Over a year ago I mentioned the HHMI Biointeractive site and its excellent videos. I just viewed another one on the same site. This one is called The Biology of Skin Color, and it is another winner. It is narrated by Nina Jablonski, who did much of the research cited in the video. (Those of you who use my Biology: Concepts and Investigations 3/e book can find a synopsis of her research in section 25.6.)
The video begins by showing off the rich variety of human skin colors, then it briefly explains the physics of light reflection. Jablonski then talks about the role of melanin: Like a “little parasol over the nucleus,” melanin protects the skin’s DNA from mutations induced by ultraviolet radiation.
The next chunk of the video explains how Jablonski and her geographer husband discovered the correlation between solar UV exposure and skin pigmentation in indigenous peoples from around the world. Jablonski introduces a colleague who sequenced DNA and discovered that a particular pigmentation-related gene, MC1R, shows little variation in Africa, suggesting strong selection against alleles conferring light skin. What is the selective pressure? Jablonski quickly rejects skin cancer as an explanation; instead, she argues that dark pigments protect folate (a vitamin that’s essential to reproductive success) from breakdown by UV radiation.
The next question is, why aren’t we all dark-skinned? Jablonski explains that humans need some exposure to UV radiation to make vitamin D. The selective pressure for dark skin therefore abates at higher latitudes, where UV radiation is weaker. As evidence, she points to indigenous people from northern latitudes, who have darkly pigmented skin but also consume diets that are high in vitamin D.
Overall, the video depicts skin color variation around the world as an evolutionary balancing act. Now, thanks to global-scale migration, many of us have pigmentation that’s not a good match for where we live. We have to use cultural adaptations — like sunscreen and vitamin D supplements — to compensate.