Evolutionary history is more than skin deep
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 look quite fit in the everyday meaning of the word, but have low evolutionary fitness because it produces far fewer offspring than other members of its species. If a particular gene version confers high fitness, that means that it helps organisms get offspring into the next generation—and those offspring are likely to carry copies of the helpful gene version that their parent had. Hence, gene versions that confer high evolutionary fitness are likely to become common through the action of natural selection.
Scientists reason that sometime after our lineage separated from that of the chimpanzees, dark skin must have become common because it conferred a fitness advantage. But what exactly was that advantage? One prominent idea is based on the fact that exposure to UV radiation destroys folate—a molecule that our bodies need for a wide variety of processes. For example, a lack of folate during pregnancy is known to contribute to birth defects like spina bifida. Darkly pigmented skin protects folate from being broken apart. Perhaps, as our ancestors lost their protective body hair, individuals without pigmentation genes suffered folate shortages that caused them to produce fewer and less healthy offspring. In that scenario, any person who happened to carry gene mutations that produced skin with protective pigmentation would have left behind more offspring and had a fitness advantage over those without skin pigmentation. Hence, over many generations, the genes that produce pigmented skin would have spread through our ancestral population.
It's difficult to figure out whether preventing folate destruction was the main reason that dark skin became common or whether it was some other benefit or a combination of different benefits. Now, research suggests that a factor previously written off may have helped select for pigmented skin after all: skin cancer. It's no secret that pigmentation protects us from skin cancer. African Americans are about 1/10th as likely to be diagnosed with a malignant melanoma and less than 1/50th as likely to be diagnosed with nonmelanoma skin cancer as are white Americans. However, this was not thought to be an important factor in the evolution of dark skin because the vast majority of these skin cancers strike when a person is well past reproductive age. This may be devastating to the health of the victim, but it has little impact on his or her evolutionary fitness. After all, evolutionary fitness is all about how many offspring one leaves behind in the next generation. If a disease strikes after a person has already reproduced to their full potential, it generally has little effect on evolutionary fitness. Because of that, few scientists thought that protection from skin cancer could have significantly contributed to the evolution of skin pigmentation in our ancestors. However, now that assumption is being brought into question.
The new study brought together many different lines of prior research focused on skin cancer rates among albino Africans living in central Africa—the same area where our ancestors first evolved pigmented skin. Most people with albinism carry mutations that cause them to produce no or very little melanin in their skin. As one would expect, these individuals run a much higher risk of developing skin cancer than do normally pigmented individuals. In fact, the risk is so high that many of them develop life threatening skin cancers before and during their reproductive years. In one study of more than 500 Tanzanians with albinism, nearly all died of skin cancer before the age of 40. Overall, the data suggest that more than 90% of albino individuals living near the equator in Africa will die in their 30s or before, mainly because of skin cancer. In other words, skin cancer does have the potential to cut short an individual's reproductive years—and hence, could have an impact on evolutionary fitness.
What does all this indicate about our African ancestors? Some scientists reason that there are a lot of similarities between our pale-skinned ancestors living on the African savannah and modern-day Albinos living in central Africa. If the evolutionary fitness of modern albinos suffers because of skin cancer, the problem likely would have been even worse for our ancestors, who didn't benefit from protective clothing, shelter, or any medical advances. So perhaps, skin cancer was a selective factor in the evolution of pigmentation after all. It may have been inappropriately discounted because most of the previous data were focused on skin cancer rates and deaths among lightly pigmented people living in areas that receive less of the sun's radiation.
More research will be required to untangle all the potential reasons that natural selection favored dark skin in our ancestors, but whatever the reasons, we can be sure that it did—and that as modern humans fanned out across the globe, they experienced a wide variety of environments that favored many different skin tones. Today, we see evidence of this complex evolutionary history in both our genes and our unique appearances.
Discussion and extension questions
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Understanding Evolution © 2016 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California