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 Tâ13910 and East African Gâ13907 LP alleles might have arisen because of a common domestication event of the cattle whereas the Câ3712-Gâ13915 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 Tâ13910 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 Gâ13907 allele and its relationships to the domestication of milk-producing species within Africa. In our study, we did not detect the LP-associated allele, Câ14010, found recently among Tanzanians and Kenyans, but reviewing the data from Tishkoff et al. would reveal that the Câ14010 allele most likely originated on different background allele, most likely H1 in our data set....Our age estimate of the Gâ13915 allele of â¼4095 (±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 Tâ13910 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 Câ14010, detected in East African populations....
p-ter at GNXP (who pointed me to this paper) has some reflective thoughts:
...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!
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There is a reason for drinking milk: "Milk beats soy for muscle gain"
http://dailynews.mcmaster.ca/story.cfm?id=4626
I don't see how there's any reason to drink milk within context of this blog post other than it's a source of nutrition. From the point of view of us wildtypes, drinking milk is for weirdo mutants. So hah.
This wildtype thinks Dunbar has sour grapes.
I remember reading theory that we evolved our ability to digest milk and gluten not because of their nutritional benefits but because they had a drug-like sedative effect that allowed people to adapt from a hunter-gatherer lifestyle to a settled one.
I remember reading theory that we evolved our ability to digest milk and gluten not because of their nutritional benefits but because they had a drug-like sedative effect that allowed people to adapt from a hunter-gatherer lifestyle to a settled one.
highest frequencies are in northern europe. low frequencies in sicily for LP. no frequency in china. so unlikely as a necessary causal factor.
Dunbar: you should see my muscles...
Anyway, the early heardsmen were living in an environment where physical prowess was useful. If drinking milk gave them an upper edge over ordinary cattle thieves, it was an advantage they could use when playing with girls.
i've heard that 1/3 of the nutritive value of milk is in lactose. without LP you lose that, and you have a higher risk of gas & diarrhea. i think that explains the selective advantage.
I hate these crap just-so stories. If indeed milk did give its drinkers superior muscles, one would also have to assume that girls would sexually prefer muscled men. So instead of a straightforward and immensely plausible adaptive explanation like lactase digestion of milk has nutritive effect, you posited a sexual selection hypothesis that has little grounding in reality. Furthermore, the study you (Lassi) pointed to demonstrated the greater efficacy of milk in promoting muscle accrual than ordinary soy-protein beverages; notably, the study did not say that milk drinkers have bigger muscles fiat. Such a suggestion would be like me saying Asians live longer because they drink tea and thus have tons of antioxidants in them-- little empirical reality and total disconnect from available evidence to conclusion demonstrates the ridiculousness of my attempt at satire. Got it?
Furthermore, the study you (Lassi) pointed to demonstrated the greater efficacy of milk in promoting muscle accrual than ordinary soy-protein beverages; notably, the study did not say that milk drinkers have bigger muscles fiat.
Bigger muscles than who? I hate these crap just-so stories.
The study says that by drinking milk people can grow more muscles. In a stone age society that counts. I don't mean just fighting. Many tasks to get a daily meal benefit from better physical shape, and in difficult times (e.g. winter) it improves chances of survival. The girls would be the first to notice. They aren't stupid.
You only partially get it. Populations do not drink milk because they can grow more muscles; they drink milk because it's a convenient source of nutrition. Some pastorals take milk as a food source to the extreme, like the Maasai. Regardless, adaptations that can exploit milk more efficiently can beneficial in pastoral societies, and can quickly be fixed at high frequency. In societies that do not need to exploit milk, or perhaps do not culturally use animal milk, lactase persistence is subject to genetic drift and many populations can lose lactase persistence (like Chinese people, who eat rice and don't exploit animal milk). The salient point is that the levels of lactase persistence has nothing to do with girls being more attracted to milk drinkers for several reasons: 1) the whole bit I just said about the adaptiveness of lactase persistence in milk-drinking populations, 2) milk-drinkers would actually have to be in better shape than non-milk drinkers in the same population, and 3) girls would have to actually be attracted to brawny milk-drinkers and somehow procreate more. Reason number 2 would be the weakest for my argument in the sense that I agree milk can be an extra source of nutrition, and more well-fed individuals tend to be physically more fit than less well-fed individuals in the same population, but I could just substitute any food for milk and the result will be the same. Interestingly enough, the performance of athletes at the Olympic games probably has little correlation with whether they drink milk. I could go on poking holes, but it suffices to say that an article on nutrition has very very little to do with the evolution of lactase persistence.
Gah, that went on longer than it should have.
lactase persistence is subject to genetic drift and many populations can lose lactase persistence
there might have been strong constraint against LP among chinese.... (from what i hear)
Dunbar: "Reason number 2 would be the weakest for my argument in the sense that I agree milk can be an extra source of nutrition, and more well-fed individuals tend to be physically more fit than less well-fed individuals in the same population, but I could just substitute any food for milk and the result will be the same."
You didn't read the abstract:
"Our findings clearly show that milk proteins are a superior source of protein in producing muscle mass gains in response to weightlifting."
Not just a quantity issue. Milk has better quality. That's the point I'm trying to make.
According to Cochran and Harpending (The 10,000 Year Explosion), the real advantage in being able to digest milk is of numbers: dairying yields 5 times as many calories per acre as herding animals for meat, so populations that can digest milk can carry more population than non milk-digesters on the same area of land. In any conflict between them, numbers would give the advantage to the dairying people.