been ardently evolving during 3.8 billion of those journeys, give or take a few hundred million years. For the vast majority of that time anything alive on Earth was no larger than a single cell. Had we been around to see this life, we would have missed it since it is invisible to the naked eye. But of course, if it hadn’t come first, we, and everything else alive on Earth, would have been out of the question.

No matter how hard we try, we cannot begin to fathom the changes, iterations, and alterations our planet has undergone since it first came to rest, collided and molten, in its current orbit. Our minds aren’t built to handle numbers that large or experiences that alien. And in this book we won’t try. Instead we will focus on a hiccup in that Brobdingnagian history, but a crucial hiccup, especially from your particular point of view—the past seven million years. Because that is when the first humans came into existence.

Compared with the other smaller, rocky planets in our solar system, Earth has always been especially capricious. During its lifetime it has been hot and molten, usually wet, sometimes cold, at other times seared. Occasionally large sectors have been encased in ice; other epochs have seen it covered with gargantuan swamps and impenetrable rain forests inhabited by insects larger than a Saint Bernard. Deserts have advanced and retreated like marauding armies, while whole seas and oceans have spilled this way and that. Its landmasses have a way of wandering around like pancakes on a buttered griddle, so that the global map we find familiar today was entirely different a billion years ago and has spent most of the time in between resolutely rearranging itself, often with interesting results for the creatures trying to survive its geologic schemes.

Charles Darwin and Alfred Russel Wallace figured out by the mid-nineteenth century that these incessant alterations explain why Earth has spawned so many varieties of life. Random revisions in the DNA of living things, together with erratic modifications in environments, have a way of causing, over time, whole new and astonishing forms of life to emerge. Darwin, when after years of thought and hand-wringing finally got around to writing
On the Origin of Species
, called this “descent by means of natural selection,” which was to say that creatures change randomly (by mechanisms unknown to him; he and everyone else in 1859 were ignorant of mutating genes or spiraling ladders of DNA) and either endure in their environment, or not. If the mutations that shift their traits help the organism to survive, he surmised, then it breeds more offspring and the species continues, passing the new traits on. If not—and this has been the case with 99.99 percent of all of the life that has ever evolved—then the life form is, as scientists like to say, “selected out.” Darwin figured that different environments favored different mutations, and over time—incomprehensibly
long periods of time—different organisms diverged like the branches of a tree, each shoot putting increasing distance between the others around it until, eventually, you find your self with varieties of life emerging as different as a paramecium and Marilyn Monroe.

So, though we may have started out with single prokaryotic cells and stromatolitic mats of algae nearly four thousand million years ago, eventually the world found itself brimming, in stages, with lungfish, slime mold, velociraptors, dodo birds, salmon and clown fish, dung beetles, ichthyosaurs, angler fish, army ants, bighorn sheep, and—after almost all of the time that ever was had passed—human beings—peculiarly complicated animals, with big brains, keen eyes, gregarious natures, nimble hands, and more self-aware than any other creature to have ever come down the evolutionary pike.

Of the twenty-seven species of humans that we have so far found that once walked Earth, ours is the most favored line for the simple reason that, so far at least, we have avoided the genetic trash bin. Given evolution’s haphazard ways, we might just as well have ended up a blowhole-breathing water mammal, a round-eyed, nocturnal marsupial, or a sticky-tongue anteater obsessed with poking its wobbling proboscis into the nearest nest of formicidae.
^a
We could even have
become
the formicidae for that matter.

Or we might have become extinct.

But as it turns out—and lucky for us—we emerged from the jungles of Africa, came to stand upright, clustered ourselves into close-knit packs, gave up our front paws for hands, grew thumbs, took up reformed, meat-eating diets, developed tools, and, in a remarkably short time—as evolutionary events go—rearranged the world right down to its molecules and right up to its climate. Today we are even manipulating the DNA that makes us possible in the first place—a case of evolution evolving new ways to evolve. (Think about that for a moment.)

We did not pop out of the jungles of Africa in our current form like Athena from the head of Zeus, big-brained, tool-laden, and ready for modern life. We came in stages, part of a vast and jumbled experiment largely driven by our home planet’s fickle ways. Between six and seven million years ago Africa’s rain forests began to shrink slowly. Earth was a different place from today, but not radically so. If there
were time-traveling, Earth-orbiting satellites to provide a global view of our planet then, it would look pretty much the same as the one we see on the Weather Channel today. India would be in place mostly, though still slowly plowing into Asia, creating the Himalayas. Australia would be roughly located where we find it now. The Mediterranean would be a touch larger. Partially submerged, the boot of Italy would not look quite so bootlike, and the Bosporus strait along with sectors of the Middle East would be inundated, though soon enough the closure of the strait at Gibraltar would transform the Mediterranean into a vast plain of salt flats, marshes, and brackish lakes.

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.