“Neanderthal” admixture seems to be higher in West Africans than in East Africans. How come? (Source)
When modern humans began their expansion from a small core somewhere in East Africa, the continent probably had several different archaic populations.
It now seems that one of them was related to the Neanderthals in Europe. In an ongoing study of Neanderthal admixture in present-day humans, John Hawks has found an apparently higher level of admixture in the Yoruba of Nigeria than in the Luhya of Kenya (see chart above).
This is counter-intuitive, as Hawks himself notes. The closest source of admixture would have been Neanderthals in what is now Israel (and probably elsewhere in the Middle East). Genes from that source should have first introgressed into northeastern Africans.
Perhaps this “Neanderthal” admixture actually came from a related archaic population that was already established in Africa. As modern humans spread west from East Africa after c. 60,000 BP, they would have partially intermixed with these archaic hominins before finally replacing them. There would thus be an east-to-west cline of increasing archaic admixture.
This would be consistent with the finding that dental traits are more “ancestral” in West Africa than in East Africa. When Irish (2011) compared dentitions from west, central, east, and south Africa, ranging in age from the late Pleistocene to the mid-1950s, the early Holocene Kenyans and Tanzanians were the ones that had the fewest ancestral traits of the Sub-Saharan African Dental Complex.
This may also explain why the level of Neanderthal admixture is almost the same throughout Eurasia (although John Hawks has found slightly higher levels among Europeans than among Asians). Perhaps this “Neanderthal” admixture was simply admixture with a Neanderthal-like population within Africa itself. Modern humans would have picked it up on their way out of the continent.
But this raises another question. Why is “Neanderthal” admixture lower in present-day sub-Saharan Africans than in present-day Eurasians? Perhaps there was another archaic population, very different from the Neanderthals, that modern humans encountered only in Africa. This might be the quasi Homo erectus represented by the Broken Hill (Kabwe) skull. It might also be the population that accounts for about 2% of the present-day African gene pool and that seems to have split away from the ancestors of modern humans some 700,000 years ago (Hammer et al., 2011). This secondary archaic source may have proportionately reduced the admixture from these “African Neanderthals.”
References
Hawks, J. (2012). Which population in the 1000 Genomes Project samples has the most Neandertal similarity? John Hawks Weblog, February 8
http://johnhawks.net/weblog/reviews/neandertals/neandertal_dna/1000-genomes-introgression-among-populations-2012.html
Hammer, M.F., A.E. Woerner, F.L. Mendez, J.C. Watkins, and J.D. Wall. (2011). Genetic evidence for archaic admixture in Africa, Proceedings of the National Academy of Science (USA), 108, 15123-15128, www.pnas.org/cgi/doi/10.1073/pnas.1109300108
Irish, J.D. (2011). Afridonty: the “Sub-Saharan African Dental Complex” revisited, American Journal of Physical Anthropology, 144(supp. 52), 174
When modern humans began their expansion from a small core somewhere in East Africa, the continent probably had several different archaic populations.
It now seems that one of them was related to the Neanderthals in Europe. In an ongoing study of Neanderthal admixture in present-day humans, John Hawks has found an apparently higher level of admixture in the Yoruba of Nigeria than in the Luhya of Kenya (see chart above).
This is counter-intuitive, as Hawks himself notes. The closest source of admixture would have been Neanderthals in what is now Israel (and probably elsewhere in the Middle East). Genes from that source should have first introgressed into northeastern Africans.
Perhaps this “Neanderthal” admixture actually came from a related archaic population that was already established in Africa. As modern humans spread west from East Africa after c. 60,000 BP, they would have partially intermixed with these archaic hominins before finally replacing them. There would thus be an east-to-west cline of increasing archaic admixture.
This would be consistent with the finding that dental traits are more “ancestral” in West Africa than in East Africa. When Irish (2011) compared dentitions from west, central, east, and south Africa, ranging in age from the late Pleistocene to the mid-1950s, the early Holocene Kenyans and Tanzanians were the ones that had the fewest ancestral traits of the Sub-Saharan African Dental Complex.
This may also explain why the level of Neanderthal admixture is almost the same throughout Eurasia (although John Hawks has found slightly higher levels among Europeans than among Asians). Perhaps this “Neanderthal” admixture was simply admixture with a Neanderthal-like population within Africa itself. Modern humans would have picked it up on their way out of the continent.
But this raises another question. Why is “Neanderthal” admixture lower in present-day sub-Saharan Africans than in present-day Eurasians? Perhaps there was another archaic population, very different from the Neanderthals, that modern humans encountered only in Africa. This might be the quasi Homo erectus represented by the Broken Hill (Kabwe) skull. It might also be the population that accounts for about 2% of the present-day African gene pool and that seems to have split away from the ancestors of modern humans some 700,000 years ago (Hammer et al., 2011). This secondary archaic source may have proportionately reduced the admixture from these “African Neanderthals.”
References
Hawks, J. (2012). Which population in the 1000 Genomes Project samples has the most Neandertal similarity? John Hawks Weblog, February 8
http://johnhawks.net/weblog/reviews/neandertals/neandertal_dna/1000-genomes-introgression-among-populations-2012.html
Hammer, M.F., A.E. Woerner, F.L. Mendez, J.C. Watkins, and J.D. Wall. (2011). Genetic evidence for archaic admixture in Africa, Proceedings of the National Academy of Science (USA), 108, 15123-15128, www.pnas.org/cgi/doi/10.1073/pnas.1109300108
Irish, J.D. (2011). Afridonty: the “Sub-Saharan African Dental Complex” revisited, American Journal of Physical Anthropology, 144(supp. 52), 174