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Ancient Genomes Reveal Scandinavian Population History

NEW YORK – A team of researchers from Sweden, Iceland, and elsewhere has used ancient DNA to look at Scandinavian population history and migrations into the region from the eastern Baltic region, British-Irish Isles, and southern Europe over some 2,000 years, from the 1st century Roman Iron Age to the 19th century.

Across the time points included in the study, "[d]ifferent processes brought people from different areas to Scandinavia," co-senior and co-corresponding author Anders Götherström, a researcher affiliated with Stockholm University and the Center for Paleogenetics in Stockholm, said in a statement.

As they reported in Cell on Thursday, the researchers performed whole-genome sequencing on samples from 77 ancient individuals, including 13 that had been profiled in the past, spanning pre-Viking, Viking, Late Viking, Medieval, and post-Medieval periods.

After weeding out contaminated or low-quality sequences, they were left with new sequences for four dozen ancient Scandinavians found at boat burial, chamber burial, and archaeological sites, such as a ringfort linked to the Sandby borg migration period from the years 400 to 550 and a Swedish warship known as Kronan that sank off the country's southeastern coast in the late 1600s.

The team analyzed the new sequence data alongside genome sequences from 249 ancient samples profiled previously and genotyping profiles for 16,638 individuals in present-day populations, characterizing migrations and gene flow in the region that is now Norway, Sweden, Denmark, and Gotland.

"We find regional variation in the timing and magnitude of gene flow from three sources: the eastern Baltic, the British-Irish Isles, and southern Europe," the authors reported.

In particular, the investigators saw non-local ancestry in ancient individuals that waned over time, leaving modern populations with more modest eastern Baltic and British-Irish ancestry levels than those documented in the Viking period.

"[W]e start to see differences in the levels and origin of non-local ancestry across the different regions and periods of Scandinavia," Stockholm University's Ricardo Rodríguez-Varela, the study's first and co-corresponding author, said in a statement.

Prior to the Viking period, for example, the researchers saw Neolithic and Mesolithic Scandinavian hunter-gatherer ancestry, along with relatively low levels of non-local ancestry. During the Viking period, on the other hand, their results revealed extensive gene flow from individuals to the west with ancestry from the British-Irish Isles across Scandinavia. In contrast, Late Viking sites in central Sweden and Gotland appeared to have higher-than-usual ancestry from mainly female migrants from the eastern Baltic region.

"Although still evident in modern Scandinavians, levels of non-local ancestry in some regions are lower than those observed in ancient individuals from the Viking to Medieval periods," Rodríguez-Varela explained. "This suggests that ancient individuals with non-Scandinavian ancestry contributed proportionately less to the current gene pool in Scandinavia than expected based on the patterns observed in the archaeological record."

When they focused on post-Medieval samples, meanwhile, the investigators saw diminishing gene flow from non-local populations, which seemed to contribute less to the ancestry of more contemporary individuals.

Even so, their results pointed to Uralic language-related ancestry proportions that varied across populations falling on a north-to-south gradient in present-day populations in Norway and Sweden, consistent with past studies pointing to enhanced Uralic ancestry in northern Scandinavia.

"It is not possible to tell from our results whether the north-south [genetic] cline existed in some form before the Viking period, as none of the 25 pre-Viking period individuals have substantial levels of Uralic ancestry," the authors wrote, noting that "additional individuals from the pre-Viking period are needed to provide more definitive evidence."