Craniofacial variation of the Xiongnu Iron Age nomads of Mongolia reveals their possible origins and population history

This paper examines Iron Age Mongolia during a time when nomadic tribes created the world's first steppe empire in Inner Asia. These aggregated tribes, known as Xiongnu (3rd century BC to the 2nd century AD), came to define steppe polity construction, later used by the Mongol Empire under the reign of Genghis Khan. They moved extensively over the eastern steppe and interacted, both in trade and intermarriage, with peoples from southern Siberia to Xinjiang. However, the Xiongnu as a people are relatively unknown to scholars, as they did not possess a written language. This study assesses Xiongnu population history and biological structure by analyzing craniofacial diversity via geometric morphometrics. Twenty-four coordinate cranial landmarks were used to test relationships among groups in the region and infer potential biological origins. The Relethford-Blangero R-matrix method was used to test hypotheses of phenotypic variation resulting from microevolutionary processes. This study hypothesizes biological continuity among Xiongnu individuals extending into modern Mongolian populations. Alternatively, long-range gene flow from adjacent geographic regions might suggest a complex population structure among the Xiongnu indicative of multiple populations controlling administrative functions. Results indicate the Xiongnu were potentially composed of at least two biologically distinct groups. Individuals from the elite cemetery of Borkhan Tolgoi (Egiin Gol) share their ancestry with a Bronze Age population from western Mongolia, and possibly, to a later migration of Turks, who came to rule the eastern steppe from the 6th to 8th centuries AD. The Xiongnu also evidence biological similarity with nomads from the Mongol Empire during the medieval period and modern Mongolians, as well as modern and ancient Central Asian, Chinese, and Siberian groups. These results are similar to ancient DNA studies that suggest a mix of Eastern and Western Eurasian haplogroups in the Xiongnu while also achieving consensus with models of steppe polity formation proposed by archaeologists who suggest local ties to extra-local groups through interactive exchange networks.