Scientists believed that the first people appeared on the ice-bound Tibetan plateau or the Tibetan plateau 15 thousand years ago. New genomic analysis suggests this data could be quadrupled.
The first people who decided to reach the Tibetan plateau, which is often called the "roof of the world", faced the most harsh environment that existed at that time on our planet. Its average altitude is over 4500 meters, it is a cold and deserted place, and the amount of oxygen there is half that at sea level. Initially, scientists believed that the first people appeared there 15 thousand years ago, but new genetic and archaeological evidence suggests that the first appearance of a person there could have occurred much earlier - perhaps even 62 thousand years ago, that is, in the middle of the last ice age. A deeper knowledge of the history of migration and population growth in this region can help uncover the mysteries of the origins of the Tibetans, as well as offer clues to understandinghow people adapted to the conditions of low oxygen content at high altitudes.
According to a study recently published in the American Journal of Human Genetics, researchers now better understand the history of the settlement of the Tibetan Plateau by sequencing entire genomes of 38 ethnic Tibetans and comparing the results with genomic sequencing of other ethnic groups. “The result was a complex set of evidence of prehistoric migration,” says Shuhua Xu, a population genetics specialist at the Institute of Biological Sciences of the Chinese Academy of Sciences. “The big surprise was the antiquity of sequenced Tibetan DNA,” Xu continues. "They may be related to ancestors who lived from 62 thousand years ago to 38 thousand years ago, and perhaps this data refers to the earliest settlement of this plateau."
When the impact of the Ice Age intensified after the first migration, the genetic mixing between Tibetans and other peoples stopped and continued for tens of thousands of years, indicating that migration to the Tibet area was reduced to a minimum at that moment. “The migration routes were probably blocked by ice,” Xu says. "It was difficult for even the most powerful hunter-gatherers to walk through them."
However, approximately in the period from 15 thousand years ago to 9 thousand years ago - after the so-called last glacial maximum, that is, at the most severe time when the ice cover reached its highest values - thousands of people headed to Tibet. “This is the most significant wave of migration that has shaped the modern Tibetan gene pool,” Xu says. This concept fits well with some independent data, which indicate that the Tibetans in the period from 12.8 thousand years ago to 8 thousand years ago began to undergo mutations that protected them from hypoxia.
Xu's team pioneered the sequencing of the entire Tibetan genome, and "the results are impressive," says archaeologist Mark Aldenderfer of the University of California, Merced, who was not involved in the study. As a result, “additional data was obtained regarding how different populations from different directions combined their genes and, eventually, there were people whom we call Tibetans today,” he adds.
The data obtained allow us to say that 94% of the genetic material of modern Tibetans is related to modern humans - they may have appeared in this region during the second migration wave - and the rest belongs to the representatives of the extinct species. The modern part of the Tibetan genome indicates a mixed heritage: 82% of the genetic material Tibetans share with other residents of East Asia, 11% with residents of Central Asia and 6% with residents of South Asia.
In addition, Xu's team identified a specifically Tibetan segment, which turned out to be very similar to the genome of the Ust-Ishim man (modern humans who lived in Siberia 45 thousand years ago), as well as several extinct species, including Neanderthals, Denisovans and unknown groups. This segment consists of eight genes, and one of them is known to be critical for adaptation at high altitude. “This means that Tibet has always been inhabited - even during the most difficult periods in terms of climate,” says Xu.
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This idea contradicts the widespread belief that the plateau's population died out during harsh climatic periods, including during the last glacial maximum, says David Zhang, a geographer at the University of Hong Kong who was not involved with Xu's group. Alderderfer and others acknowledge that this plateau may have served as a refuge for humans during the Ice Age. “There were a lot of habitable places (for these early populations) where local conditions weren't so bad - we are talking about the valleys of large rivers on this plateau,” he says.
Supports the idea of the population of Tibet and the study presented at the 33rd International Geographic Congress, which was held in Beijing last summer. On it, a group of experts reported the earliest archaeological evidence of the presence of man, which dates back to the period from 39 thousand years ago to 31 thousand years ago. The excavation site, where many stone tools and animal remains were found, is located on the banks of the Salween River in the southeastern part of the Tibetan plateau.
Different lines of evidence now combine to point to a much older period and more permanent human presence on this plateau than previously thought, Alderderfer says. However, in his opinion, the missing pieces of the puzzle still need to be found. "More excavation is needed to close these gaps."
Jane Qiu