Monday, May 14, 2012

Origins of Nilo-Saharan and Cushitic Speakers

"Figure S15 [Tishkoff et al]:  STRUCTURE analysis of the African dataset only (121 populations) with all genetic data (848 microsatellites, 476 indels and 3 SNPs) from K = 2 - 14. Each vertical line represents an individual. Individuals are clustered by self-identified ethnic group (shown at bottom) and ethnic groups are clustered by major geographic region. The colors represent the proportion of inferred ancestry from K ancestral populations. Values for K are shown on the left and the number of similar runs (F) for the primary mode for each set of 25 STRUCTURE runs at each K value is shown on the right."

The genetic structure and history of Africans and African Americans

Tishkoff et al,  Science. 2009 May 22;324(5930):1035-44. Epub 2009 Apr 30. (Link)

From the Supplement:

Origins of Nilo-Saharan and Afroasiatic Cushitic speaking populations

"The southern/central Sudanese show high levels of both the Nilo-Saharan(red) and Afroasiatic Cushitic purple AACs from K=8-13 (Fig. S15), consistent with linguistic arguments suggesting a long history of extensive contact and gene flow ~20,000-10,500 ya, along the western edges of the Ethiopian highlands.  The history of regional interactions between Nilo-Saharans and Cushites 5,000-1,000 ya in southwestern Sudan and adjacent parts of Uganda and Kenya were likely to have reinforced the genetic patterns observed in the STRUCTURE analyses.

"Our data support the hypothesis based on linguistic, archeological, mtDNA, and Y chromosome data, that the Sahel has been a corridor for bi-directional migration between eastern and western Africa.  We observe the highest proportion of the "Nilo-Saharan AAC” in the southern/central Sudanese populations (Nuer, Dinka, Shilluk, Nyimang), with decreasing frequency from northern Kenya (e.g. Pokot) to northern Tanzania (Datog, Maasai). From K = 5-13, all Nilo-Saharan speaking populations from Kenya, Tanzania, southern Sudan, and Chad cluster with west-central Afroasiatic Chadic speaking populations (Fig. S15).  These results are consistent with linguistic and archeological data, suggesting a possible common ancestry of Nilo-Saharan speaking populations from an eastern Sudanese homeland within the past ~10,500 years, with subsequent bi-directional migration westward to Lake Chad and southward into modern day southern Sudan, and more recent migration eastward into Kenya and Tanzania ~3,000 ya (giving rise to Southern Nilotic speakers) and westward into Chad ~2,500 ya (giving rise to Central Sudanic speakers).  A proposed migration of proto-Chadic Afroasiatic speakers ~7,000 ya from the central Sahara into the Lake Chad Basin may have caused many western Nilo-Saharans to shift to Chadic languages.  Our data suggest that this shift was not accompanied by large amounts of Afroasiatic gene flow. Analyses of mtDNA provide evidence for divergence ~8,000 ya of a distinct mtDNA lineage present at high frequency in the Chadic populations and suggest an East African origin for most mtDNA lineages in these populations."

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