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Ancient Caucasus Populations Profiled

NEW YORK (GenomeWeb) – An international team led by investigators in Germany has characterized ancient population patterns in the Caucasus and steppe ancient DNA across some 3,000 years, providing a look at the advent of groups that subsequently migrated to Eurasia.

As they reported online today in Nature Communications, researchers from the Max Planck Institute for the Science of Human History and elsewhere sequenced 45 individuals from the Caucasus who died between around 3,500 and 6,500 years ago. Their results pointed to a genetic split between populations in the Caucasus and those in the nearby steppe region, although northern Caucasus individuals were genetically similar to those in the south.

"We assume that in the wake of the Neolithic period … when a more sedentary lifestyle with domesticated animals and plants was established, populations from the southern Caucasus spread over the mountains to the north and there met with nomadic populations from the Eurasian steppe," co-senior and co-corresponding author Wolfgang Haak, an archeogenetics and biological science researcher affiliated with the MPI-SHH and the University of Adelaide, said in a statement.

"The genetic boundary corresponds in principle to the ecological and geographical regions: the mountains and the steppe," Haak added. "Today, on the other hand, the Caucasus mountains themselves are more of a barrier to gene flow."

In contrast, the team noted that populations from the Eurasian steppe area to the northeast and northwest had additional sources of ancestry: representatives from a Yamnaya-related steppe groups appeared to carry farming ancestry from the west, for example, while an earlier steppe group with elaborate burial mounds, called the Steppe Maykop, lacked genetics from the farming group, and instead had some ancestry resembling Paleolithic Siberians, ancient Native Americans, and modern-day North Asians.

"These are exciting and surprising findings, which highlight the complexity of the processes that lead to the formation of Bronze Age steppe pastoralist," first author Chuan-Chao Wang said in a statement. Wang was a postdoctoral researcher at the Max Planck Institute for the Science of Human History when the study was done. He is currently an anthropology and ethnology researcher at Xiamen University.

Such results suggest Eurasia "was the site of many exciting chapters in human prehistory that are still shrouded in mystery," co-senior and co-corresponding author Johannes Krause, director of the Max Planck Institute's archaeogenetics department, said in a statement, adding that the team aims to explore these issues "in close collaboration with archaeologists and anthropologists."

Prior studies have demonstrated that the Caucasus region, which stretches from the Black Sea to the Caspian Sea and encompasses present day Russia, Azerbaijan, Armenia, Georgia, Iran, and Turkey, was a source of Eastern and Caucasus hunter-gatherer ancestry for groups migrating to Europe and parts of Asia. But the history of the populations in the Caucasus itself has been less well studied.

In an effort to clarify past population dynamics in the area, the researchers did shotgun sequence-based screening — followed by in-solution enrichment-based sequencing that targeted more than 1.2 million nuclear DNA SNPs and the mitochondrial genome — on 59 ancient samples the Caucasus region. After tossing out contaminated samples or samples from first-degree relatives, they were left with genetic data at 30,000 or more SNPs for 45 individuals.

When the team analyzed the data alongside other ancient and modern samples sequenced previously, it found that the newly sequenced individuals fell into two main genetic clusters in the region: a group of western Eurasian steppe individuals and a cluster of ancient individuals in the southern Caucasus that resembled Bronze Age Armenian and current Caucasian populations.

Along with archeological clues from the region, the genetic profiles made it possible for the researchers to tease apart genetic relationships over time in cultural groups from across the Caucasus and steppe, including the advent of a Yamnaya culture on the steppe some 4,400 to 5,300 years ago that migrated to Eurasia.

"Irrespective of the early branching pattern, the spread of some or all of the [Proto-Indo-European] branches would have been possible via the North Pontic/Caucasus regions and from there, along with pastoralist expansions, to the heart of Europe," the authors concluded. "This scenario finds support from the well attested and widely documented 'steppe ancestry' in European populations and the postulate of increasingly patrilinear societies in the wake of these expansions."

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