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.
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:
- Lowered mortality due to protection from sunburn and skin cancer
- Enhancing survival through camouflage in forested environments
- The antimicrobial properties of melanin in highly infectious environments
- 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.