Over the past few weeks I’ve talked about the relationship of genes & biology to culture. First I noted the likely impact of the evolutionary arms race between our adaptive immune system and plagues & endemic infectious diseases upon the course of human history. Second, I pointed to the utility of phylogenetics in giving us another tool through which we have a window onto the past. But what about culture’s affect upon our genes? The canonical example is probably lactose tolerance, or more precisely lactase persistence. In this case the adoption of dairy culture has resulted in a radical alteration of a common mammalian phenotype, the non-persistence of lactase production, the enzyme which breaks down milk sugar (lactose), into adulthood. A large proportion of modern humans exhibit lactase persistence across vast swaths of western Eurasia and northern Africa. Not only that, but it seems a new feature of our species, a trait of the last 10,000 years driven by the proactive adoption of a cattle culture across diverse societies at different times. It is also one of the strongest, if not the strongest, signature of natural selection within the genomes of many groups (e.g., the recent genomic tests of selection use the area around LCT as a control to make sure that they’re working!). There’s now a new paper out in The American Journal of Human Genetics which report a new allele extant at high frequencies among Arab peples, Independent Introduction of Two Lactase-Persistence Alleles into Human Populations Reflects Different History of Adaptation to Milk Culture. Here’s an important part from the discussion:
…This result would justify the hypothesis that the European T−13910 and East African G−13907 LP alleles might have arisen because of a common domestication event of the cattle whereas the C−3712-G−13915 allele in Arabia most likely arose due to the separate domestication event of camels. This slightly far-reaching proposal is analogous to the previous interpretations: the presence of the LP T−13910 allele among three North African Berber populations (from Morocco and Algeria) has been taken as a genetic evidence for shared origin of the dairy culture between North African populations and Eurasians…Additional analyses of the East African samples could shed light on the origin of the G−13907 allele and its relationships to the domestication of milk-producing species within Africa. In our study, we did not detect the LP-associated allele, C−14010, found recently among Tanzanians and Kenyans, but reviewing the data from Tishkoff et al. would reveal that the C−14010 allele most likely originated on different background allele, most likely H1 in our data set….
Our age estimate of the G−13915 allele of ∼4095 (±2045) years in the Arabian Peninsula would suggest that the introduction of this LP variant might be associated with the domestication of the Arabian camel more than 6000 years ago…An analogous concept for the major European mutation was also supported by maximum likelihood analysis for the T−13910 allele, which likely arose after the domestication of cattle 5,000-10,000 years ago…Interestingly, similar age estimates were observed also for the LP mutation C−14010, detected in East African populations….
…With these assumptions, one predicts that lactose tolerance has arisen around 25 times since it became advantageous. Given that we’re talking about less than ten thousand years since dairy farming, that’s quite remarkable.
The relevant parameter here is the mutational target size–if lactose tolerance could only be caused by a change at one particular base pair in humans, it would never have arisen independently so many times. But with a mutational target so large, and a selection coefficient so strong, it becomes inevitable that any culture that developed dairy farming would eventually develop lactose tolerance. But it still seems amazing to me that it happened so quickly!