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Ancient Genome Analyses Retrace Eurasian Steppe Population Dynamics

NEW YORK (GenomeWeb) – In a pair of papers published today, researchers from Denmark, the UK, and elsewhere have looked at population migrations and interactions in the Eurasian steppe before and during the Early Bronze Age, the Iron Age, and the Medieval period.

From the population dynamics identified in each study, it appears that the "vast majority of the genetic makeup of contemporary people in this 8,000-kilometer-long stretch across Europe and Asia has really mainly been formed within the last thousand years," Eske Willerslev, a geogenetics researcher affiliated with the University of Copenhagen, the University of Cambridge, and the Wellcome Trust Sanger Institute, said during a telephone press briefing this week. "It tells us that part of our world was created very, very recently."

For one of the studies, appearing online in Nature, the researchers performed low-coverage whole-genome sequencing on DNA from more than 100 ancient human tooth or bone samples, retracing the migrations of the Scythian, Hun, and East Asian populations into the steppe over the last 4,500 years or so. They also conducted array-based genotyping on more than 500 individuals from 16 present-day populations in Central Asia, Altai, Siberia, and the Caucasus to further contextualize the ancient findings.

"We have already, in previous work, established that there was a migration of Europeans into the Asian steppe — the so-called Yamnaya people — around 5,000 years ago," Willerslev said. "But obviously, today, the people living in Central Asia and Western Asia are mainly of Asian descent. We wanted to understand what happened from the Bronze Age and further up in time."

With that in mind, collaborators at the Danish National High-Throughput Sequencing Centre used Illumina HiSeq 25000 instruments to sequence 137 samples — ranging in age from 1,500 to 4,500 years old — to an average depth of one-fold, and subsequently considered genetic clustering patterns and ancestry tracts for these samples in their analysis.

When the researchers focused on the Scythian population, a group of horse-riding, Iranian language-speaking warriors that resided across the steppe region roughly 2,200 to 2,800 years ago, for example, they found diverse genetic ancestry patterns despite their cultural uniformity.

The team also detected ancestry from Late Bronze Age herder populations, European farmers, and hunter-gatherers from southern Siberia. But in the western steppe, there appeared to be enhanced ancestry from Bronze Age European groups, while Asian ancestry ticked up in the Eastern steppe.

Co-author Peter de Barros Damgaard, a geogenetics researcher at the University of Copenhagen, noted that there have been conflicting hypotheses for Scythian population origins proposed in the past. One theory asserts that the population descended from a western region near the Black Sea, while another one puts the Scythians' origins in present-day Siberia.

"Our research kind of reconciles this whole picture and shows that … they kind of originated in both areas, and that they are transitioning cultures to the Scythian organization, which is a stateless confederation of warrior tribes," de Barros Damgaard said during the call.

Moving forward in time, the researchers retraced Scythian population replacement by a westward-migrating Hun population with ancestry from Xiongnu nomads. They noted that the East Asian populations with Turkic culture and language, in turn, supplanted the Hunnic Empire, ultimately leading to the populations that are currently found in the region.

"This process was changing the steppe … from being of mainly Western Eurasian genetic ancestry to becoming of East Asian genetic ancestry," Willerslev said. "It was also changing the steppe in terms of being Indo-European [language family] speaking into becoming Turkic-speaking people."

For a related study published online today in Science, Willerslev, de Barros Damgaard, and other investigators from centers around the world used whole-genome sequencing for dozens of ancient individuals stretching back 11,000 years to explore even earlier population patterns in the steppe region and beyond, providing a population dynamics perspective informed by insights into horse domestication in the Botai region during the Copper to Bronze Age transition around 5,000 years ago.

"[Horse domestication] is really what creates the extreme [population] dynamics off the Eurasian steppe," Willerslev said, noting that the population that domesticated the horse was eventually replaced by the European Bronze Age expansions into the steppe that were documented in the first paper.

The work published in Science focused on Yamnayna population expansions into Europe and Asia from the western steppe. Along with high-coverage genome sequencing on dozens of modern-day individuals from Central Asia and elsewhere, the team conducted array-based genotyping on 140 individuals from populations in northern Pakistan, and genome sequencing on 74 Mesolithic, Neolithic, Copper Age, Bronze Age, and Iron Age samples from Eastern Europe, Central Asia, and Western Asia.

Among other population dynamics, the researchers saw relatively modest genetic impacts from Yamnaya and Afanasievo population expansions into Inner Asia, contrasting with the genetic contributions such expansions made in Europe — a difference suspected of stemming from horse husbandry by hunter-gatherer groups in the Botai region. 

Indeed, de Barros Damgaard said, the two population papers highlight "how intimately tied the human trajectory is to the use of [the] horse, starting with domestication, which allowed these groups to grow in numbers."