The first humans venturing onto the Tibetan Plateau, often called the “roof of the world,” faced one of the most brutal environments our species can endure. At an average elevation of over 4,500 meters, it is a cold and arid place with half the oxygen present at sea level. Science has long held that humans did not set foot in this alien place until 15,000 years ago, as suggested by archaeological evidence of the earliest known settlement on the northeastern fringe of the plateau 3,000 meters above sea level. But now new genetic data indicate this may have occurred much earlier—possibly as far back as the last ice age, 62,000 years ago.
A better understanding of modern Tibetans’ genetic mix and diversity could help reconstruct the history of migration and population expansion in the region, and may help unravel the mystery of the ethnic origins of Tibetans—and of how humans have adapted to low-oxygen conditions at high altitudes.
For the new study, researchers sequenced the entire genomes of 38 ethnic Tibetans and 39 Han Chinese (the country’s majority ethnic group), and compared the results with published genomic sequences of other ethnic groups around the world—information that allowed the team to pinpoint the common genetic origin of different populations and to get a better grasp on the history of migration in Tibet.
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“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. Xu’s work was published in September in the American Journal of Human Genetics, and presented at the American Society of Human Genetics’ annual meeting in Vancouver. 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. “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. 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. The modern part of the Tibetan genome shares 82 percent similarity with East Asians, 11 percent with Central Asians, and 6 percent with South Asians. “Among all ethic groups, Han Chinese are most closely related to Tibetans,” Xu says.
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. That idea contradicts the commonly held notion that any early plateau dwellers would have been eliminated during harsh climate intervals such as LGM and another period known as the Younger Dryas between 12,900 and 11,600 years ago, says David Zhang, a geographer at the University of Hong Kong, who was not involved in Xu’s research.
In 2002 Zhang and a colleague published a controversial study in Geophysical Review Letters showing marks of hands and feet from at least six individuals in rocks that were once soft mud, which was dated to 20,000 years ago at 4,000 meters above sea level in the heart of Tibet. Based on this they theorized that people were living in Tibet at the height of LGM, but the lack of archaeological finds near the site has cast doubt on this. “Many people don’t think it’s possible,” Aldenderfer says. “But there were plenty of places for [those early populations] to live where local conditions weren’t that bad, such as the big river valleys on the plateau.” The handprints and footprints were uncovered near one of the plateau’s many hot springs, which could have served as refuges for plants, animals and humans, he adds.
Two independent archaeological studies presented at the 33rd International Geographical Congress, held in August in Beijing, also support the antiquity of Tibet settlement as suggested by Xu’s genetic data. 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. The site, rich with stone tools and animal bones, lies at 2,500 meters above sea level at the bank of the Salween River. “This may represent one of the first steps of human colonization on the plateau,” Dong says. “Those hunter-gathers might then expand to the inner plateau along the river valley.”
The second study pushes back the dates of human settlement above 4,000 meters by 4,000 years. 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. “This may have helped the plateau dwellers to survive the harsh conditions at such high elevations nearing the end of the Younger Dryas,” he says.
“It’s increasingly clear that there has been much earlier and much more persistent human occupation of the plateau than we previously thought,” Aldenderfer says. He stresses, however, that pieces are still missing from the puzzle: “More excavations are required to close those gaps.”