Lactase persistence
[1] In some human populations though, lactase persistence has recently evolved[2] as an adaptation to the consumption of nonhuman milk and dairy products beyond infancy.
The distribution of the lactase persistence (LP) phenotype, or the ability to digest lactose into adulthood, is not homogeneous in the world.
[3][4][11] In Africa, the distribution of lactase persistence is "patchy":[12][13][3] high variations of frequency are observed in neighbouring populations, for example between Beja and Nilotes from Sudan.
[4] High percentages of lactase persistence phenotype are found in traditionally pastoralist populations like Fulani and Bedouins.
[3][15] Lactase persistence is prevalent in Nguni and certain other pastoralist populations of South Africa as a result of the dairy they consume in their diet.
Lactase persistence amongst Nguni people is, however, less common than in Northern European populations because traditionally, their consumption of dairy came primarily in the form of amasi (known as Maas in Afrikaans), which is lower in lactose than fresh, raw milk as a result of the fermentation process it goes through.
[citation needed] The combination of pastoralism and lactase persistence genes would have allowed individuals the advantage of niche construction, meaning they would have had less competition for resources by deriving a secondary food source, milk.
Rather than having to raise and slaughter animals, one cow or goat could repeatedly serve as a resource with fewer time and energy constraints.
As a result, no selection has perpetuated the spread of these allelic variants, and the lactase persistence genotype and phenotype remains rare.
[1] For example, in East Asia, historical sources also attest that the Chinese did not consume milk, whereas the nomads who lived on the borders did.
China is particularly notable as a place of poor tolerance, whereas in Mongolia and the Asian steppes, milk and dairy products are a main nutrition source.
The nomads also make an alcoholic beverage, called airag or kumis, from mare's milk, although the fermentation process reduces the amount of lactose present.
[48] In addition, factors such as metabolic traits, socioeconomic status, lifestyle, and fertility were found to be unrelated to the findings, indicating that health improvements for these women were due to dairy products consumption and exhibited lactase persistence.
Human populations differ in the prevalence of genotypic lactase persistence, phenotypic lactose tolerance, and habitual milk consumptions.
[citation needed] One study suggested that lactase persistence was selected for parallel to malaria resistance in the Fulani from Mali.
Genetic studies suggest that the oldest mutations associated with lactase persistence only reached appreciable levels in human populations in the last 10,000 years.
[56] 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,[57] 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.
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.
[62] In Central Asia, the causal polymorphism for lactase persistence is the same as in Europe (T*13910, rs4988235), suggesting genetic diffusion between the two geographical regions.
[11] This, in turn, could also be an indirect genetic proof of early domestication of horses for milk products as recently attested from archaeological remains.
[11] In South Asia, the dominant causal polymorphism for lactase persistence is the same as in Europe (T*13910, rs4988235), suggesting genetic diffusion between the two geographical regions.
[65][19][20][67] 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).
[1] 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.
[69] 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.
[70] Lactose malabsorption is typical for adult mammals, and lactase persistence is a phenomenon likely linked to human interactions in the form of dairying.
[71] After weaning, or the transition from being milk-fed to consuming other types of food, their ability to produce lactase naturally diminishes as it is no longer needed.
[citation needed] Some examples exist of factors that can cause the lactase persistence phenotype in the absence of any genetic variant associated with LP.
Individuals may lack the alleles for lactase persistence, but still tolerate dairy products in which lactose is broken down by the fermentation process (e.g. cheese, yogurt).
[75] Also, healthy colonic gut bacteria may also aid in the breakdown of lactose, allowing those without the genetics for lactase persistence to gain the benefits from milk consumption.
