The colors of people

Yesterday I heard an excellent talk on the evolution of skin color by Nina Jablonski of Penn State. She started by describing how little work there was on the subject when she was asked to give a talk about it in the early 1990s; although it’s an interesting topic, the subject of skin color and race was such a hot potato that people were reluctant to pick it up. Dr. Jablonski described herself as an optimist who believes that we’re socially mature enough to deal with discussions of skin color. Her talk didn’t directly refute the ideologies involved in racism, but was an excellent example of how to sidestep ideology entirely and present the evolution behind skin color variation in humans as a fascinating story of science and history, with an abiding biochemical tension at the heart of it.

The tension has to do with the human body’s love-hate relationship with ultraviolet radiation. In general, UV is deleterious to us (and to many other living things); in particular, it destroys the essential B vitamin folate and causes various sorts of tissue damage and DNA damage. However, certain wavelengths of UVB radiation are essential to the formation in the skin of vitamin D, which helps our bodies absorb calcium and can be very difficult to get enough of from dietary sources. Thus, we mostly could do very well without UV radiation, except that we need vitamin D, not only for bone health but for a host of other reasons.

(A side note on our current take on this problem: For years dermatologists have focused on the bad side of UV radiation, urging the use of sunscreen; also, the lifestyle, in industrialized countries in particular, has tended toward more time spent indoors and less out in the sun, resulting in lower UV exposure. I’ve recently read a bit on vitamin D and depression, mostly because I’m being treated for a severe vitamin D deficiency. I’ve been contemplating whether to continue protecting my skin at all times from UV (especially important because I grew up in a sunny climate at a time when sunscreen was not anywhere near as ubiquitous as it is today) or to let at least a little of the sunshine in so as to boost my vitamin D levels. Jablonski says she advises people to follow the advice of dermatologists to protect their skin and consider dietary supplementation to meet their vitamin D needs; some dermatologists, she reported, are now recommending prudent sun exposure, which means exposing only parts of the body that normally don’t get much sun. One dermatologist asked her to please encourage people to sun their buttocks.)

Exposure to UV radiation is the single most important factor in explaining human skin color, the story of which is based on the presence in our bodies of varying levels of melanin. This nifty chemical produces skin pigmentation and protects against the negative effects of UV radiation. The earliest members of genus Homo were darkly pigmented, living in an area with abundant UVR, but then, in time, we moved to areas with different levels of UV radiation. UVR is generally most abundant at the equator and tapers off at higher latitudes, with several variations. In areas of higher altitude, like the Himalayas and the Andes, UV exposure is higher because there is less protective atmosphere between the earth and the sun. Also, equatorial areas that typically have a lot of cloud cover get less UVR than noncloudy equatorial regions.

As the human habitat has expanded out of Africa and over virtually the entire globe, humans have moved to a variety of UV regimes, and we have evolved accordingly. There’s no single optimal skin color; in a given region, the optimum level of pigmentation depends on the balance between keeping UVR-related damage and mortality to a minimum while maximizing vitamin D production in the skin. Jablonski made two key points: 1) Skin color is not a good indicator of race or genetic grouping because both light and dark pigmentation have evolved independently multiple times (light pigmentation evolved at least twice in humans, for example, and once in Neanderthals; dark pigmentation also likely evolved multiple times). 2) Skin color is an excellent subject for teaching people about evolution, being one of the best examples of evolution in humans, and easily visible to all as a part of everyday life.

The story is full of fascinating little quirks; for example, Tibetans, although they live at high altitude, are not as darkly pigmented as you would expect for the amount of UVR there, because humans moved into that part of the world relatively recently (I think she said it was within the last three or four thousand years), and they came with heavy clothes and structures that protected them from the sun. As a result, they don’t actually get a huge amount of UV exposure, and need to stay relatively pale to get their vitamin D. Another interesting tidbit is that in most human populations (if not all, my notes are unclear on this point, sorry), the women tend to be more lightly pigmented than the men. This may be related to women’s childbearing role; for example, a successful pregnancy requires quite a lot of calcium. However, sexual selection may also play a role in areas where men preferentially choose lighter-skinned women, exaggerating a difference originally caused by natural selection.

Jablonski also explained why some people tan more easily than others. It turns out that some populations, e.g., some of those that live around the Mediterranean, have developed the facility to regulate the melanin content of their skin relatively easily in response to environmental conditions; i.e., they tan easily. She also addressed the question of why the Inuit are as highly pigmented as they are, despite living at such high latitudes. They are exposed to a great deal of reflected UVR bouncing off the snow, so they need the protection from UV, and they also consume some of the most vitamin D-rich foods on the planet, namely, the blubber of marine mammals, so they can afford to miss out on some of the vitamin D production in the skin that lighter pigmentation would allow.

The questions after the talk were all good, in particular one about why there’s (almost always) a broader range of hair colors associated with lighter skin pigmentation than with darker. The reason has to do with the variety of genetic differences that can combine to create light skin color; lighter skin color can be associated with a variety of genetic combinations and thus hair colors. In areas where dark skin is advantageous, however, dark hair is also often advantageous, so the two traits tend to be linked. Jablonski described how we learned that dark hair can be advantageous in a sunny climate from studying thermoregulation in birds. The black feathers of crows, for example, are good at dissipating heat. This is counterintuitive to anyone who has ever had a car with a dark vinyl interior, but heat penetrates white plumage (and I’m guessing is then transmitted readily to the bird), whereas black plumage traps the heat and then dissipates it when a breeze blows or when the bird puffs up its feathers, as crows evidently do in hot weather.

It’s stories like this that make the natural world such a rewarding and engaging object of study, and I wholeheartedly endorse Jablonski’s recommendation that the story of skin color be used as an educational tool for anyone who is teaching about evolution. I think she was addressing mostly professional scholars and educators, but anyone, especially parents and others involved in the care and tending of young minds, can learn more and share this story. Some online resources include a recent NPR story and a story from Discover Magazine. Jablonski has also written a book, Skin: A Natural History, which has some information on skin color. I leave it to you to decide whether to sun your buttocks, but if you want to teach young people, or any people, about human evolution, I highly recommend that you consider using the story of skin color as a wonderful, accessible teaching story.