Ancient DNA sheds light on Tibetan ancestry

Chinese paleoanthropologists have discovered that people living on the Qinghai-Tibet Plateau for the past 5,000 years share a single origin derived from a northeast Asian group that mixed with a separate, unknown group.

The history of permanent settlement on the plateau, with its cold, and arid environment, and the origins of the many genetic adaptations found in modern Tibetans at high altitudes are of interest to researchers.

Despite extensive archaeological evidence, DNA samples from ancient humans were previously limited to those taken from a thin slice of the southwestern Himalayan highlands.

The team, led by Fu Qiaomei from the Institute of Vertebrate Paleontology and Paleoanthropology with the Chinese Academy of Sciences, sequenced the genomes of 89 individuals dating from 5,100 to 100 years ago found at 29 archaeological sites across the large plateau, which is one of the highest inhabited zones above sea level and one of the harshest environments settled.

In the study, which was published in the latest edition of the peer-reviewed journal Science Advances, researchers reported that the oldest specimen containing the unique genetic components found in modern Tibetans was unearthed at a ruin in the northeast of the plateau.

Approximately 80 percent of the genetic components were related to people living in what is today northern China between 9,500 to 4,000 years ago, while the remaining 20 percent were attributed to an unknown population.

The findings suggest that the introduction of northeast Asian ancestry to the plateau took place alongside the population expansion in northern China during the Neolithic Era.

"This pattern is found in populations prior to the arrival of domesticated crops on the plateau," Fu said, adding that this early wave of migration was not associated with the arrival of grain growers, which took place later.

The study also revealed that the unique genetic traits appear to have been continuous over time. One example is the EPAS1 allele carried by many Tibetans.

An allele is any of two or more variants of a gene that may occur in a given place in the chromosome and is responsible for alternative characteristics.

EPAS1 is thought to have originated in an extinct species of ancient humans known as the Denisovans, and was likely acquired as the Tibetans adapted to lower oxygen levels on the plateau.

The passing on of the EPAS1 allele has become more frequent over the last 2,800 years on the Qinghai-Tibet Plateau, undergoing marked expansion over the past 700 years, in concordance with the natural trend toward the survival of the fittest, according to the study.

But when and where this adaptive allele first entered the ancestral Tibetan population is still unknown.

"The arrival of this variant must have occurred sometime prior to 5,100 years ago in an ancestral population that contributed to all plateau populations," Fu said.

Subsequent genetic shifts show that human migration and interaction within and between highland and lowland populations have influenced genetic mixing over time, allowing the ancestral population to diversify rapidly.

After further comparison, Fu's team identified three distinct genetic patterns appearing about 2,500 years ago. They belong to different Tibetan populations found in the northeastern, southern/central, and southern/southwestern regions of the plateau.

The study showed that people living in the southern part of the plateau began to display a significant degree of genetic similarity starting about 3,000 years ago, suggesting that the Yarlung Zangbo Valley was an important migration corridor at the time.

From about 4,700 years ago, northeastern populations received an influx of additional northern East Asian ancestry at lower elevations.

Although modern Tibetans have differing levels of gene migration compared to lowland East Asian populations, this is a result of recent trends of human migration as the pattern is not observed in populations dating from 1,200 to 800 years ago.

The study also found genetic influences from further afield in local populations. Individuals living in the fifth and 12th centuries in Shigatse in the southern part of the plateau were found to be genetically related to people from Central Asia.

Populations in the western part of the plateau showed partial Central Asian ancestry as early as 2,300 years ago, according to Wang Hongru, the paper's co-author and a professor at the Agricultural Genomics Institute in Shenzhen, Guangdong province.

Also, the inhabitants of Qamdo and Nyingchi some 2,000 years ago were genetically related to inhabitants of the southern part of East Asia.

"This is the largest study of ancient genetics on the plateau to date," said another of the paper's co-authors, Lu Hongliang, who is from Sichuan University.

Fu added, "With these findings, we now have a much better understanding of an important period of human history in Asia."

Xinhua