Vitamin D deficiency and skin color are two biological topics I've focused on a lot. The latter may have a relationship to the former insofar as light skin is better at synthesizing Vitamin D at low radiation levels (i.e., at high latitudes). Additionally, some of the genes that are under recent natural selection (within the last 10,000 years), such as OCA2 & SLC24A5, are related to pigmentation (specifically, the lightening of skin or eye color). At my other blog I've reproduced some ethnographic data which shows deviation from expectation of skin color in various populations assuming that UV radiation is the independent variable (and generating the slope using the observed values of indigenous populations). It won't be a surprise to some of you that I suspect that perhaps Vitamin D deficiency due to a shift toward a high carbohydrate diet might have resulted in lighter skin emerging to compensate for the loss of natural intake via food. But is Vitamin D deficiency that bad? Rickets is an extreme case, and there is evidence for its influence in increasing mortality in relation to chronic end-of-life disease (cancer, etc.). There is some data which suggests birth defects, and that's pretty important, as it hits fitness at a critical point. But there's something else.... Epidemic influenza and vitamin D:
...Solar radiation triggers robust seasonal vitamin D production in the skin; vitamin D deficiency is common in the winter, and activated vitamin D...acts as an immune system modulator, preventing excessive expression of inflammatory cytokines and increasing the 'oxidative burst' potential of macrophages. Perhaps most importantly, it dramatically stimulates the expression of potent anti-microbial peptides, which exist in neutrophils, monocytes, natural killer cells, and in epithelial cells lining the respiratory tract where they play a major role in protecting the lung from infection. Volunteers inoculated with live attenuated influenza virus are more likely to develop fever and serological evidence of an immune response in the winter. Vitamin D deficiency predisposes children to respiratory infections. Ultraviolet radiation (either from artificial sources or from sunlight) reduces the incidence of viral respiratory infections, as does cod liver oil (which contains vitamin D). An interventional study showed that vitamin D reduces the incidence of respiratory infections in children. We conclude that vitamin D, or lack of it, may be Hope-Simpson's 'seasonal stimulus'.
The article above is focused on why flu (and cold) breaks out during winter; the basic model is that elevated Vitamin D synthesis during periods of high radiation (e.g., summer) tends to beef up the immune system and so allow one to globally resist infection more effectively. What does this have to do with skin color evolution? Simple: agriculture increased population densities greatly and so resulted in the rise of a host of new infectious diseases which could now be incubated far more easily. In other words, I am suggesting here that one of the reasons that light skin might have emerged is that it served as a survival edge in a society which was being exposed to new variants of infectious disease. If you take a close look at the table on my other post you will note that many of the populations which are far lighter than expectation are those which have long experience of agriculture, are at extreme population densities and are characterized by endemic nutrient deficiencies (i.e., the typical Japanese or Indian peasant consumed far less protein than an English peasant around 1500). ...that's all for now....
