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The evolution of skin color

Posted on: May 7, 2012   by  Brant Cebulla

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The variation and evolution in skin color and pigmentation are subjects that are still not fully understood or agreed upon. In a recent paper, Professor Nina Jablonski of Penn State University declares, “Skin color has been observed and studied by philosophers and scientists for over two millennia and yet there is still much about it that is not known.” But there are theories.

Jablonski NG. The evolution of human skin colouration and its relevance to health in the modern world. J R Coll Physicians Edinb. 2012 Mar;42(1):58-63.

Any theories regarding skin pigmentation have to consider evolution of both dark pigmentation under high UV radiation (UVR) conditions and evolution of depigmentation under low UV radiation conditions. As we all know, there is a patterned distribution of skin color all over the world, where the lighter your skin color, the further away from the equator your genome evolved.

For the evolution of dark skin pigmentation, there are four major hypotheses proposed to date. They are:

  1. Lowered mortality due to protection from sunburn and skin cancer
  2. Enhancing survival through camouflage in forested environments
  3. The antimicrobial properties of melanin in highly infectious environments
  4. Protection of folate metabolism against deficiencies caused by UVR

Jablonski argues that the last theory has the most merit. Folate deficiency interferes with normal development and causes birth defects, thus creating the survival need for adequate folate levels. While folate comes from diet, UVA exposure harms levels of folate’s main serum blood form, 5-methylhydrofolate. Darker pigmentation is able to absorb and scatter UVA and in result, lessen folate destruction. Therefore under high UVR conditions in Africa, it was necessary to have dark skin to maintain folate sufficiency.

On the other hand, there is still the question why skin pigmentation became lighter under low UVR conditions, in places outside the tropics. One theory is that light skin pigmentation evolved to permit easier vitamin D production in the skin.

The less melanin in the skin (lighter skin), the less your skin can “deflect” UVB, allowing your body to more easily produce vitamin D in response to sun exposure. In conditions where there wasn’t as much UVR as the tropics, the body needed to adapt to be able to produce an equal amount of vitamin D, despite having less UVB to work with. And so skin lightened.

This theory is not new, but perhaps gaining more merit now that research is discovering the importance of vitamin D beyond bone health.

In summary, skin pigmentation in modern humans in theory is the result of a dual need to both protect the body from UVR and utilize UVR for vitamin D production. This concept can be applied to sun exposure recommendations. Sun exposure is both beneficial and harmful. There is a dual need to both protect yourself from the sun by avoiding burning, but also expose yourself for vitamin D production.

2 Responses to The evolution of skin color

  1. Karsten

    Hi Brant,

    Interesting article, which reminds me of something I’ve been wondering about for awhile. Assuming that Vitamin D production has evolved to be regulated by sun exposure in order to protect us against skin cancer, that suggests that there must be some trade off involved in high vitamin D levels, even at the levels naturally attainable by long periods of exposure to the the sun. Otherwise, why wouldn’t the body just keep our vitamin D at those high levels at all times, regardless of the amount of sun exposure?

    While I’m aware that light skinned life guards can attain very high levels (100+ ng/ml?) of 25(OH)D, it seems to me that a tighter upper bound ought to be obtainable by observing the range of vitamin D levels for dark skinned individuals. Presumably the darkest Africans also display a range of vitamin D levels when comparing moderate and extreme UV exposures (or they wouldn’t have evolved a UV controlled production mechanism). In that case, it might be reasonable to assume that the 25(OH)D levels of a dark skinned individual who has been subjected to a long period of excessive sun exposure might be at a level that is on the balance detrimental for purposes not including the prevention of skin cancer. I wonder what that level would be.

    • Brant Cebulla

      Thanks Karsten. Some interesting points you have brought up. The lifeguard study found that 8 lifeguards had levels between 53-79 ng/ml. A recent study found that hunter gather tribes in Africa have levels between 23-68 ng/ml with an average of 45 ng/ml. Highest level to date on sun exposure alone is about 90 ng/ml.

      From an evolution standpoint, these are the levels the body adapted to; so in theory, if we knew nothing else about the way vitamin D behaved in the body, these would be the recommendations. Our recommendations (50 ng/ml, range of 40-80 ng/ml) reflect this. By your suggestions, this would be a little tighter (maybe 45 ng/ml, 30-65 ng/ml). We do feel these are good recommendations for the time being, until more research gives us a more “accurate” picture.

      Interestingly, however, is that it seems like the body can and does use more, if given. Topical vitamin D analogues are a good example. These activated vitamin D creams treat psoriasis and give cells more vitamin D-like-compounds than they would typically have to work with.

      Also relevant to this discussion is Vieth’s theory on declining 25(OH)D levels. He theorizes that falling 25(OH)D levels are associated with poor health outcomes and it is why u-shaped association findings are found at northern latitudes but not equatorial latitudes. This would suggest that our bodies do indeed find equatorial levels preferable to more variable, fluctuating levels at higher latitudes.

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