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. (Figure 2 from Li et al. in BMC Genetics 2015 16:78. Image used under a CC-BY 4.0 license.)
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Within a nondescript Bronze Age cemetery first discovered by Swedish archaeologists in 1934 and rediscovered by the Xinjiang Archaeological Institute in 2000, researchers have found the oldest and best-preserved mummies in the Tarim Basin area of China. Their skeletal remains, along with unprecedented artifacts, are helping solve the longstanding question of the origins of human settlement in a politically contested area of China.
Contemporary occupants of the Tarim Basin, a geographical area in the Xinjiang Uyghur autonomous region of northwest China, are both biologically and culturally diverse. The region borders numerous countries and was historically a part of the Silk Road trade route between the West and the East, so people and artifacts have moved through the Tarim Basin for thousands of years. But the origins of the inhabitants of the basin have been questioned.
One hypothesis suggests that the earliest settlers of this part of Asia were nomadic herders from the steppes of Russia and Kazakhstan, while the other suggests that people came first from the oases of Bactria, or modern Uzbekistan, Afghanistan, and Turkmenistan. While both hypotheses have support in archaeological findings such as burial customs, clothing styles, and animal bones, previous genetic evidence from human remains, which came from a cemetery called Gumugou on the eastern edge of the Tarim Basin, was inconclusive.
Writing in the journals BMC Genetics and BMC Biology, Chunxiang Li, an ancient DNA specialist at Jilin University, and colleagues report on their analysis of human remains from the Xiaohe tomb complex also on the eastern edge of the basin. Dating to about 4000 years before present, the site boasts notable artifacts like “numerous large phallus and vulva posts made of poplar, striking wooden human figurines and masts, well-preserved boat coffins, leather hides,” as well as grain and other preserved organic material, they write. More importantly, Xiaohe has produced the oldest, best-preserved mummies in the Tarim Basin, ideal for testing hypotheses about the origins of these people, and the site spans a millennium, making it ideal for looking into population interaction after initial settlement.
In a Feb. 18, 2011 photo, the Beauty of Xiaohe, a mummy discovered in the Tarim Basin in far western China, is shown at the “Secrets of the Silk Road” exhibit at the University of Pennsylvania Museum of Archaeology and Anthropology in Philadelphia. The exhibit is scheduled to run through until March 15. Philadelphia is the final stop before the artifacts return to China. (AP Photo/Matt Rourke)
From the earliest layer of burials, Li and colleagues tested 20 individuals who produced affinities with 5 different mitochondrial DNA haplogroups, or major branches on the female side of the genetic family tree. “The dominant haplogroup,” they write, “in the Xiaohe people was the East Eurasian lineage C” which corresponds with a likely origin in South Siberia. But there were also “two West Eurasian mtDNA haplogroups H and K.” In looking more closely at the lineages and mutations, however, Li and colleagues noted that several of the samples had mutations that are either rare in modern people or are not found in modern gene banks. They further analyzed Y chromosome haplogroups to attempt to identify major branches of the male line. But all seven males in the study belonged to a haplogroup that is widely distributed throughout Eurasia.
“Considering the presence of haplogroups H and K in the Xiaohe people and the geographical distribution of shared sequences, we conclude that the west Eurasian component observed in the Xiaohe people originated from western Europe, and maternal ancestry of the Xiaohe people might have close relationships with western Europeans,” Li and colleagues note. By the Bronze Age, the people buried at Xiaohe cemetery were already “admixed,” coming together millennia earlier in Siberia and Mongolia.
In order to delve more deeply into population movement along the Silk Road, Li and colleagues examined dozens more samples from three later time periods at Xiaohe. Again, the most common mtDNA haplogroup was C, suggesting origins in southern Siberia. These more recent burial layers “confirmed that the origin of the mitochondrial lineages is more widespread,” the researchers write, including six west Eurasian lineages, five east Eurasian lineages, and one Indian lineage. In particular, the “west Eurasian genetic components in the Xiaohe people corroborate the ‘steppe hypothesis’.”
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 literature (Figure 1 from Li et al. BMC Genetics 2015 16:78. Used with a CC-BY 4.0 license.)
Reconstructing a possible route by which the Tarim Basin was populated, Li and colleagues write that “people bearing the south/west Asian components could have first married into pastoralist populations and reached North Xinjiang through the Kazakh steppe following the movement of pastoralist populations, then spread from North Xinjiang southward into the Tarim Basin across the Tianshan Mountains, and intermarried with the earlier inhabitants of the region, giving rise to the later, admixed Xiaohe community.”
The populations from the Russian steppe seem to have contributed more genetically to this population than did the populations from the oases of Bactria. “The groups reaching the Tarim Basin through the oasis route,” the researchers note, “may have interacted culturally with earlier populations from the steppe, with limited gene flow, resulting in a small genetic signal of the oasis agriculturalists in the Xiaohe community.”
The story of the Tarim mummies is compelling because of their incredible preservation and their striking array of diverse biological features. In unfurling their DNA, researchers like Li and colleagues may finally have solved the mystery of their origins.
For more information on DNA analysis from the Xiaohe cemetery, please see: Li et al. 2015. Analysis of ancient human mitochondrial DNA from the Xiaohe cemetery: insights into prehistoric population movements in the Tarim Basin, China. BMC Genetics 16:78. DOI 10.1186/s12863-015-0237-5. Li et al. 2010. Evidence that a West-East admixed population lived in the Tarim Basin as early as the early Bronze Age. BMC Biology 8:15. DOI 10.1186/1741-7007-8-15.
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Kristina Killgrove is a bioarchaeologist and university professor. For more osteology news, follow her on Twitter (@DrKillgrove) or like her Facebook page Powered by Osteons.
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