Analysis of ancient mitochondrial DNA gives insights into population movements in the Tarim Basin, China

12/06/2016

Fig. 1

Map of Eurasia showing the location of the Xiaohe cemetery, the Tarim Basin, the ancient Silk Road routes and the areas occupied by cultures associated with the settlement of the Tarim Basin. This figure is drawn according to literatures

Fig. 2

a Fourth layer of the Xiaohe cemetery showing a large number of large phallus and vulva posts; b A well-preserved boat coffin; c Female mummy with European features; d Double-layered coffin excavated from the Xiaohe cemetery

Chunxiang Li, Chao Ning, Erika Hagelberg, Hongjie Li, Yongbin Zhao, Wenying Li, Idelisi Abuduresule, Hong Zhu and Hui Zhou

BMC Genetics201516:78

DOI: 10.1186/s12863-015-0237-5

Abstract

Background

The Tarim Basin in western China, known for its amazingly well-preserved mummies, has been for thousands of years an important crossroad between the eastern and western parts of Eurasia. Despite its key position in communications and migration, and highly diverse peoples, languages and cultures, its prehistory is poorly understood. To shed light on the origin of the populations of the Tarim Basin, we analysed mitochondrial DNA polymorphisms in human skeletal remains excavated from the Xiaohe cemetery, used by the local community between 4000 and 3500 years before present, and possibly representing some of the earliest settlers.

Results

Xiaohe people carried a wide variety of maternal lineages, including West Eurasian lineages H, K, U5, U7, U2e, T, R*, East Eurasian lineages B, C4, C5, D, G2a and Indian lineage M5.

Conclusion

Our results indicate that the people of the Tarim Basin had a diverse maternal ancestry, with origins in Europe, central/eastern Siberia and southern/western Asia. These findings, together with information on the cultural context of the Xiaohe cemetery, can be used to test contrasting hypotheses of route of settlement into the Tarim Basin.

Background

The Tarim Basin in the Xinjiang region of China is situated on the Silk Road, the collection of ancient trade routes that for several millennia linked China to the Mediterranean (Fig. 1). The present-day inhabitants of the Tarim Basin are highly diverse both culturally and biologically as a result of extensive movements of peoples and cultural exchanges between east and west Eurasia [1, 2, 3]. Archaeological and anthropological investigations have helped to formulate two main theories to account for the origin of the populations in the Tarim Basin [4, 5, 6, 7, 8, 9, 10, 11, 12]. The first, so-called “steppe hypothesis”, maintains that the Tarim region experienced at least two population influxes from the Russo-Kazakh steppe. The earliest settlers may have been nomadic herders of the Afanasievo culture (ca. 3300–2000 B.C.), a primarily pastoralist culture derived from the Yamna culture of the Pontic-Caspian region and distributed in the Eastern Kazakhstan, Altai, and Minusinsk regions of the steppe north of the Tarim Basin (Fig. 1) [9, 12, 13, 14, 15]. This view is based on the numerous similarities between the material culture, burial rituals and skeletal traits of the Afanasievo culture and the earliest Bronze Age sites in the Tarim Basin, such as Gumugou (ca. 3800 BP), one of the oldest sites with human burials in Xinjiang [8, 9, 11, 12, 16]. These first settlers were followed by people of the Late Bronze Age Andronovo cultural complex (ca. 2100–900 B.C.), another pastoralist culture derived from the Yamna culture, primarily distributed in the Pamirs, the Ferghana Valley, Kazakhstan, and the Minusinsk/Altai region (Fig. 1) [8, 9, 11, 12, 15, 16]. This is signaled by the introduction of new material culture, clothing styles and burial customs around 1200 B.C. The second model, known as the “Bactrian oasis hypothesis”, also postulates a two-step settlement of the Tarim Basin in the Bronze Age, but maintains that the first settlers were farmers of the Bactria–Margiana Archaeological Complex (or BMAC, also known as the Oxus civilization) (ca. 2200–1500 B.C.) west of Xinjiang in Uzbekistan (north Bactria), Afghanistan (south Bactria), and Turkmenistan [17], followed later by the Andronovo people from the northwest (Fig. 1) [5, 7]. This model emphasises the environmental similarities between the Xinjiang and Central Asian desert basins, and suggests that certain features, including the irrigation systems, wheat remains, woolen textiles, bones of sheep and goats, and traces of the medicinal plant Ephedra found in Xinjiang could be evidence of links with the Oxus civilization [5, 7, 16]. These contrasting models can be tested using DNA recovered from archaeological bones. Previous genetic evidence on the origin of the earliest settlers was based on the analysis of mtDNA from burials at the Gumugou cemetery in the eastern edge of the Tarim Basin. In that study, researchers sequenced the first mtDNA hypervariable region (HVRI), but the results were inconclusive [18]. The discovery of another Bronze Age site of a similar age to Gumugou, with many well-preserved mummies, including individuals with European facial features, provided a unique opportunity to obtain genetic evidence about the first settlers of the Tarim Basin [19, 20, 21].

We describe here the analysis of mtDNA from human remains recovered from the Xiaohe tomb complex, an important Bronze Age site in the eastern edge of the Tarim Basin (40°20′11″N, 88°40′20.3″E) (Fig. 1). Discovered originally in 1934 by the Swedish archaeologist Folke Bergman, it was subsequently lost, but rediscovered in 2000 by a team from the Xinjiang Archaeological Institute using global positioning equipment. The cemetery was excavated between 2002 and 2005, and consisted of five strata with radiocarbon dates ranging from 4000 to 3500 years before present (¹⁴C yBP) [19, 22]. The site has many notable features, including numerous large phallus and vulva posts made of poplar, striking wooden human figures and masks, well-preserved boat coffins, leather hides, wheat and millet grains, and many artifacts (Fig. 2). Importantly, it contains the oldest and best-preserved mummies so far discovered in the Tarim Basin, possible those of the earliest people to settle the region. Genetic analysis of these mummies can provide data to elucidate the affinities of the earliest inhabitants, and help understand later patterns of human migration in the Eurasian continent.

The necropolis consisted of five layers of burials spanning half a millennium, offering the opportunity to determine the extent of interactions between the people of Xiaohe and other populations after the original settlement of the Tarim Basin. Did the people remain comparatively isolated or did they intermarry with newcomers? In an earlier study, we analysed DNA recovered from the deepest and oldest layer of burials of the Xiaohe site, the fifth layer, corresponding to the earliest inhabitants. Our results revealed that the first settlers carried both European and central Siberian maternal lineages. These findings agreed with the archaeological evidence for a close connection to the Afanasievo culture of the steppe north of the Tarim Basin, in other words with the “steppe hypothesis” [23]. We describe here the analysis of the maternal lineages of individuals recovered from the remaining four burial layers, and discuss the results in the context of the contrasting views on the settlement and migration patterns of the Tarim Basin.

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