Evolution of Skin Color
I recently had the great pleasure of reading an article on the evolution of human skin color1. The biological history is an interesting topic, as the color of one’s skin has profound implications in our cultural and political systems to this day.
A look at the general distrubtion of skin color across the earth tells us something seemingly intuitive. The darkest skinned peoples are found near the earth’s equator, with lighter and lighter skinned peoples as you approach the poles. Like most people, you and I probably assumed that these colors were protection against skin cancer caused by damaging UV rays. Likewise, this was the assumption scientists had for quite some time.
It sounds good, but it doesn’t really work. The problem is that skin cancers are likely to show up and kill a person only after they’ve had plenty of time to leave behind offspring. If you’re leaving behind fit offspring, natural selection won’t magically cause a beneficial shift that leads to a longer or healthier life. This should also be inuitive. Think of how many diseases or ailments afflict us well after our reproductive years.
So if skin cancer is not a sufficient reason for the extra melanin observed in dark skinned peoples, what is?
The answer is the vitamin B folate (folic acid). Researchers observed that in light skinned peoples, UV rays break down this essential vitamin. This is particularly harmful to pregnant women, who have children with various neurological defects. Folate is also essential for DNA synthesis, so any cells that divide quickly (like sperm cells) need sufficient folate levels to function properly.
Why then do peoples become more light skinned toward the poles? The answer is the opposing selective force caused by another essential nutrient — vitamin D. Sufficient levels of UV rays are necessary to convert cholesterol into a vitamin D precursor in your skin cells. These precurser molecules are then further modified in your liver to their active vitamin D state, in a process I might have understood had I not been so depressingly hopeless in organic chemistry.
Along the equator where skin pigments are dark, UV levels are sufficiently high to produce the necessary levels of vitamin D. But where UV rays are insufficient, selection has favored the loss of dark pigments.
The authors propose that their finding also explains another observed characteristic of skin color. Women are lighter skinned than men. (They cite the difference at 3-4%). While sexual selection may have a part in this, they suggest its beginnings stem from women’s high calcium needs throughout their reproductive years. A good supply of Vitamin D is necessary for calcium absorption.
Most interesting are examples of people that do not fit the expected skin color from the latitudes they inhabit. Take the Inuit as an example. Not only did they recently migrate to their present area within the last 5,000 years, but their foods (mainly fish and marine mammals) are extremely rich in vitamin D. Thus they do just fine with darker skin at their extreme northern latitudes.
It’s a fascinating question - why do we look different when in truth (genetically), we’re so much alike. It’s answer, like so many in biology, lies in our evolutionary past.
More importantly, this story serves as a wonderful example of evolution’s importance. Without it, biology is largely reduced to memorization. These cells do that. That vitamin does this. It’s not that the cells and vitamins aren’t important (try living without them!), it’s just that without evolution, they lack their context and all perspective is lost.
And this is exactly the crisis we face in many of our schools. All perspective is lost.
1 Jablonski, Nina G., and George Chaplin. 2002. “Skin deep.” Scientific American 287 (4) (October): 74-82. (pdf format)
Wow, Jay, that’s interesting stuff. I’ve often wondered about that myself. So is this fact now, or just a theory?
Mary, perhaps like everything in science, this is an explanation that is considered far more useful in explaining the data than past understandings we’ve had on the subject. And this explanation is submitted for the approval of other scientists who work in these areas. The official stance in science on what’s true or not, is largely a consensus activity.
It’s so hard to get people who want absolute unchanging truths to understand. Like any understanding in science, the explanation for skin color is always up for review. One day someone may find problems with this interpretation that I’ve presented and offer a better explanation. Once again, it’s up to the rest of the scientific community whether the new explanation is found necessary or convincing.
So to answer your question more directly, we could call the distrubtion of different skin colors across the globe as facts — easily verifiable sets of data. Any explanation that seeks to explain these facts will always be a theory.
In this case, I think many scientists find the authors explanation convincing.
Nice post. I was aware of the Vitamin D story — including the additional factoid that cheeks, which are more exposed to light than other parts in polar regions, are pink because they are even more transparent — but not the folate side of things. Thanks.
I couldn’t agree more. As Dobzhansky summed it up for us in 1973, “nothing in biology makes sense except in the light of evolution”