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.”