Lactase persistence cause adaptation
Lactase persistence and malaria resistance
One study suggested that lactase persistence was selected for parallel to malaria resistance in the Fulani from Mali. Proposed mechanisms are: nutritional advantage of milk; low content of p-aminobenzoic acid compared to non-milk diets; intake of immunomodulators contained in milk.Evolutionary history
According to the gene-culture coevolution hypothesis, the ability to digest lactose into adulthood (lactase persistence)  became advantageous to humans after the invention of animal husbandry and the domestication of animal species that could provide a consistent source of milk. Hunter-gatherer populations before the Neolithic revolution were overwhelmingly lactose intolerant, as are modern hunter-gatherers. Genetic studies suggest that the oldest mutations associated with lactase persistence only reached appreciable levels in human populations in the last 10,000 years. This correlates with the beginning of animal domestication, which occurred during the Neolithic transition. Therefore, lactase persistence is often cited as an example of both recent human evolution and, as lactase persistence is a genetic trait but animal husbandry a cultural trait, gene-culture coevolution in the mutual human-animal symbiosis initiated with the advent of agriculture.Depending on the populations, one or the other hypothesis for the selective advantage of lactase persistence is more relevant: In Northern Europe, the calcium absorption hypothesis might be one of the factors leading to the strong selection coefficients, whereas in African populations, where vitamin D deficiency is not as much of an issue, the spread of the allele is most closely correlated with the added calories and nutrition from pastoralism.Several genetic markers for lactase persistence have been identified, and these show that lactase persistence has multiple origins in different parts of the world (i.e. it is an example of convergent evolution). In particular, it has been hypothesized that the T*13910 variant appeared at least twice independently. Indeed, it is observed on two different haplotypes: H98, the more common (among others in the Finnish and in the Fulani); and H8 H12, related to geographically restricted populations. The common version is relatively older. The H98 variant – most common among Europeans – is estimated to have risen to significant frequencies about 7,500 years ago in the central Balkans and Central Europe, a place and time roughly corresponding to the archaeological Linear Pottery culture and Starčevo cultures.
The T*13910 variant is also found in North Africans. Thus it probably originated earlier than 7500 ya, in the Near East, but the earliest farmers did not have high levels of lactase persistence and did not consume significant amounts of unprocessed milk.Some hypotheses regarding the evolutionary history of lactase persistence in given regions of the world are described below.Africa
The situation is more complex in Africa, where all five main lactase persistence variants are found.The presence of T*13910 alleles among the Khoe pastoralists is ascribed to gene flow from Europe. However, the presence of other alleles signals gene flow from East Africa.It has been hypothesized that the G*13915 variant dispersed from the Middle East, in association with the domestication of the Arabian camel.The G-14009 mutation is based in Ethiopia.The G*13907 variant is concentrated among Afroasiatic speakers in Northeast Africa.The C*14010 allele is today most common among pastoralist groups inhabiting eastern Africa, from where it is thought to have spread along with pastoralism into parts of southern Africa. Ultimately, the C*14010 lactase persistence variant is believed to have arrived from the Sahara in areas that were previously inhabited by Afroasiatic-speaking populations. This was deduced from the existence of animal husbandry- and milking-related loanwords of Afroasiatic origin in various Nilo-Saharan and Niger-Congo languages, as well as from the earliest appearance of processed milk lipids on ceramics which were found at the Tadrart Acacus archaeological site in Libya (radiocarbon-dated to c. 7,500 BP, close to the estimated age of the C*14010 mutation).The evolutionary processes driving the rapid spread of lactase persistence in some populations are not known. Among some populations inhabiting East Africa, lactase persistence has gone from negligible to near-ubiquitous frequencies in just 3000 years, suggesting a very strong selective pressure. Some studies also proposed that selection for lactase persistence is not as strong as supposed (soft selective sweep), and that its strength varies a lot depending on particular environmental conditions.
Post animal domestication, individuals gained the ability to tolerate lactose after weaning from infancy. This offered a crucial advantage to humans through natural selection by creating genetic variances.Neolithic agriculturalists, who may have resided in Northeast Africa and the Near East, may have been the source population for lactase persistence variants, including –13910*T, and may have been subsequently supplanted by later migrations of peoples. The Sub-Saharan West African Fulani, the North African Tuareg, and European agriculturalists, who are descendants of these Neolithic agriculturalists, share the lactase persistence variant –13910*T. While shared by Fulani and Tuareg herders, compared to the Tuareg variant, the Fulani variant of –13910*T has undergone a longer period of haplotype differentiation. The Fulani lactase persistence variant –13910*T may have spread, along with cattle pastoralism, between 9686 BP and 7534 BP, possibly around 8500 BP; corroborating this timeframe for the Fulani, by at least 7500 BP, there is evidence of herders engaging in the act of milking in the Central Sahara.
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