Valley or ducked into the canopies of the humid rain forests around the Congo basin, where the Mbuti and Bantu now live.
The story is not as geographically contained or as neat as it sounds. Some populations of early modern humans are known to have wandered back into the Sahara—a lush landscape then, crisscrossed with finger lakes and rivers—and eddied backward into local pools of humanoids, coexisted and even interbred with them, perhaps generating evolutionary backcrosses. As Christopher Stringer, the paleoanthropologist, described it, “In terms of modern humans, this means that . . . some modern humans have got more archaic genes than others. That does seem to be so. So it leads us on to ask again: what is a modern human? Some of the most fascinating ongoing research topics in the next year or two will be homing in on the DNA that some of us have acquired from Neanderthals. . . . Scientists will look at that DNA and ask, is it functional? Is it actually doing something in the bodies of those people? Is it affecting brains, anatomy, physiology, and so on?”
But the long march went on. Some seventy-five thousand years ago, a group of humans arrived at the northeastern edge of Ethiopia or Egypt, where the Red Sea narrows to a slitlike strait between the shrugged shoulder of Africa and the downward elbow of the Yemeni peninsula. There was no one there to part the ocean. We do not know what drove these men and women to fling themselves across the water, or how they managed to cross it (the sea was shallower then, and some geologists have wondered whether chains of sandbar islands spanned the strait along which our ancestors hopscotched their way to Asia and Europe). A volcano had erupted in Toba, Indonesia, about seventy thousand years ago, spewing enough dark ash into the skies to launch a decades-long winter that might have precipitated a desperate search for new food and land.
Others have proposed that multiple dispersals, prompted by smaller catastrophes, may have taken place at various times in human history. One dominant theory suggests that at least two independent crossings occurred. The earliest crossing occurred 130,000 years ago. The migrants landed in the Middle East and took a “beachcomber” route through Asia, hugging the coast toward India and then fanning out southward toward Burma, Malaysia, and Indonesia. A later crossing happened more recently, about sixty thousand years ago. These migrants moved north into Europe, where they encountered Neanderthals. Either route used the Yemeni peninsula as its hub. This is the true “melting pot” of the human genome.
What is certain is that every perilous ocean-crossing left hardly any survivors—perhaps as few as six hundred men and women. Europeans, Asians, Australians, and Americans are the descendants of these drastic bottlenecks, and this corkscrew of history too has left its signature in our genomes. In a genetic sense, nearly all of us who emerged out of Africa, gasping for land and air, are even more closely yoked than previously imagined. We were on the same boat, brother.
What does this tell us about race and genes? A great deal. First, it reminds us that the racial categorization of humans is an inherently limited proposition. Wallace Sayre, the political scientist, liked to quip that academic disputes are often the most vicious because the stakes are so overwhelmingly low. By similar logic, perhaps our increasingly shrill debates on race should begin with the recognition that the actual range of human genomic variation is strikingly low—lower than in many other species (lower, remember, than in chimpanzees). Given our rather brief tenure on earth as a species, we are much more alike than unlike each other. It is an inevitable consequence of the bloom of our youth that we haven’t even had time to taste the poisoned apple.
Yet, even a young species possesses history. One of the most penetrating powers of genomics is its ability to organize even closely related genomes into classes and subclasses. If we go hunting for discriminatory features and clusters, then we will, indeed, find features and clusters to discriminate. Examined carefully, the variations in the human genome will cluster in geographic regions and continents, and along traditional boundaries of race. Every genome bears the mark of its ancestry. By studying the genetic characteristics of an individual you can pinpoint his or her origin to a certain continent, nationality, state, or even a tribe with remarkable accuracy. It is, to be sure, an apotheosis of small differences—but