2010/07/20

Why are we different colors?

The most striking difference among human populations, or “races” (a loaded term that I discuss in WEIT), is skin color.  As we all know, and as the following diagram shows, populations from tropical areas have darker skin than do populations from higher latitudes:

Fig. 1.  Distribution of skin pigmentation among human populations (from Encylopedia Brittanica)

What causes this variation?  Various theories have been suggested, including sexual selection for different skin tones in different places, but because of the strong correlation between pigmentation and amount of sunlight, biologists have suggested that pigmentation differences are a result of natural selection imposed by different light levels in different places.  Here’s the amount of light (measured as total annual UV) in different areas of the world:

Fig. 2 Annual UV exposure on Earth (from NASA via Natural Frequency).

The idea that pigmentation evolved as an adaptation to light levels is supported by the observation of facultative changes in pigmentation that occur with higher exposure, that is, tanning.  Light-skinned people get darker when exposed to more sun.

These differences among populations almost certainly represent more than one evolutionary event.  First, although we don’t have fossil skin from our African hominin ancestors like Homo erectus, it’s likely that it was dark, as are populations there now.  But even earlier ancestors may have been lighter. If you look at our closest relatives, chimps and gorillas, you see that their skin (at least those parts under the hair) is unpigmented.  Only the exposed parts are pigmented.  The ancestral color of humans, then, was probably light, and then, as we became “naked apes,” evolved to a darker shade. (The evolution of hairlessness in our species is another matter, perhaps involving our ability to sweat.)

Then, as the presumably dark populations of humans moved into the Middle East and Europe, they evolved lighter skin color.  But when those populations colonized Australia, skin color got dark again. This almost certainly happened, too, when humans moved from northern Asia across the Bering Strait and down into the Americas: those populations that reached Central and South America likely re-evolved dark pigmentation.

What were the selective pressures that caused these changes?  For a long time I accepted the “classic” story that was taught in school:  populations getting more sunlight evolved darker skin as protection against UV-induced melanomas and the toxic effects of too much vitamin D3, which is produced only by sunlight striking the skin.  In low-light areas, skin evolved a lighter shade because we need fair amounts of vitamin D3 to build strong bones (without it, children get rickets, which is why foods like milk often have added vitamin D).  Thus, dark-skinned ancestors in the tropics would have reduced vitamin D toxicity and fewer melanomas, while lighter ancestors in the temperate zones would have stronger bones.  This could cause differential mortality or reproduction that would explain the differences in pigmentation.

The problem with this story is just that—it’s a story.  Although this scenario sounds plausible, there wasn’t much hard evidence supporting it, at least not when I was in school.  A recent paper in PNAS by Nina Jablonski and George Chaplin summarizes the latest evidence and comes to some different conclusions about the evolution of human skin color.  Their findings:

Why tropical populations are darker. Probably not because of melanomas. The authors dismiss this:

Sunburn and skin cancer have negligible effects on reproductive success (7, 18). Nonmelanoma skin cancers are common in older individuals from modern lightly pigmented populations inhabiting sunny climes, but they are rarely fatal or incapacitating (20). Melanoma afflicts younger individuals and is often fatal, but it is much rarer than nonmelanoma skin cancers.

They also point out that most human skin cancers result from light-skinned individuals moving to the tropics, something that wouldn’t have occurred in our ancestors.  They conclude that “the effects of skin cancers on reproductive success in humans today are modest, and were probably statistically inconsequential before rapid, long-distance travel and migration.”

Jabonski and Chaplin also criticize the idea that too much vitamin D was another selective force: they say that “overproduction of vitamin D was refuted as the primary cause of the evolution of dark pigmentation by the discovery that hypervitamosis D due to sun exposure is physiologically impossible because of photochemical regulation.”

This was all news to me, and I was glad to hear it.  I won’t be telling my students so much about melanoma and vitamin toxicity when discussing the evolution of skin color.

If it’s not skin cancer or hypervitaminosis, then, what were the selective pressures? Jablonski and Chaplin suggest that it’s the quantity of folate (folic acid), one of the B vitamins that plays a crucial role in biosynthesis, including DNA synthesis and repair. In a paper published in 2000 which I had not previously seen (citation below), the authors floated the idea that because folate can be destroyed by sunlight in the blood vessels of the skin, the skin evolved to be darker to keep folate from being destroyed.  In the present paper, they specifically mention that an absence of folate could cause neural tube defects (NTDs) in human embryos, serving as a potent selective pressure.

I find the evidence for this theory intriguing but a bit thin.  The authors say that there is evidence of a “protective effect of dark pigmentation against folate depletion,” but looking up the studies cited I see only a correlation between “racial” origin and NTDs.  This might be due to factors other than pigmentation.   The authors seem on stronger ground when claiming that folate protects against neural tube defects, several studies show that supplementing women’s diets with folic acid leads to a significantly lower production of NTDs in their offspring.

Why populations experiencing less solar radiation are lighter. Here the authors pretty much accept the idea that selection is based on the need for vitamin D3, which not only builds bones, but plays a role in the immune response, cell proliferation, and functioning of the brain, heart, and pancreas.  Since folate is produced in the epidermis and dermis, it seems plausible that the skin got lighter to ensure adequate amounts of this important vitamin.  But again the evidence is not conclusive.  The strongest, cited in the 2000 paper, is that recent dark-skinned migrants from tropical to temperate areas (Ethiopia to Israel, India to the UK) tend to suffer from vitamin D deficiencies.

Why there is tanning? Jablonski and Chaplin suggest that tanning was an adaptation that evolved not to protect long-distance human migrants (since they didn’t exist in our ancestors), but to protect populations at intermediate latitudes where there is much greater seasonal variation in the amount of UV.  Seasonally variable pigmentation would be useful in protecting against folate depletion in high-UV seasons while allowing vitamin D synthesis during periods of low UV.

The work of Jablonski and Chaplin is intruiging, I’ll certainly be telling my students about the “folate hypothesis,” as well as about the evolution of tanning.  But I’ll insist that these ideas are tentative.  Studying the adaptive significance of human racial variation is a difficult task for two reasons.  First, we can’t do experiments on humans, except for medically-related ones like giving pregnant women folic acid.  We certainly can’t move people wholesale from place to place and look at the connection between their traits and their fitness, though we can in some cases take advantage of fortuitous migrants. In fruit flies, which are larger in more northern areas than in tropical areas, we can actually do experiments, and show that large body size evolves in the laboratory when we rear flies under cold conditions.

The other problem is that traits distinguishing human populations evolved a long time ago (between 60,000 and 10,000 years ago, when humans moved out of Africa and colonized North America and Australia), and we can only speculate about selective forces that occurred so long ago. (That said, the geographic distribution of UV light hasn’t changed much since then!)

It’s curious, but understandable, that we know so very little about the evolution of population differences in our own species.  We do know a bit about skin color, but as for nose shape, hair texture, eye folds, and body build—the other traits that distinguish human populations—our ignorance is deep.

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Jablonski, N. G. and G. Chaplin. 2010.  Human skin pigmentation as an adaptation to UV radiation.  Proc. Nat. Acad. Sci. USA 107(supp.):8962-8968.

Jablonski, N. G. and G. Chaplin. 2000.  The evolution of human skin coloration. J. Human Evolution 39:57-106.

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