A reader pointed me to a paper in The Proceedings of the Royal Society B, Was skin cancer a selective force for black pigmentation in early hominin evolution?. My initial reaction was to dismiss this argument, because cancer is obviously a late-in-life disease, and therefore it would not be a major selective hit. Rather, I found Nina Jablonski’s argument in Skin: A Natural History, persuasive. Basically she suggests that chemical processes triggered by ultraviolet radiation result in the destruction of folate, and this in turn leads to elevated miscarriage rates due neural tube defects. Anything that impacts direct female reproductive output is a candidate from strong natural selection, so the thesis was highly persuasive.
But after reading the paper I do think the argument that carcinomas can reduce the fitness of humans with light skin in the tropics has merit. The author uses the case histories of albinos in Africa, who tend to develop serious health issues by their twenties. Obviously white skin is not albinism, but if the suboptimal function and lethality for albinos is correct, I can’t but help think that light-skinned hunter-gatherers on the Malthusian margin would have an even tougher go.
We can frame evolutionary question with some molecular genetic inferences. It seems that strong constraint (selection) impacted the region around MC1R 1-2 million years. The model is that ancient hominins shifted toward the savanna, lost their fur due to thermo-regulation needs, and then evolved dark skin to protect themselves against the radiation. What I think needs to be acknowledged is that it could have been that multiple forces resulted in the shift toward dark skin, which probably occurred concomitantly with the loss of fur.