Ancient DNA study reveals population history of West Tibetan Plateau

According to a study published in Current Biology May 22, the genetic components of the ancient populations in the western Tibetan Plateau are the closest to the ancient populations in the southern Tibetan Plateau, and their main genetic components have been maintained over the past 3,500 years. In addition, these ancient populations in the western Tibetan Plateau had complex and frequent interactions with ancient populations within and outside the plateau.

The study was conducted by the team of Prof. Fu Qiaomei of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences and archaeologists from the Tibetan Institute for the Preservation and Research of Cultural Relics and other institutions.

Ngari Prefecture, located in the western part of the Tibetan Plateau, is a junction connecting the Tibetan Plateau, South and Central Asia and a potential migration channel for early human populations. Previous research has confirmed that genetic components associated with Central Asian populations influenced this region about 2,300 years ago, but the genetic history of earlier populations in this region was unclear.

In this study, researchers sequenced whole-genome nuclear data from a total of 65 individuals from six sites dating from 3500 to 300 years ago in Ngari Prefecture. The newly sequenced data covers six archaeological sites, including Gebusailu, Laga, Sangdalongguo, Pulanduowa, Qulongsazha and Guge Ganshi Cave.

By combining recently sequenced data with previously published ancient genomic data, the evolutionary history of populations on the western Tibetan Plateau over the past 3,500 years has now been reconstructed.

This study is a systematic, long-term genomic study of ancient populations in the western Tibetan Plateau. It is of great importance for an in-depth understanding of the history of interactions between ancient populations of the western highlands and ancient populations within the highlands and in neighboring South Asia and Central Asia, according to the researchers.

Genetic characteristics of ancient populations of the western highlands and their interactions with ancient populations within the highlands

Previous understanding of the genetic history of the ancient populations of the Western Highlands was extremely limited. Based on just one site dating back 2,300 years, the researchers speculated that there was genetic influence between the populations of the Western Highlands and Central Asia. However, the exact time, place and extent of this impact were unknown.

In this study, researchers reconstructed the evolutionary history of populations on the Western Plateau over thousands of years using genetic information from different ancient Tibetan populations.

They showed that the ancient Ngari populations from 3,500 years ago had genetic components similar to the southern highland populations and did not carry additional genetic components from Central or South Asia.

This genetic makeup has been stably maintained in northwestern Ngari prefecture for over a thousand years: The genetic makeup of the late Gebusailu and Laga populations from 2300 years ago is almost the same as that of the Gebusailu population from 3500 years ago.

In other words, the Ngari population during this period mainly carried the genetic components of ancient populations from the southern highlands, while genetic components from Central and South Asia were introduced much later.

After that, starting from 2300 years ago, Central Asian components appeared in some places in northwestern Ngari Prefecture, but until 150 years ago, the proportion of Central Asian components was still less than 15%. Overall, local Ngari genetic components remained dominant from 3500 to 150 years ago.

The researchers also discovered complex population interactions and migrations within the highlands. The genetic components of ancient populations in the southern and western parts of the plateau 3500 years ago were very similar, indicating that population migrations and south-west expansion may have occurred earlier.

In addition, compared to the population in northwestern Ngari, the populations from 1800 to 1600 years ago from the Kongque River to the upper reaches of the Xiangquan River in southeastern Ngari were further influenced by ancient populations from the southern plateau. This suggests that the people of the southern plateau may have begun to expand westward again before the Tubo Empire expanded westward in the early 7th century.

In short, the interactions between the ancient populations of the southern and western highlands were much more complex than recorded in historical documents. As early as 1,800 years ago, multiple human migrations from east to west took place between the two sites, and a gradient of genetic components introduced by the expansions can be observed in these populations from east to west.

Interactions between ancient populations on the Western Plateau and beyond

Although Central Asian components began to influence some Ngari populations around 2300 years ago, the proportion of Central Asian components in the genomes of these populations is low (<5%).

On the one hand, these results provide genetic evidence for cultural interactions between ancient populations on the plateau and ancient populations in neighboring Xinjiang, Central Asia, and South Asia. On the other hand, they also reflect that such cultural interactions between the highlands were accompanied by limited population migrations or mixing.

Beginning 2,000 years ago, genetic components related to South Asia also influenced the region of Ngari Prefecture. Interestingly, the genetic components related to South Asia that influenced the ancient Ngari populations originated from mixed populations that settled in Central Asia. These populations are genetically related to Central Asian populations, but are culturally typical of South Asian populations. Previous research has suggested that they were immigrants from the Indus Valley Civilization who settled in Central Asia.

This is the first identification of genetic components linked to South Asia in ancient populations on the Tibetan Plateau, providing clues to interactions between ancient populations on the Plateau and those associated with the Indus Valley Civilization.

In the 9th century AD, with the fall of the Tubo regime, descendants of the Tubo royal family established the Kingdom of Guge in present-day Ngari Prefecture. The Kingdom of Guga failed only in the 17th century. As a continuation of the Tubo lineage, the Guge population is an important window into the genetic characteristics of the last Tubo people.

The sample of ancient people collected by the researchers from the Guge cave showed the influence of Central Asian populations, and the share of Central Asian components was as much as 31%. This mixing event is estimated to have occurred around 1353 AD, coinciding with the prosperity of the Guga Kingdom.

This new result shows that in addition to its historical ties with South Asian kingdoms, the Guge kingdom also had population interactions with people from Central Asian kingdoms.

Kinship Practices in Early Western Highland Communities

The researchers also examined kinship among individuals from early Western Highlands sites. In the 2,300-year-old Gebusail tomb, a family structure of the second generation, consisting of a father and two sons, was discovered.

At the same time, individuals at this site and the 1,800-year-old Pulanduowa site were mostly male, and the diversity of Y-chromosome haplotypes was clearly lower than that of mitochondrial haplotypes, indicating that both may have been communities dominated by paternal inheritance.

The 1,600-year-old tomb of Qulongsazha includes a family line in which second- and third-generation descendants are related by maternal kinship, suggesting the role of maternal kinship in the family line. Combined with the sex ratio and paternal and maternal genetic diversity, this pattern reflects a relatively balanced pattern of paternal and maternal inheritance.

Nevertheless, this study is a preliminary investigation of kinship practices at early plateau sites, and future research with more samples will shed more light on the details and diversity of ancient Tibetan community structure.

Due to the high altitude and harsh natural environment of the western Tibetan Plateau, anthropological and archaeological research in this region is relatively limited. In particular, there are large gaps in previous research on the evolutionary history of the early ancient populations of the West Tibetan Plateau and their interaction with ancient populations in neighboring areas.

Using a combination of genetics and archaeology, the researchers thoroughly explored the evolutionary history of ancient populations on the Western Plateau over the past 3,500 years. While the major genetic components of the ancient populations of the Western Plateau show continuity, these populations had complex and frequent interactions with ancient populations in the neighboring Southern Plateau region, Central Asia, and South Asia beyond the Plateau.

Since the Tibetan Plateau is a bridge between East, Central and South Asia, the results of this study are important for understanding the genetics of ancient populations on the plateau and the interactions of these populations with ancient inhabitants of Central and South Asia.