Genetic evidence for multiple episodes of population mixture in southern and eastern African history. J. Pickrell1, N. Patterson2, P. Loh3, M. Lipson3, B. Berger3, M. Stoneking4, B. Pakendorf5, D. Reich1,2 1) Department of Genetics, Harvard Medical School, Boston, MA; 2) Broad Institute, Cambridge, MA; 3) Department of Mathematics and Computer Science, MIT, Cambridge, MA; 4) Department of Evolutionary Genetics, Max Plank Institute, Leipzig, Germany; 5) Laboratoire Dynamique du Langage, CNRS and Université Lyon Lumière.

   The history of southern Africa involved interactions between indigenous hunter-gatherers and a range of populations that moved temporarily or permanently into the region. The influence of these interactions on the genetic structure of current populations remains unclear. Here, using patterns of linkage disequilibrium inferred from genome-wide genetic data, we show that there are at least two admixture events in the history of Khoisan populations (southern African hunter-gatherers and pastoralists who speak non-Bantu languages with click consonants): one involving populations related to Niger-Congo-speaking African populations, and one that introduced ancestry most closely related to west Eurasian (European or Middle Eastern) populations. We date this latter admixture event to approximately 900-1,800 years ago, and show that it had the largest demographic impact in the subset of Khoisan populations who speak Khoe-Kwadi languages. A similar signal of west Eurasian ancestry is present throughout eastern Africa; in particular, we also find evidence for two admixture events in the history of several Kenyan, Tanzanian, and Ethiopian populations, the earlier of which involved populations related to west Eurasians and which we date to approximately 2,700 - 3,300 years ago. We thus suggest that west Eurasian ancestry entered southern Africa indirectly through eastern Africa. These results demonstrate how large-scale genomic datasets can inform complex models of population movements, and highlight the genomic impact of largely uncharacterized back-to-Africa migrations in human history.

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