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Ancient Genomes Unravel Events in Human Settlement of Baltic Sea Region

NEW YORK (GenomeWeb) – By analyzing more than three dozen ancient genomes from the Baltic Sea region, researchers have been trying to put together a picture of the prehistoric population history of the area. Sweden, they found, was likely settled in two waves, one from the north and one from the south.

The eastern Baltic and Scandinavia were first settled by foragers about 11,000 years ago as glacial ice sheets there retreated, and farmers arrived in the region about 6,000 years ago during the Early Neolithic Period, 1,000 years later than in Central Europe.

To tease out further ancient population movements in the region, a Max Planck Institute for the Science of Human History-led team of researchers sequenced and analyzed the genomes of 38 individuals that lived between 7,500 BCE and 200 BCE in what's now Lithuania, Latvia, Estonia, northwest Russia, and Sweden, as they reported in Nature Communications.

"Our analyses support a dynamic population history of the Baltic Sea region, where populations did not remain in 'genetic stasis' despite the late adoption of agrarian subsistence strategies when compared to the rest of Europe," Max Planck's Johannes Krause and his colleagues wrote in their paper.

The researchers examined skeletal remains found at 25 different archaeological sites in the Baltics. While they analyzed the DNA of 106 sets of human remains, they focused on 41 samples with good DNA preservation for shotgun sequencing or SNP capture. In the end, they generated genome-wide data for 38 individuals with an average coverage of between 0.02-fold and 8.8-fold on targeted SNPs.

These 38 samples included Mesolithic hunter-gatherers from northwestern Russia, a farmer from southern Sweden from 4,000 BCE, and Baltic Bronze Age individuals from Latvia and Lithuania, among others.

A principal components and ADMIXTURE analysis of these samples, in combination with previously published ancient samples from the Eastern Baltic and about 1,000 modern samples, found the Mesolithic foragers of Northern Europe fell into three clusters: eastern hunter-gatherers, Scandinavian hunter-gatherers, and western hunter-gatherers.

The Russian foragers from the researchers' samples fell within the eastern hunter-gatherer cluster, they noted. The ADMIXTURE analysis indicated that the eastern hunter-gatherers harbored a genetic component that's been found in Neolithic farmers from Iran and Steppe populations from the Bronze Age, indicating shared ancestry among these groups.

The Scandinavian hunter-gatherers, meanwhile, appeared to be intermediate between the eastern and western hunter-gatherers in the PCA analysis.

"Eastern hunter-gatherers were not present on the eastern Baltic coast, but a genetic component from them is present in Scandinavia. This suggests that the people carrying this genetic component took a northern route through Fennoscandia into the southern part of the Scandinavian peninsula," Krause added in a statement. "There they genetically mixed with western hunter-gatherers who came from the south, and together they formed the Scandinavian hunter-gatherers."

This is in line with what was reported earlier this month in PLOS Biology. An Uppsala University-led team found that two hunter-gather groups likely moved to Scandinavia by following distinct southern and northeastern routes.

In this new study, Krause and his colleagues also reported that the introduction of farming to Scandinavia coincided with a shift in ancestry and occurred about 4,000 BCE, earlier than previously thought. These early Scandinavia farmers had increased levels of ancestry in common with Levantine and West Anatolian farmers. This suggested to the researchers that the first farmers in Scandinavia migrated there from Central Europe, bringing their farming technology and Anatolian ancestry with them.

They also noted that the Siberian and East Asian-related ancestry present today in northeastern Europe appears to have arrived after the end of the Bronze Age. Identifying the population that introduced that ancestry could be a next step, the researchers said.