We next analyzed UK Biobank population structure in conjunction with ancient DNA samples. Modern European populations are known to have descended from three ancestral populations: Steppe, Mesolithic Europeans and Neolithic farmers 21,22 . We projected ancient samples from these three populations as well as ancient Saxon samples 24 onto the UK Biobank PCs (Figure 3, Supplementary Figure 4, see Online Methods). These populations were primarily differentiated along PC1 and PC3, indicating higher levels of Steppe ancestry in northern UK populations.
Additionally, the lack of any ancient sample correlation with PC2 suggests that Welsh populations are not differentially admixed with any ancient population in our data set, and likely underwent Welsh-specific genetic drift. We confirmed these findings by projecting pan- European POPRES 26 samples onto the UK Biobank PCs (see Online Methods, Supplementary Figure 5) noting that of the continental European populations, Russians (who have the most Steppe ancestry) lie on one side and Spanish and Italians (who have least) 22 lie on the other side along PC1 and PC3, and that none of the continental European populations projected onto the same regions as the Welsh on PC2 and PC5.
In addition to the impact of ancient Eurasian populations, we know that the genetics of the UK has been strongly impacted by Anglo-Saxon migrations since the Iron Age 24 , with the Angles arriving in eastern England and the Saxons in southern England. The Anglo-Saxons interbred with the native Celts, which explains much of the genetic landscape in the UK. We analyzed a variety of samples from Celtic (Scotland and Wales) and Anglo-Saxon (southern and eastern England) populations from modern Britain in conjunction with the PoBI samples 20 and 10 ancient Saxon samples from eastern England 24 in order to assess the relative amounts of Steppe ancestry. [. . .] We consistently obtained significantly positive f4 statistics, implying that both the modern Celtic samples and the ancient Saxon samples have more Steppe ancestry than the modern Anglo-Saxon samples from southern and eastern England. This indicates that southern and eastern England is not exclusively a genetic mix of Celts and Saxons. There are a variety of possible explanations, but one is that the present genetic structure of Britain, while subtle, is quite old, and that southern England in Roman times already had less Steppe ancestry than Wales and Scotland.
Population structure of UK Biobank and ancient Eurasians reveals adaptation at genes influencing blood pressure. Kevin Galinsky, Po-Ru Loh, Swapan Mallick, Nick J Patterson, Alkes L Price doi: http://dx.doi.org/10.1101/055855
Note: I find it unlikely the pattern they observe is a holdover from Roman times. I suspect it will turn out the decrement of Steppe ancestry in England stems from a continual trickle of continental genes into England over the past 1000 years (from which the fringes of the British Isles were comparatively isolated).
Also supports my impression that the Wellcome Trust paper still overestimated the degree of Iron Age British admixture in modern England (given that the authors had assumed a simple two-way admixture, while the authors of the above preprint provide evidence "southern and eastern England is not exclusively a genetic mix of Celts and Saxons").