A new paper on the arXiv quantifies and dates the West Eurasian admixture in east Africa, and uncovers the presence of such admixture even in the Khoe-San of southern Africa. It appears that the admixture first occurred in East Africa about ~3ky ago, and reached southern Africa about ~1.5ky ago.
It is quite remarkable that different waves of migration converged into southern Africa from different directions: west African farmers and west Eurasian-admixed east African pastoralists. We should count ourselves lucky that the Khoe-San were discovered when they did: a few centuries more, and they too might have followed the fate of other populations finding themselves at the losing side of a technology differential, their culture lost, and their DNA preserved only as fragments in the gene pools of the more successful groups.
Ancient west Eurasian ancestry in southern and eastern Africa
Joseph K. Pickrell et al.
The history of southern Africa involved interactions between indigenous hunter-gatherers and a range of populations that moved into the region. Here we use genome-wide genetic data to 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 involved populations related to Niger-Congo-speaking African populations, and the other 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 Khoisan populations that 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 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 reconstruct the allele frequencies of the putative west Eurasian population in eastern Africa, and show that this population is a good proxy for the west Eurasian ancestry in southern Africa. The most parsimonious explanation for these findings is that west Eurasian ancestry entered southern Africa indirectly through eastern Africa.