Many of the marks that evolutionary history has left on our bodies are invisible. Lactose tolerance, a predisposition towards diabetes, genes that contribute to breast cancer, and many other inconspicuous traits are legacies of the paths that our ancestors took as they left or stayed in Africa between 60 and 125 thousand years ago. However, other markers of these unique evolutionary histories are perfectly obvious, perhaps most notably skin color. It's clear that people whose ancestors hail from different parts of the earth have differently colored skin and that this is related to how much of the sun's radiation hits that part of the planet. The less radiation, the lighter the native population's skin color tends to be. This is a great example of recent evolution in human populations. But what if we go back deeper in our evolutionary history, back to when all of humanity lived in Africa? At that time, all humans had darkly pigmented skin. A new study sheds light on how and why this skin pigmentation evolved.
Where's the evolution?
Humans have different skin colors because we have different amounts and kinds of the pigment melanin in our skin. Our closest living relatives, the chimpanzees, have pale skin without melanin underneath their dark fur, and almost certainly the ancestor that we share with chimps did too. So how did the early members of the human branch of the tree of life get from hair-covered light skin to hairless dark skin? Researchers have many competing hypotheses about what sort of natural selection caused dark skin to evolve. In all of these hypotheses, the notion of evolutionary fitness is important.
In evolutionary terms, fitness indicates not how physically fit or healthy an organism is but how effective an organism carrying particular gene versions is at getting offspring into the next generation. So, for example, an animal carrying genes that cause it to expend little energy on reproduction and lots of energy on building muscles might...