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

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 someplace 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 pointis 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