Shades of Skin

As a freshman high school student, I remember this question popping up during the physics class on radiation:

When black catches heat faster and leaves faster (opposite to white),why didn't people who live in hot areas and especially work under sun didn't evolve to have lighter skin to avoid sun strokes, and it only becomes only darker under the sun?

My physics teacher did not know the answer to that. He asked the questioner to consult the biology teacher, which in all probability was forgotten. High school kids have short attention spans.

As I grew older and delved into the subjects of evolution, pathology and anthropology, I have found the answer to the question. And to that high school kid from 7 years ago, I write this answer.

You are indeed correct. The colour black absorbs radiation better and emits radiation better, while white is a great reflector. But you are correct from only a physical point of view. This, however, is a physiological dilemma.

Although darkly-pigmented skin absorbs about 30 to 40% more sunlight than lightly-pigmented skin, dark skin does not increase the body's internal heat intake in conditions of intense solar radiation. Solar radiation heats up the body's surface and not the interior. Furthermore, this amount of heat is negligible compared to the heat produced when muscles are actively used during exercise.

Evolutionarily, increased body temperature from long wavelength radiation of the sun causing heat strokes has been the least of our concerns. Humans are excellent at maintaining a constant body temperature and losing excess heat. Fur traps heat— so we lost our body hair. We have high performing sweat glands in millions, which are among the best in the animal kingdom. The blood vessels under the skin dilate to increase blood flow, so that the excess heat is carried by the blood from the muscles and lost through the skin. That is why our skin feels hot after exercise.

Combining all these factors, why this dude—

—became the top dog in his food web is clear.

We can outrun almost any animal in the animal kingdom except maybe horses. No, not in speed. Endurance. There are many animals which are far faster and nimble than us. But they can do that only for short bursts— they have no way of losing the heat they built up except sit in a shade and pant. But us humans? We can keep on going for immense distances before tiring. The prehistoric hunter gatherer could track his prey over long distances and tire it out before putting it down.

So now that we have cleared how magnificently efficient our heat loss systems are, lets move over to why people of tropics or those who spend longer time under the sun have darker skin.

Dark skin is caused by the pigment present in skin, melanin (especially eumelanin). Melanin is produced in specialized cells called melanocytes, which are found at the lowest level of the epidermis. Melanin is stored inside small membrane-bound packages called melanosomes. People with naturally-occurring dark skin have melanosomes which are clumped, large and full of eumelanin. For the same body region, light- and dark-skinned individuals have similar numbers of melanocytes (there is considerable variation between different body regions), but pigment-containing organelles, called melanosomes, are larger and more numerous and more melanin rich in dark-skinned individuals.

Why do sun exposed people have more melanin?

Ultraviolet radiation. Specially UVB.

UV radiation is extremely damaging in large amounts, especially the amount it is present in tropical regions. UV rays cause folate deficiency in man, which is manifested as extreme fatigue, lethargy, muscle weakness, peripheral neuropathy (tingling in extremities) and anemia. Folate deficiency in pregnant mothers lead to neural tube development defects in embryos.

UVB light also causes DNA damage― binding thymine base pairs next to each other in genetic sequences to bond together into pyrimidine dimers, a disruption in the strand, which reproductive enzymes cannot copy. It causes sunburn and increases skin cancer risk over time.

But UV is not all bad either. At lower concentrations, UV interacts with a molecule called 7-dehydrocholesterol in the skin, converting it into vitamin D3, the active form of vitamin D. Vitamin D promotes calcium absorption in the gut and maintains adequate serum calcium and phosphate concentrations to enable normal mineralization of bone. Without it, you’d have been riddled with rickets and osteoporosis and bent like a bow with tetany.

Melanin present in dark skin acts as a regulator of UVB. By absorbing the right amounts of UVB it maintains a steady enough flow of UV to maintain vitamin D production but low enough not to damage DNA and folic acid. A four-fold difference in naturally-occurring dark skin gives seven to eight-fold protection against DNA damage.

To track the history of light and dark skins, lets take a little trip. When the light-skinned early hominids migrated from the shadowy tropical rain forests to the African Savannah, there was an evolutionary pressure on them to develop darker skin. By the time Homo sapiens came to existence, they were all universally dark skinned. When humans migrated to the colder temperate climates with lower UVB radiation content, there was an opposite evolutionary pressure to develop light skin to oppose vitamin D deficiency. You could argue that native tribes of the arctics, like the Inuits and Eskimos have darker skin despite living in extremely cold UV poor regions, but you need to consider the fact that their primary diet is seafood— fatty fish rich in vitamin D. As such, they never felt the pressure of evolution forcing them to develop lighter skin.

There is also a hypothesis that higher melanin content in the skin acts as defence against fungal and parasitic infections, which are rampant in the tropics— but its credibility remains disputed. (Melanin is an antimicrobial agent in many insects though.)

And that is why sun loving folks have darker skin. Evolution is fun, isn’t it?