Human Origins / Archaeology

One day last fall, in the home freezer of Spencer Wells, there were these things: a large leg of lamb, a few quarts of milk, and underneath, DNA samples from 2,500 people in Central Asia. Wells is an anthropological geneticist and an energetic collector of DNA, especially Y chromosomes. He lived then in an old stone house outside Geneva, but he was raised in Lubbock, Texas. His own Y chromosome, like his name, hails from Connecticut—an ancestor was governor there in the 17th century. Before that, Wells’s chromosome came from southern England, and before that, maybe 30,000 years ago, it came from Central Asia. From then and there to here and now, it was passed on, like an indelible stain, by a thousand fathers to a thousand sons, one after the other, until it ended up in Wells’s father, a Lubbock lawyer, and then in Wells.

The DNA samples in the freezer, then, are samples of Wells’s own roots—and of those of a good part of humanity. Before Wells collected the samples, the region was pretty much terra incognita, genetically speaking. Now some geneticists see it as a second font of human diversity. In Wells’s view, the grasslands of Central Asia, so reminiscent of the East African savannas with their abundance of big game, are where the human race fattened up after it left Africa, 50,000 or 60,000 years ago. “It was essentially a meat locker,” he says. “Loads of food. And that allowed them to build up the population density to then go out and move westward and then eastward.”

The westward branch of humanity entered Europe; the eastward branch eventually crossed the Bering Strait and entered North America, and there the two branches met again in 1492. By that time they had come to seem very different from each other. Traces of how human beings had fanned out across the planet, acquiring superficial racial differences along the way, are written in our DNA and especially in the Y chromosome.

Before long, the record of that ancient migration will begin to vanish. Our ancestors took tens of thousands of years to spread around the planet; people today move from Lubbock to Geneva or from Tamil Nadu to Texas in hours. In the process they wipe out genetic clues to the past. Think of our genes as the vestiges of an ancient library in which geneticists are trying to piece together and decipher the books; now think of that ruin being paved over for a new airport. Archaeologists would want to mount a rescue dig—exactly what anthropological geneticists are doing these days. That, along with a young man’s taste for adventure, is what has repeatedly sent Wells bouncing across the Central Asian steppes in a Land Rover. The DNA he  has brought back records not just our distant history but also our more recent past—and in particular what happened around 800 years ago, when a prodigious fornicator named Genghis Khan splashed into the gene pool like a cannonball.

Wells is a tall, handsome man in his thirties, with strawberry blond hair and a chiseled face that quickly turns ruddy in the sun. Words stream out of him without a trace of a Texas accent—after Lubbock and before Geneva he went to Harvard and Stanford. He felt bottled up in Lubbock, he says, and is drawn to places like India, where you step out of a taxicab to face cows and crowds and people of many colors speaking strange languages: “It’s incredible, and it’s overwhelming. I love the feeling of being immersed.”

At Harvard, where he got his Ph.D., Wells studied fruit flies with Richard Lewontin and became interested in understanding the reasons for genetic variation within a particular group. A population crashes due to disease, for instance, and is then restarted by a few individuals, or a few individuals migrate to a new, uninhabited region and start a new population. In both cases the genes of the founders become prevalent in the new population, even if they confer no particular selective advantage. “So much of what we see in the DNA, in genetic variation, is due to population events,” says Wells. “Which is great, but I’m not interested in the population history of fruit flies. I am, however, very interested in the population history of humans.”

Lewontin’s advice was to go west, to Stanford, to work with Luigi Luca Cavalli-Sforza, the father of anthropological genetics. When Wells arrived at Stanford in 1994, Cavalli-Sforza’s lab was just plunging into studies of the Y chromosome. Two researchers there, Peter Underhill and Peter Oefner, had recently invented a technique for rapidly finding DNA mutations—markers—at the same point in the genomes of two different people. The invention proved useful for tracing human migration. Most spontaneous mutations do neither harm nor good but simply accumulate in the genome, one at a time, as they are passed from one generation to the next. A mutation shared by everybody, therefore, must have arisen in everybody’s common ancestor. The mutation marks the trunk of that population’s family tree. Each successive mutation identifies a branching point, right out to the twigs at the tip of the tree, which represent individual humans.

Forensic geneticists use large numbers of markers to isolate and identify one of those twigs in a murder case. Population geneticists focus mostly on the bigger branches. A mutation that is near-universal in Asia, for instance, but near-absent in Africa is most likely a sign that somewhere in deep time a small group of founders with that marker left Africa and started a new population in Asia.

What complicates the picture, as it complicates so many things, is sex. DNA comes in chromosomes, and chromosomes come in matched pairs, and when a body makes a sperm cell or an egg, the two chromosomes in a pair recombine, exchanging large chunks of DNA. Over time, each chromosome becomes a patchwork of contributions from innumerable ancestors, both male and female. A recombined chromosome might tell you, for example, that your Ice Age ancestors came from Central Asia and a later ancestor was governor of Connecticut, but it would be missing their passage through England. Its story wouldn’t make much sense.