Excerpt from The Genetics of Health,by Sharad P. Paul

Humans and their closest living relatives, apes like chimpanzees and bonobos, were separated from their common ancestor around six million years ago, and with this severance came the differentiation into many species of humans or apes. This common ancestor was covered with dark hair and had light skin underneath. Around four million years later, or two million years ago, Homo erectus migrated out of Africa.

By now, this ancestor had a larger brain and more developed hand skills, but this brought with it certain problems. The body had to work harder to keep the brain cool (just as a computer needs a cooling fan) as physical activity and the use of hand tools increased. Although Homo erectus was not the first hominid to walk upright, he walked faster and migrated farther than other hominids, which also required cooling. To cool, he needed to sweat. To sweat properly, he required many sweat glands and a naked body. Aboriginal people—such as indigenous Australians or the Negritos who inhabit the Andaman Islands in India or the Philippines—have particularly high concentrations of sweat glands when compared to people unacclimatized to hot weather.

Apes, like chimpanzees, have black hair but light pinkish skin. As man evolved from apes, he lost the thick hair and emerged rather naked. This evolution into a naked ape necessitated a darkening of the epidermis to protect against UV damage. Skin cancers, wrinkling, and signs of aging typically spring to mind as the worst effects of UV damage. But issues like cancer or beauty are not as important from an evolutionary point of view; in all its wisdom, Nature knows that beauty doesn’t last, but a species has to. The scientific explanation for the darkening of skin of our ancestors in Africa is that the sun causes photolysis, the destruction of folic acid (folate), a metabolite essential to preventing birth defects. Therefore, to preserve folic acid, and thereby the species’ reproductive ability, skin evolved to become dark.

How does skin darkening—that is, increased melanin production— protect against UV damage? As a general rule, for solar radiation to produce biochemical damage, it needs to be absorbed. Melanin acts as an optical and chemical filter that reduces penetration of all UV wavelengths into subepidermal tissues. As an optical filter, it works through a scattering effect. This is made apparent by the fact that there are very few instances of folic acid deficiency in Africa, even when nutrition is relatively poor, pointing to highly melanized skin protecting against UV-induced folic acid damage.

It is no coincidence that neural tube defects, such as spina bifida and anencephaly, which are both caused by a deficit in folic acid, are far less common in the tropics, among dark-skinned people, than they are in the more temperate climates. It is also no coincidence that countries in Asia and Africa have larger populations when compared to Western countries, even with their poorer living standards and the fact that less healthcare is available during pregnancy, because reproductive issues are not as prevalent due to folic acid issues. When I graduated from medical school in India twenty-five years ago, medical doctors were not trained to routinely advocate folic acid for women planning to conceive, whereas it was already routine in Western countries, due to the much higher risk of neural tube defects.

Sunlight destroys folic acid, but it produces vitamin D, which the body needs in order to absorb calcium. Therefore the darker skinned you are, the higher your inherent folic acid levels, but correspondingly you end up with a reduced capacity to absorb vitamin D.

As the first modern humans migrated out of Africa into Europe one hundred thousand years ago, their skin lightened to allow more sun penetration to facilitate the production of vitamin D, because of the lower annual sunshine hours than in Africa. In colder, polar climates, it was even more important to produce enough vitamin D, as the sun was absent for long periods. But the lack of sun created its own problems. When dark-skinned people arrived in Europe, many developed rickets due to vitamin D deficiency, as dark skin was not transparent enough to absorb vitamin D easily.

Rickets does not only cause skeletal problems and bone deformities, but also infertility. This was a double whammy, both from a sexual selection and a natural selection point of view. Deformed individuals were less likely to be chosen as mates, and infertility ultimately bred them out. Skin, therefore, lightened to enable enough vitamin D production. There were good reasons for dark skin in Africa, but in European climes, it just did not make sense.

In dark skin that evolved in Africa, vitamin D exists in the form of vitamin D3, a precursor to vitamin D. This is the beauty of evolution’s anticipatory intelligence. As skin gradually became dark in Africa over millions of years, the body could foretell a shortage of vitamin D that would occur due to the darkness of skin and produced high levels of preformed vitamin D.

Later, as white-skinned people migrated east from Europe, reached the Indian subcontinent, and ventured farther south into Asia, to more tropical climates, the skin again darkened to preserve folic acid. However, this darkening came after an initial period of lightening in Europe and therefore happened over thousands of years, as recently as four thousand to ten thousand years ago. Therefore, the body did not have time to hoard pre-vitamin D, as this darkening of skin was an adaptive response to preserve folic acid under the tropical sun, not an evolutionary response as had occurred in Africa. Therefore, Asians, especially those from the Indian subcontinent, have very low vitamin D levels.

To confirm this evolutionary theory, Martine Luxwolda studied vitamin D levels in African populations, especially in locations where early man originated and where the population was still on a traditional diet. Although she knew she would find high values, it was a surprise that the values were quite so high. Luxwolda says:

The present study indicates a mean 25(OH)D concentration of 115 nmol/l, as based on traditionally living populations with sun exposure habits that might be comparable to our African ancestors before the out-of-Africa diaspora.

In their study of the “traditional people” in Tanzania, Luxwolda and her team found that this population had 115 nmol per liter (nmol/ l) of vitamin D (more specifically, serum 25-hydroxyvitamin D) on average compared to the 30–60 nmol/l that Westerners had. In contrast, virtually all Indian subcontinental folk are vitamin D deficient. And because vitamin D helps muscle strength and injury recovery, the more inherent vitamin D, the better natural athletic prowess. This is why Africans and African Americans excel in athletics and may explain why India, with over a billion people, has never won even one Olympic track-and-field medal!

Thus, while the original dark color of skin in Africa was evolutionary (to preserve folic acid), once early humans had lost much of their body hair, other dark skin colors that formed were adaptive, depending on their environment. Quoting Charles Darwin again, in The Descent of Man:

Of all the differences between the races of man, the colour of the skin is the most conspicuous and one of the best marked. It was formerly thought that differences of this kind could be accounted for by long exposure to different climates; but Pallas first showed that this is not tenable, and he has since been followed by almost all anthropologists. This view has been rejected chiefly because the distribution of the variously coloured races, most of whom have long inhabited their present homes, does not coincide with corresponding differences of climate.

One of the discrepancies with the evolutionary theories based on the sun as a double-edged sword when it comes to vitamin D and folic acid is that we would expect the Inuit people to be lighter skinned than the Scandinavians, but they are darker skinned. This is where diet played a part in the adaptive response.

The Inuit diet is extremely rich in vitamin D due to consumption of salmon and oily fish. The European diet, in contrast, was cereal based, and so vitamin D-deficient. Over time, European skin had to adapt and become lighter to easily absorb vitamin D, whereas Inuit skin did not: cod liver oil, for example, has around 1,200 IU of vitamin D per tablespoon, and salmon has around 300 IU per ounce. If you compare that with European diets, cereals (if unfortified) contain no vitamin D and cheese only has around 4 IU per ounce. The Baltic countries—Latvia, Lithuania, and Estonia—have among the palest-skinned people in Europe because early settlers found that the warmer climates allowed them to farm grain, whereas neighboring countries like Norway and Iceland ate salmon, whales, and sharks and so had higher vitamin D levels. In my TEDx Talk, I pointed out that this was the scientific explanation behind Miss Norway having a better tan than Miss Estonia at a Miss World pageant (of course all in the interest of scientific research!).

Diet also can explain the caste system in India. The upper class Brahmins are vegetarians and therefore eat a largely cereal- and dairy-based diet. People who eat fish, such as fishermen, who were considered lower castes, had higher vitamin D levels, and over centuries, the skin of the upper castes lightened to try and absorb more vitamin D, a process that takes several centuries.

While hominids appeared 2.4 million years ago, descendants of modern man left Africa only one hundred thousand years ago. However, evolutionary responses take much longer. This is why African people in Luxwolda’s study had higher vitamin D levels than Europeans even though they did not avoid sun or stay indoors like modern Asians. The evolutionary clock runs much slower than that.

So the emergence of different skin colors was essentially a tale of two vitamins—folic acid and vitamin D. We now know why skin color changed and what causes the different shades: melanin. But what were the genes that mediated this? Genes control the entire flow of hormones, vitamins, and enzymes.

There is an important skin color gene called SLC24A5, which was a major factor in color change between African and European populations. SLC24A5 is essentially a transporter (specialized proteins that facilitate transport of certain substances across cell membranes) that, in humans, is encoded by the corresponding SLC24A5 gene. While researchers are unsure how this actually regulates skin color, it seems to have something to do with the movement of calcium into cells, and we can now see the link between vitamin D, calcium, and sunlight. The SLC24A5 gene comes in two variants, dark and light. People with two dark versions (DD) are black, and people with two light versions (LL) are white—for example, people with DD would have been more likely to develop rickets when compared to those with LL versions of the SLC24A5 gene. Of course you could have combined versions and end up brown skinned.

Fish like sticklebacks often change color when they move from freshwater rivers, estuaries, and saltwater as a form of camouflage. Such changes are mediated by another gene called KITLG. It transpires that humans with two copies of the African form of the KITLG gene have darker skin color, when compared to people with one or two copies of the new KITLG variant that is common in Europe. This makes the KITLG gene one of the key regulators of skin color adaptation in both fish and humans. The change in color is regulated by the same gene in different animals, from the lowly stickleback to modern humans. This is the real “dermocracy” of skin color. What it tells us is that evolution and skin color are not merely words we encounter in a book or museum; they are glimpses into the story of our natural origins.

When nature selected these two vitamins to do battle, it perhaps forgot to reconcile with ignorant human minds—the evolutionary call was a duty, and when the order came to propagate a species, skin responded. After all, biology has no bias; humans do.

Learn more about  The Genetics of Health.

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