Geography and Skin Color

Professor Jared Diamond, of UCLA, has written a review in the journal Nature regarding human skin color. The usual explanation for why humans have different skin color had to do with geography. Those populations living closer to the equator had darker skin to protect them against sunlight. This view has many exceptions, however, and was therefore incomplete. More recent studies have shed light on the subject (no pun intended).

A couple of scientists, Jablonski and Chaplin, identified new criteria to gauge what skin color should be based on geography. Instead of using latitude, they examined ultraviolet radiation (UVR) on the earth's surface. While it is true that UVR is less at higher latitudes, there are exceptions to this. High altitudes tend to have more UVR even at high latitudes, and UVR decreases with increased water vapor due to clouds, rain, or humidity. So, simply looking at latitude is not enough.

What Jablonski and Chaplin found was a series of trade-offs to UVR exposure. UVR is harmful in that it breaks down many compounds in the body (through photolysis). Of particular importance is vitamin B folate. A positive consequence of UVR is the synthesis of vitamin D, which everyone requires. So, skin color evolved to balance these two physiological properties.

Again, there are exceptions, but they are understood and make sense in light of this balance. The Inuit have dark skin despite living in high latitudes. This was due to the fact they consumed marine mammals, rich in vitamin D, and therefore selection had no need to push them in the direction of lighter skin. Modern Inuit now suffer severe vitamin D deficiencies since their subsistence strategies have changed (i.e. supermarkets for food).
Similar examples are provided, where human migration of some populations has not given them time to evolve skin colors appropriate to their location. But there are examples that cannot be explained just by the vitamin trade-offs, such as the Tasmanian aborigine's. Here, the authors link to sexual selection. Skin provides a visual cue to someone's age, health, and ancestry, not to mention a place for decoration. This might override the selective pressure to evolve skin tones better adapted to the location, especially if diet provides the necessary compensation.

It is scientific studies such as these that demonstrate how ridiculous racism is, and the arguments for lesser or inferior races. Hopefully, as Diamond points out, more researchers will tackle the issue of skin color so that we might better understand our origins and distribution. Diamond, J. 2005. Geography and skin colour. Nature 435: 283-284.

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