I2-M438 and Scandinavian Hunter-Gatherers
Haplogroup I2-M438 was formed ca. 25500 BC, and the modern European population has a TMRCA ca. 19900 BC. Individuals from Mesolithic Scandinavia, which define the so-called Scandinavian Hunter-Gatherer (SHG) ancestry, show exclusively I2-M438 lineages. In samples of Mesolithic SHG, together with those of WHG and EHG ancestry (but for those of the Balkans), mtDNA haplogroups U5 and U2 are prevalent [Mathieson et al. 2017].
The ice sheet retracted from northern Europe allowed for the colonisation of the Scandinavian Peninsula from about 9700 BC, according to the archaeological record, both in southern and northern Scandinavia, while ice still dominated the interior.
The peninsula seems to have been colonized first from the south by peoples from central Europe, related to late-glacial lithic technology (direct blade percussion technique), which brought West Hunter-Gatherer ancestry with them.
Genomic studies show an invasion from the north-east by peoples from east Europe, who brought East Hunter-Gatherer ancestry. Their technology is probably represented in the ‘pressure blade’ technique found in northern parts of Scandinavia.
This left a paradoxical pattern of increased EHG ancestry in north and west Scandinavia, and WHG ancestry in east and central Scandinavia, which correlates with Baltic samples. It has also been shown that selection drove the unique combination of light skin and hair and varied blue to light-brown eye colour, as part of the adaptation to different environment, in contrast with WHG who had the specific combination of blue eyes and dark skin[Günther et al. 2017].
Scandinavian Hunter-Gatherer (SHG) ancestry, therefore, represents a mixed group tracing parts of their ancestry to both WHG and EHG.
Many ancient DNA samples of I2-M438 haplogroups are found since the Palaeolithic, and two main branches seem to have divided early: I2a1b2-L621 lineages are found mainly in the Balkans, and I2a2a-M223 – distributed through central Europe – seems to have followed the expansion of Italo-Celtic and Germanic, and were therefore possibly integrated with R1b1a1a2a1a-L151 lineages since the Bell Beaker complex.
Image modified from Günther et al. (2017). «Mesolithic samples and their genetic affinities – (A) Map of the Mesolithic European samples used in Günther et al. (2017). The pie charts show the model-based estimates of genetic ancestry for each SHG individual. The map also displays the ice sheet covering Scandinavia 10,000 BP (most credible (solid line) and maximum extend (dashed line) ). Newly sequenced sites are shown in bold and italics. Additional European EHG and WHG individuals used in this study derive from sites outside this map. (B) Magnified section of genetic similarity among ancient and modern-day individuals using PCA featuring only the Mesolithic European samples». Original under a CC-BY-NC 4.0 International license.
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