Lactase Persistence - Evolutionary History

Evolutionary History

The ability to digest lactose into adulthood (lactase persistence) would have only been useful 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 ten thousand years. 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.

Several genetic markers for lactase persistence have been identified, and these show that the allele has multiple origins in different parts of the world (i.e. it is an example of convergent evolution). The version of the allele most common amongst 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 approximately corresponding to the archaeological Linearbandkeramik and Starčevo cultures. From there, it most probably spread eastwards as far as India. Likewise, one of the four alleles associated with lactase persistence in African population, is also probably of European origin. Since North Africans also possess this version of the allele it is probable that it actually originated earlier, in the Near East, but that the earliest farmers did not have high levels of lactase persistence and, subsequently, did not consume significant amounts of unprocessed milk. Lactase persistence in Sub-Saharan Africa almost certainly had a separate origin, probably more than one, and it is also likely that there was a separate origin associated with the domestication of the Arabian camel. None of the mutations so far identified have been shown to be causal for the lactase persistence allele, and it is thought that there are several more yet to be discovered.

The evolutionary processes driving the rapid spread of lactase persistence in some populations are not known. In some East African ethnic groups lactase persistence has gone from negligible to near-ubiquitous frequencies in just three thousand years, suggesting a very strong selective pressure. But some models for the spread of lactase persistence in Europe attribute it primarily to a form of genetic drift. Competing theories on why the ability to digest lactose might be selected for include nutritional benefits, milk as a water source in times of drought, and increased calcium absorption helping to prevent rickets and osteomalacia in low-light regions.

Roman authors recorded that the people of northern Europe, particularly Britain and Germany, drank unprocessed milk. This corresponds very closely with modern European distributions of lactose intolerance, where the people of Britain, Germany, and Scandinavia have a high tolerance, and those of southern Europe, especially Italy, have a lower tolerance.

In east Asia, historical sources also attest that the Chinese did not consume milk, whereas the nomads who lived on the borders did. Again, this reflects modern distributions of intolerance. China is particularly notable as a place of poor tolerance, whereas in Mongolia and the Asian steppes mare milk is drunk regularly. This tolerance is thought to be advantageous, as the nomads do not settle down long enough to process mature cheese. Given that their prime source of income is generated through horses, to ignore their milk as a source of nourishment would be foolish. The nomads also make an alcoholic beverage, called kumis, from mare milk, although the fermentation process reduces the amount of lactose present.