Convergent adaptation of human lactase persistence in Africa and Europe
"Archeological evidence suggests that cattle domestication originated in southern Egypt as early as ~9,000 years ago but no later than ~7,700 years ago and in the Middle East ~7,000-8,000 years ago28, consistent with the age estimate of ~8,000-9,000 years (95% c.i. ~2,200-19,200 years) for the T-13910 allele in Europeans. The more recent age estimate of the C-14010 allele in African populations, ~2,700-6,800 years (95% c.i. ~1,200-23,000 years), is consistent with archeological data indicating that pastoralism did not spread south of the Sahara and into northern Kenya until ~4,500 years ago and into southern Kenya and northern Tanzania ~3,300 years ago28,29. The ability to digest milk as adults is likely to be adaptive owing to the increased nutritional benefits from milk (carbohydrates as well as fat, protein and calcium) and also because milk is an important source of water in arid regions2,28,30,31. Considering the symptoms of lactose intolerance, which includes water loss from diarrhea, individuals who had the lactase persistence–associated alleles and could tolerate milk could have had a very strong selective advantage2. This is supported by our high estimates for the selection coefficient (s = 0.035-0.097). Because the selective force, adult milk consumption, is associated with the cultural development of cattle domestication, the recent and rapid spread of the lactase persistence–associated alleles, together with the practice of pastoralism in East Africa, is an excellent example of ongoing adaptation in humans32 and coevolution of genes and culture3."
"We observe the oldest age estimates of the C-14010 allele, ~6,000-7,000 years (95% c.i. ~2,000-16,000 years), in the Kenyan Nilo-Saharan and Tanzanian Afro-Asiatic populations (Table 1). We also observe an old age estimate in the Tanzanian Sandawe, but its low frequency suggests it was introduced via recent gene flow (Supplementary Discussion). However, we cannot distinguish with certainty whether this allele first arose in the Cushitic-speaking Afro-Asiatic populations, who are thought to have migrated into Kenya and Tanzania from Ethiopia ~5,000 years ago33 and practice a mixture of agriculture and pastoralism, or in the Nilotic-speaking Nilo-Saharan populations, who are thought to have migrated into Kenya and Tanzania from southern Sudan within the past ~3,000 years33 and are strict pastoralists28. These results are consistent with both linguistic34 and genetic data (F.A.R. and S.A.T., unpublished data) indicating cultural exchange and genetic admixture between these groups. The absence of C-14010 in the southern Sudanese Nilo-Saharan–speaking populations suggests that this allele either originated in or was introduced to the Kenyan Nilo-Saharan populations after their migration from southern Sudan. Regardless of the population origins of the C-14010 allele, it spread rapidly throughout the region along with the cultural practice of pastoralism, consistent with a demic diffusion model of genetic and cultural expansion35."