Wells argues that 30,000 years ago has to be the wrong date. The evidence is another marker, M242, that he and Mark Seielstad of Harvard identified. It arose after M45 but before M3, in the Asian population that was bound for America; Native Americans have M242, and so do some people still living in Central Asia. The Ice Age ancestors of Native Americans must have had that marker when they crossed the Bering Strait, and so the time of M242’s first appearance puts an upper limit on the time of their passage. “We can definitely rule out a date prior to 20,000 years ago,” says Wells.
Hammer and his Arizona colleagues, anthropologists Stephen Zegura and Tatiana Karafet, have recently confirmed that result. Their own Y chromosome collections tell them that the genetic separation of the Asian and American populations occurred no earlier than 17,000 years ago. And they think they have narrowed down a source region, an Asian ancestral home for Native Americans—the Altai Mountains of southwestern Siberia and western Mongolia.
The Altai is a remote region of 14,000-foot alps, deep river valleys, and large high-altitude lakes. “I think that has been a place where people have been for a long time,” Hammer says, “and have spawned many descendant populations. And I think some of those descendants ended up in the Americas.”
The Paleolithic migration into Europe, like the Paleolithic migration into North America, may have also departed from the Altai region, although it may have been from elsewhere in Central Asia or other locations. After 1492, in any case, those two great rivers of humanity, which had diverged 30,000 years earlier, began to converge again in America, and their waters commingled. Hammer estimates that 17 percent of Native American men today have Y chromosomes inherited from Europeans. (In African American men the European admixture may be from 5 to 30 percent.) These percentages show that history, and not just natural selection, has a big effect on the human gene pool—and that conquerors tend to spread their Y chromosomes.
EURASIAN JOURNEY
Graphic by Matt Zang (enlarge 68k) |
Genghis Khan was born east of the Altai Mountains, at the northern edge of the vast Mongolian steppe in 1162. His biographers agree, unsurprisingly, that he was driven by a lust for power. He was also driven by lust. Rashid ad-Din, vizier to a later khan, quotes Genghis as having said, “Man’s greatest good fortune is to chase and defeat his enemy, seize his total possessions . . . use the bodies of his women as a nightshirt and support, gazing upon and kissing their rosy breasts, sucking their lips which are as sweet as the berries of their breasts.” Rashid says Genghis brought a new wife home from every campaign, maintaining a harem of 500. His interest in sex was enduring. In his sixties and ailing, he crossed the Gobi Desert to massacre the Tanguts and died on that campaign. According to one legend, he was killed by the Tangut queen, Kurbelzhin, who injured his sex organ during intercourse.
The conqueror’s body was carried back to Mongolia, but his tomb has never been found. A team of archaeologists led by John Woods of the University of Chicago has spent several summers searching for it. It is not just the physical remains of Genghis Khan that are missing, says Woods. Although he fathered a huge empire, there is no artifact that can be definitively linked to him. There may be something else, however. The idea first came to Tatiana Zerjal while she was looking at an odd pattern on her computer screen.
After the 1998 expedition with Wells, Zerjal returned to Oxford, where her adviser, geneticist Chris Tyler-Smith, then had his lab. While Wells screened the Central Asian Y chromosomes for single-nucleotide mutations such as M45, Zerjal searched for a different DNA variable called a microsatellite. A microsatellite is a short, repetitious sequence of DNA—CACACACA, for instance—in which the number of repetitions can change from one generation to the next and often does.
When a Y chromosome is passed from father to son, the chance that a specific single nucleotide will change from, say, T (thymine) to A (adenine) is on the order of one in a few tens of millions. But the chance that a given microsatellite will change from, say, 11 CA (cytosine and adenine) repeats to 12 is on the order of one in a few hundred. That’s why a particular pattern of microsatellites can profile a particular individual—to show, for instance, as Tyler-Smith and Zerjal had done shortly before embarking on their Central Asian work, that Thomas Jefferson had fathered a son by his slave Sally Hemings and that his Y chromosome had been passed down to a man living in Pennsylvania today. In the human family tree, says Tyler-Smith, “the single-nucleotide polymorphisms give you the trunk and the main branches. The microsatellites give you the twigs at the end.”
As Zerjal screened Central Asian Y chromosomes for 16 different microsatellites, one combination showed up repeatedly. It was far more common than expected, and men all over Central Asia had it—which is also not what you’d expect. “Suddenly, I thought, ‘Wow, this is Genghis Khan,’ ” says Zerjal. At first, Tyler-Smith says, “We thought it was more or less a joke.”
But as Zerjal worked through more than 2,000 Y chromosomes, the joke wouldn’t die. On her computer diagram of how the chromosomes might be related, fully 8 percent of them clustered together in a starlike pattern, meaning they had either the identical set of microsatellites or one that differed at just one of the 16 locuses. Those mutations were most diverse in Mongolia, indicating that the original star-cluster chromosome had come from there. And judging from the small number of mutations it had accumulated, it came from there only about a thousand years ago.
