It could not have spread so fast and so far by chance, yet natural selection made a poor explanation: The full sequence of the Y chromosome has been determined, but it doesn’t seem to do much except make the bearer male. Zerjal and Tyler-Smith started reading up on Mongolian history. They noticed that the vast range of the distinctive star-cluster chromosome corresponded almost exactly to the extent of Genghis Khan’s empire. The only outlier is a small ethnic group called the Hazaras, who live in northern Pakistan, which Genghis never conquered. The puzzling chromosomes are more frequent in the Hazaras today than in any other population, even the Mongols. But the Hazaras migrated into Pakistan from neighboring Afghanistan only in the 19th century, and they brought with them an interesting oral tradition: They claim to be direct descendants of one of Genghis Khan’s battalions. Some even claim, with genealogies to back them up, to be direct descendants of Genghis Khan himself. The Hazaras don’t refute the case, says Tyler-Smith; they cinch it. “It was a conclusion that was forced upon us.”
That doesn’t mean those Central Asian men have inherited some interesting trait from Genghis Khan, like his fierceness or his lust. To the extent that such traits are genetic at all, they probably involve many genes, none of which is likely to be on the Y chromosome. And although the rest of Genghis Khan’s genome has certainly, if Zerjal and Tyler-Smith are right, made an outsize contribution to the Central Asian gene pool, it has been chopped to bits and mixed in so thoroughly over the centuries by genetic recombination that no one today is likely to have his whole suite of genes for any particular trait. What they have, in his intact Y chromosome, is more like an invisible birthmark.
Genghis Khan was not necessarily the first to have it; its rough age of 1,000 years suggests he inherited it from an ancestor, perhaps a great-great-great-grandfather. Nor was he the only one to spread it: His brothers, sons, grandsons, and some of his cousins would have had the same Y chromosome. His sons and grandsons ruled the empire he built; one grandson, Kublai, was emperor of China. Presumably they enjoyed sexual opportunities similar to Genghis’s, and some were just as vigorous about exterminating competition. According to one chronicle written a century after Genghis’s birth, there were more than 20,000 people of his lineage “living in the comfort of wealth and affluence.”
To some geneticists, the whole story seems incredible. “It’s complete conjecture!” says Underhill. “There are no living relatives of Genghis Khan that anyone can document, as they did for Thomas Jefferson. And the other problem I have is they estimate the age of that lineage to be about a thousand years—it could be easily 3,000 years old, depending on which mutation rate you use. You could have 3,000 years of this chromosome dispersing across Central Asia. You don’t need to invoke Genghis Khan screwing every woman in sight. It just doesn’t compute with me.”
“What is the alternative?” asks Tyler-Smith. “We know from the genetics that this pattern originated in Mongolia or nearby a thousand years ago or some similar time. So the alternative to its being spread by Genghis Khan is that his Y chromosome, despite his reported 20,000 descendants, is not visible in the genetic record now, but that of another person has spread in this unprecedented way. To me that is just less plausible. I think it was his military ability that allowed it to spread. If it hadn’t been for that, it would have been just another low-frequency chromosome.”
The fastest and most famous case of evolution by natural selection is the case of the British peppered moths: In the 19th century, as mills and factories began to darken the air with soot, a rare all-black mutant quickly became more common in Britain than the normal white moth with black spots because it was less conspicuous to predators. Genghis Khan’s Y chromosome, says Tyler-Smith, spread at a comparable rate through Central Asia. “It shows that a kind of social selection can operate in humans, whereby people inherit status and the reproductive advantage that goes along with that,” he says. “It can have a large effect on the genetics.”
Man does not evolve by natural selection alone. Darwin even believed that sexual selection—in choosing mates, we choose the genes we pass on to the next generation—was a more important source of “differences in external appearance between the races of man.” That idea has never really been tested, says Wells. In recent decades geneticists, beginning with Richard Lewontin, Wells’s adviser, have clearly showed how insignificant the genetic differences among races are: The diversity within any single population is far, far greater. “But, by God,” says Wells, “I can tell the difference between somebody who comes from the Outer Hebrides and someone who comes from Cambodia. They look different.” Science has yet to explain that.
How humans settled the planet, in prehistoric and historic times, and how they came to be so diverse, are interesting questions for anthropological geneticists to tackle, if only those questions can be freed from their association, in some people’s minds, with racism and colonialism, and if only the geneticists can get enough support. But they face another more difficult problem: globalization. People today are feverishly uprooting themselves and their Y chromosomes, moving, as it were, from Cambodia to the Outer Hebrides, from their yurts into the nearest multiethnic city. All the lineages Wells has found in Asia, he likes to say, could probably be found in a single nightclub in New York City’s East Village, engaged in precoital rituals. “Socially, I think it’s fantastic,” Wells says. “It just makes my life bloody difficult.” A geneticist taking samples in that nightclub would be like a wine lover who never gets to see the labels: He would find tremendous diversity, but without geographic context it would be meaningless. He would be powerless to understand the fantastic migrations that had brought all those different people to that one place and time.
We are headed toward a world in which we will have erased the historical record in our genes just as we have acquired the means to read it. For several years now Wells has been trying to organize an effort to do on a global scale what he already did in Central Asia—similar to the ill-starred Human Genome Diversity Project. The idea is simply to preserve a genetic snapshot of humanity. “It’s the sort of thing where, once you lose the information, you’re never going to get it back,” he says. “It is our single, unique human history, and it would be nice to know what that is as we hurtle into the future and start to change our own genetics.
“The clock is ticking. We need to get out there and do some more sampling.”
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