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Ancient Genome Sequences Spell Out Early Scandinavian Migrations

NEW YORK (GenomeWeb) – By sequencing and analyzing several Mesothilic individuals from Scandinavia, an Uppsala University-led team has uncovered evidence of two early migrations to the region by hunter-gatherer groups following distinct southern and northeastern routes.

The researchers sequenced samples from seven ancient Scandinavians — dated at between about 6,000- and 9,500-years-old — and compared the results to existing sequence data for Mesolithic Scandinavians and other ancient and modern individuals, in an effort to untangle Scandinavian colonization. The findings, appearing online today in PLOS Biology, point to at least two distinct migrations into the Scandinavian peninsula after the ice sheets of the Last Glacial Maximum began slipping away more than 20,000 years ago.

In particular, the team saw signs of a post-glacial migration from the south, representing western hunter-gatherers from an area that roughly coincides with present-day Denmark and Germany, along with a subsequent migration from the northeast that involved eastern hunter-gatherer populations with pressure blade stone tools that appear to have picked their way along northern Norway's Atlantic coast.

"These two groups met and mixed in Scandinavia, creating a genetically diverse population, which shows patterns of genetic adaptation to high latitude environments," co-corresponding authors Mattias Jakobsson, an organismal biology researcher at Uppsala University, and Stockholm University archeology and classical studies researchers Anders Götherström and Jan Storå, and their co-authors wrote.

The team noted that prior archeological clues put humans in Scandinavia's southern and northern peripheries by around 11,700 years ago, with a distinct type of pressure blade technology beginning to appear in northern Scandinavia roughly 10,200 years ago. Even so, the authors explained, "migration routes, cultural networks, and the genetic makeup of the first Scandinavians remain elusive, and several hypotheses exist based on archaeology, climate modeling, and genetics."

Adding to prior genetic studies uncovering signs of western- and eastern hunter-gatherer-related ancestry in hunter-gatherers from central and eastern Scandinavia, the investigators set out to sequence additional Mesolithic individuals from sites in western, northern, and eastern parts of the peninsula. They used Illumina HiSeq 2500 or HiSeqX instruments to sequence ancient DNA libraries established with DNA from bone or tooth samples from Mesolithic hunter-gatherer remains in southwestern Norway, northern Norway, or Sweden's Stora Karlsö or Gotland Baltic islands.

After sequencing pooled libraries, including a handful of libraries prepared using an Arbor Biosciences MYcroarray whole-genome capture method, they were left with ancient Scandinavian genomes covered to between 0.1-fold and almost 58-fold average coverage apiece.

Based on their analyses of the most complete, high-coverage genome, generated for one of the Baltic island individuals, the researchers uncovered roughly 10,600 new SNPs, including a subset shared across multiple Mesolithic Scandinavians.

When they compared the new genomes with sequences from half-a-dozen previously sequenced Mesolithic-aged Scandinavian hunter gatherers — along with dozens more Upper Paleolithic, Mesolithic, or Early Neolithic Eurasian individuals and hundreds of modern-day individuals — the investigators again saw western- and eastern hunter-gatherer similarities in all of the Scandinavian hunter-gatherers considered.

By digging further into the genetic, archaeological, and geographic information at hand, they teased out two distinct hunter-gatherer migrations into Scandinavia: a southerly western hunter-gatherer migration, followed by an eastern hunter-gatherer migration bringing pressure blade tools along the northeastern coast of Scandinavia.

The team's evidence suggests the migratory populations went on to mix, leading to high levels of genetic diversity relative to hunter-gatherer groups in other parts of Europe at the time, fueling adaptations that were beneficial in the far northern locales, such as reduced skin pigmentation.

"These groups were genetically more diverse than the groups that lived in central, western, and southern Europe at the same time," Uppsala's Jakobsson said in a statement. "That is in stark contrast to the pattern seen today where more genetic variation is found in southern Europe and less in the north."