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The much earlier divergence between Tibetans and Han Chinese makes sense because there are continuous material cultures on the plateau since 15,000 years ago, he says.The study, Aldenderfer adds, “also provides fine details of how different populations from various directions may have combined their genes to ultimately create the people that we call Tibetans.” The data show that 94 percent of the present-day Tibetan genetic makeup came from modern humans—possibly those who ventured into Tibet in the second wave of migration—and the rest came from archaic hominins such as Denisovans, Neandertals and unknown groups.A team led by archaeologist Guanghui Dong of Lanzhou University in Gansu province unveiled the earliest archaeological evidence of human presence—dating to 39,000-31,000 years ago—on the southeastern fringe of the Tibetan Plateau.

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The findings also reveal a startling genetic continuity since the plateau was first colonized 62,000 years ago.

“This suggests that Tibet has always been populated—even during the toughest times as far as climate was concerned,” Xu says.

Since that initial migration, as the ice age tightened its grip on the plateau, genetic mixing between Tibetans and non-Tibetans probably ground to a halt for tens of thousands of years—suggesting that movement into Tibet dropped to the minimum.

“The migration routes were probably cut off by ice sheets,” Xu says.

“It’s simply too harsh even for the toughest hunter-gatherers.” But about 15,000 to 9,000 years ago—after the so-called last glacial maximum (LGM), during which the Earth’s ice cover had reached its most extensive point and climate was at its harshest—people flocked into Tibet en masse.

“It’s the most significant wave of migration that shaped the modern Tibetan gene pool,” Xu says.

Qinghai Normal University archaeologist Guangliang Hou and some of his colleagues recently excavated an archaeological site dated to 11,500 years ago, which is in line with the second and more important wave of migration that Xu’s study suggests.

Hou said at the geographical congress that the site, close to a main tributary of the Yellow River, is teeming with charcoal—a telltale sign of fire use by humans.

“We can really see rapid population expansion [on the plateau] during that time.” Interestingly, he adds, this was also when the common ancestor of Tibetans and Han Chinese split—contrary to a previous study suggesting that the divergence took place as late as 2,750 years ago.

“This is the first study to sequence the entire genome of Tibetans, and the resolution is really impressive,” says Mark Aldenderfer, an archaeologist at the University of California, Merced, who was not involved in Xu’s study.

“Tibetan-specific DNA sequences can be traced back to ancestors 62,000-38,000 years ago…This represents the earliest colonization of the Tibetan Plateau,” says Shuhua Xu, a population geneticist at the Chinese Institute of Sciences’ Shanghai Institutes for Biological Sciences.

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