Y chromosomes and mtDNA in Bronze Age Tarim basin

This appears to be a published study by the same team which appeared in a recent National Geographic documentary on the Tarim mummies. The site of Xiaohe (of the current study) is earlier and to the southeast of Urumqi.

The finding that the Bronze Age population, like that of Krasnoyarsk Siberians belonged exclusively -as far as sampling allows- to Y-chromosome haplogroup R1a1 is extremely interesting as it raises the issue of when and how exactly the diverse extant Y-chromosome gene pool of Central Asia came about.

We can now confidently say that even at the early age of ~4ky BP an R1a1-bearing population of presumably western Eurasian origin had acquired a mixed mtDNA gene pool consisting of both west- and east-Eurasian mtDNA, which agrees with what was presented in the aforementioned documentary, in which many of the seemingly Caucasoid mummies had East Eurasian mtDNA.

From the paper:

The dominant haplogroup in the Xiaohe people was the East Eurasian lineage C, shared by 14 Xiaohe individuals who were associated with two different mtDNAhaplotypes (S1 and S2). According to the coding region 11969 A to G, all lineage C found in the Xiaohe people were further classified to subhaplogroup C4, which had D-loop group-specific polymorphisms at nucleotide positions (np) 16298 (T to C) and16327 (C to T) [18].

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Besides the East Eurasian lineage, two West Eurasian mtDNA haplogroups H and K were found among the Xiaohe people. H lineage is the most common mtDNAhaplogroup in West Eurasia [20], but haplogroup H with a 16260T was shared by only nine modern people in Genbank, including one Italian, one German, one Hungarian,one Portuguese, one Icelander and four English people. Haplogroup K, a western Eurasian–specific haplogroup, is mainly distributed in Europe, central Asia, and Iran [20, 21]. However, haplogroup K with 16134T, found in the Xiaohe people, has not been found in modern people to our knowledge.

...

Among the Xiaohe people, three sequences with the unique HVRI motif 16189–16192–16311 formed a subcluster (Figure 2) and were not shared by modern people. [...] The results showed that they are related neither to the West Eurasian haplogroups UK, TJ, HV, R11and R1, nor to the East Eurasian haplogroups B and F. So we designated them as haplogroup R*temporarily. Another sequence with motif 16223–16304, shared by some people fromEast Asia, India, and Europe, was assigned to haplogroup M*.


The mtDNA results are interesting because they tie in with my theory of a boreal mtDNA-U zone which has recently found further support (and extended in time) with the mtDNA testing of Kostenki. Like the Krasnoyarsk Siberians the Caucasoid component in Xiaohe is light on U. Unfortunately, we don't have enough information to place the origin of this component in a particular region of West Eurasia.

A graphical display of this theory can be seen in Figure 5 from the paper (left). Sites 8, 9 and 3 (Lake Baikal and Xiognu) only have U5 "western" mtDNA, and, as I have argued in my previous post, represent the eastern edge of the "boreal zone" to which were added non-U bearing Caucasoids from the west.

As for the Y-chromosome results:

The Y chromosome haplogroup of the sevenmales were all assigned to haplogroup R1a1a through screening the Y-SNPs at M89,M9, M45, M173 and M198 successively. Haplogroup R1a1a is widely distributed inEurasia: it is mainly found in Eastern Europe, Central Asia, South Asia, Siberia,ancient Siberia, but rare in East Asia [22-24].

The authors address the origins of the admixture:

Given the unique genetic haplotypes and the particular archaeological culture, the time of this admixture could be much earlier than the time at which the Xiaohe people were living at the site. This means that the time of their mingling was at least a 1000 years earlier than previously proposed.

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The admixture therefore probably occurred elsewhere, before immigration into the Tarim Basin. The Xiaohe people might well have been an admixture at the time of their arrival. Where did the initial admixture occur?

To answer that question, the authors identify Afanasievo and other steppe cultures as related to the Xiaohe people. This is not very surprising as the Afanasievo people were described in the anthropological literature as prominent-nosed Caucasoids of western origin, although individual skulls show Mongoloid influences.

So, it's possible that the admixture took place in Siberia, and an already admixed population found its way to Xiaohe by ~4ky BP. The important question is: what happened to the male lineages of the eastern component of this population?

Years ago, I advanced a "pendulum" theory of migrations in Eurasia to explain the fact that quite often we find in the northern belt from Europe to China populations with typically Western/Eastern Y-haplogroups accompanied by the "opposite" (Eastern/Western) mtDNA.

According to my thinking, this is due to the patriarchal nature of mobile Eurasian societies (whether nomads or hunters) in which the "clan" maintains its Y-chromosome gene pool but incorporates foreign females. Thus, the absence of non-R1a1 chromosomes can be explained by the fact that non-R1a1 male individuals were not incorporated into the "western" tribe that made its way across Eurasia from Europe to China, but Eastern Eurasian-mtDNA bearing females were gradually absorbed; such would have been plentiful among the indigenous Mongoloid populations that lived east of the Urals since the Paleolithic.

Thus, at the eastern end of this migration, we ended up with an R1a1-pure/East Eurasian mtDNA-heavy population.

Years later, the pendulum of Eurasian migration swung backwards, with some of the Asian R1a1-bearing individuals returning towards Europe (starting with the Scythians) to meet their distant cousins, this time shedding whatever east Eurasian mtDNA gene pool they had acquired, for the regular west Eurasian mtDNA gene pool that would have been reinforced in the return journey.

BMC Biology doi:10.1186/1741-7007-8-15

Evidence that a West-East admixed population lived in the Tarim Basin as early as the early Bronze Age

Chunxiang L et al.

Abstract

Background

The Tarim Basin, located on the ancient Silk Road, played a very important role in the history of human migration and cultural communications between the West and the East. However, both the exact period at which the relevant events occurred and the origins of the people in the area remain very obscure. In this paper, we present data from the analyses of both Y chromosomal and mitochondrial DNA (mtDNA) derived from human remains excavated from the Xiaohe cemetery, the oldest archeological site with human remains discovered in the Tarim Basin thus far.

Results

Mitochondrial DNA analysis showed that the Xiaohe people carried both the East Eurasian haplogroup (C) and the West Eurasian haplogroups (H and K), whereas Y chromosomal DNA analysis revealed only the West Eurasian haplogroup R1a1a in the male individuals.


Conclusion

Our results demonstrated that the Xiaohe people were an admixture from populations originating from both the West and the East, implying that the Tarim Basin had been occupied by an admixed population since the early Bronze Age. To our knowledge, this is the earliest genetic evidence of an admixed population settled in the Tarim Basin.

Source: Dienekes