Here is another mtDNA map. Haplogroup U4 in particularly common in Balto-Slavic and to a lower extent also in other Y-DNA R1a populations (including Turkic and Altaic). The highest percentages of U4 are observed among the Chuvash (16.4%) and Bashkirs (14.8%) of the Volga-Ural region, who have respectively 31% and 26% of R1a. In the Caucasus U4 peaks among the Georgians (8.4%), Dargins (6.4%) and Kumyks (6.3%), the three ethnic groups who also happen to have the highest levels of R1a in the region (9%, 13% and 8.5% respectively). U4 is only completely absent from the North Ossetians, who also lack R1a.
Likewise the Cantabrians in northern Spain mark a hotspot of U4 in southwest Europe and also have unusually high levels of R1a. It's harder to explain the higher frequencies in Catalonia, Béarn and in western Loire Valley, except that all of them have exceptionally high levels of R1b-DF27 (especially SRY2627). There may have been a founder effect on the maternal side in the Indo-Europeans who brought this subclade of R1b. The Basques did not inherit it because they preserved most of their pre-Indo-European mtDNA.
U4 is expectedly absent from Sardinia and Corsica, who both lack any R1a on the paternal side.
Oddly enough the Kurds, who have the highest levels of R1a in the Middle East, don't seem to have any U4 lineages.
It's hard to explain how the Benelux got such high percentages of U4, but that may be due to the small sample size (although the results are comparable for Belgium and the Netherlands, both around 6 to 6.5%).
UPDATE 1:
Another explanation for the elevated U4 frequency in the Benelux, western France and Catalonia is that these lineages have survived in those regions since the Mesolithic. U4 has been found in Mesolithic Portugal, Germany and Sweden. No Mesolithic DNA from the Benelux or France has been tested to date, and there is only one sample from Mesolithic Spain (U5b1 from Navarre). It has been proven that Mesolithic hunter-gatherers from Iberia and Scandinavia were very close autosomally and that their mtDNA both included U4 and U5. Their Y-DNA is unknown yet, but besides the obvious haplogroup I it is not impossible that they possessed R1a lineages as well. Those would be the oldest branches of R1a like M420, SRY1532 and perhaps M17. The Mesolithic hunter-gatherers might have eventually been surrounded by Neolithic farmers and let their women marry into rich farmers' families. The paternal R1a lineages of hunter-gatherers would have remained low while the maternal U4 lineages who had integrated the Neolithic population would have grown much faster. Sampietro et al. provides the first evidence that U4 lineages did integrate farming communities in Catalonia during the Neolithic.
The lower incidence of U4 in countries with relatively high percentages of R1a like Hungary, Romania and Bulgaria can be explained by the fact that a lot of this R1a is of non-Slavic origin, and was brought mostly by Central Asian invaders like the Huns, the Alans, the Eurasian Avars, the Magyars, the Khazars and the Bulgars. In the Balkans and Carpathians, only the Serbs have an elevated frequency of U4, which is concordant with the fact that they are more Slavic than all of their neighbours (the Croats, Bosnians and Bulgarians being Slavicised people rather than true ethnic Slavs).
UPDATE 2: a detailed page about the origins, history, distribution and subclades of haplogroup U4 is now available here.
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Originally Posted by Maciamo
