sistent with the spread of pastoralism into sub-Saharan Africa ~4.5 kya (Tishkoff et al., 2007b). The estimates of the selection coefficients of the African mutations (0.035–0.097) are among the highest reported for modern humans, and intuitively this makes sense given not only the increased nutritional value of drinking milk as an adult but also the increased source of water in regions such as the Sahara where dehydration and diarrhea can cause death.
In sub-Saharan Africa, the long-range exchange networks of Neolithic technology and associated spread of Bantu languages (which we refer to here as the “Bantu expansion” for the sake of simplicity) have had a major influence on biological and cultural diversity in sub-Saharan Africa. On the basis of archaeological and linguistic data, the Bantu languages and associated agricultural and iron age technologies are thought to have originated in Nigeria or Cameroon (Greenberg, 1972) ~5,000 years ago (Phillipson, 1975; Berniell-Lee et al., 2009) and spread relatively rapidly across sub-Saharan Africa. The extent to which this was associated with the migration of populations vs. a diffusion of language and technology among populations has been debated.
The linguistic classification of the ~600 Bantu languages is interpreted to represent several dispersals throughout sub-Saharan Africa [e.g., Vansina (1995)]. Ehret (2001) argues that proto-Bantu diverged into several daughter clades, all but one of which are spoken only in the northwestern region of the Bantu-speaking areas (i.e., western central Africa), and the other of which was a forest Savanna Bantu clade. Ehret (2001) goes on to argue that the forest Savanna Bantu clade diverged into several daughter clades, including the Savanna Bantu clade, and this diversification is linked to the spread of Bantu languages into central and southern Africa. The Savanna Bantu clade includes most of the contemporary languages spoken in eastern Africa, southeastern Africa, southwestern Africa, and the southern Savanna belt. This reconstruction supports a model in which proto-Bantu emerged in western central Africa ~5,000 years ago and diversified and spread across the rainforest for ~2,000 years before the first archaeological evidence of eastern Bantu speakers in the Great Lakes region (Ehret, 2001).
Archaeological evidence related to the Bantu expansion largely focuses on the distribution of Early and Late Iron Age sites in Africa. Phillipson argues that the Eastern Bantu languages likely arose in western central Africa around the time of the emergence of Early Iron Age artifacts consistent with cattle keeping, but that the spread of Eastern Bantu languages