stimulates the production of melanin, which is why skin tans when it is exposed to the sun.
Scientists, anthropologists, and casual observers alike have noted that, historically, darker-skinned peoples were found in latitudes close to the equator. An obvious and convenient explanation was that humans living in these latitudes developed dark skin as a protective device against skin cancer and melanoma, both of which may be caused by exposure to ultraviolet, or UV, light found in greater quantities in equatorial regions. While this theory has its merits, as melanin does act as a sunscreen, on its own it doesn’t account for those dark-skinned populations who went north gradually losing colour. If the likelihood of developing skin cancer were the only factor in determining concentrations of melanin in a population, it wouldn’t necessarily be detrimental to have darker skin in an area with lower levels of UV radiation. Other factors must have been at play.
Recent research into the colour of human skin begins with the assumption that our earliest common ancestors’ skin was fair. This assumption is partly based on the fact that our nearest relatives in terms of species, chimpanzees, have light skin. At birth, young chimps have pink skin on their faces, the palms of their hands, and the soles of their feet. As they age, their skin freckles and darkens due to exposure to the sun.
It is widely believed that as our ancestors moved out of the jungles and onto the sun-exposed savannah and began walking and running long distances, the issue of over-heating became more pressing. Gradually, as these creatures evolved to become modern-day humans, they became less hairy, with an increased number of sweat glands to allow them to more easily regulate their body temperature in that harsher climate. Long, thick hair remained on their heads as protection against their brains overheating. However, with the loss of body hair, their skins were much more exposed to the sun and the risks that it brings.
This leads neatly to the theory that our earliest light-skinned ancestors gradually evolved to have dark skin as a protection against sun cancer.
For American anthropologist Nina Jablonski and her husband, George Chaplin, a geographic information-systems specialist, the sun—cancer theory didn’t go far enough in explaining variations in skin colour. Jablonski believed that, as most sun cancers occur in people after their childbearing years, the impact on reproductive success, and therefore on evolution, would be minor. This point was crucial in her search to find other factors that would explain the prevalence of dark-skinned populations closer to the equator and fairer-skinned populations in the north.
Since the 1960s, scientists have known that UV light converts cholesterol in the skin into vitamin D. They have also known that melanin inhibits vitamin D production in the skin, and so that lighter skin allows for greater absorption of UV light for vitamin D conversion than darker skin does. Vitamin D is essential for the absorption of calcium, which, as practically every Australian school child could tell you, is essential for strong, healthy bones and teeth. People with calcium deficiency run the risk of developing diseases such as rickets and osteoporosis. Rickets mainly affects the young, as their skeletons are developing, resulting in soft or deformed bones. Osteoporosis, a risk in later years, results in brittle bones.
While vitamin D is available from some foods, including fish oil, most of us get the bulk of our requirements indirectly from sunlight, which carries UV light to our skin. Although rickets is relatively rare in Australia, in 2001, the Medical Journal of Australia identified a new high-risk group for vitamin D deficiency: dark-skinned women and women who wear traditional Islamic dress.
These women, because of the role that melanin plays in inhibiting vitamin D production in the skin, and because most clothing absorbs ultraviolet B radiation (necessary for producing vitamin D), are susceptible to this deficiency, and so, too, are their children.
Jablonski was able, through access to a NASA database of measurements of UV radiation at the earth’s surface, to establish that the middle latitudes do in fact get more UV light. Here, where historically people have dark skin, there is sufficient UV to synthesise vitamin D all year round. In subtropical and temperate regions, where people with lighter skin but with the ability to tan are historically found, all but one month of the year provided enough sunlight to synthesise vitamin D; and in the third region, near the poles, on average, there is insufficient UV radiation to produce vitamin D in the skin.
From another study, Jablonksi came across a crucial link between successful reproduction and skin colour. This study showed that folate, a substance necessary to prevent neural-tube defects like spina bifida occurring in developing foetuses, is depleted in the human body when it is exposed to sunlight. Folate is also vital for the production of viable sperm. The lighter skinned a person is, the more rapidly folate breaks down within the body.
With this study, Jablonksi had found a link between factors for successful breeding and exposure to the sun. On the exposed plains of the savannah, darker skin would have meant more success at breeding, and so natural selection would have favoured those more swarthy individuals. Jablonski’s hypothesis was further strengthened when she came across a paper written by an Argentinean paediatrician who attended the birth of three babies with neural-tube defects whose mothers had all used solariums in the early stages of their pregnancies.
Because of the large amounts of sunlight available all through the year in those central latitudes around the equator, dark-skinned people living in these regions, despite the large amounts of melanin in their skin, were able to absorb enough sunlight for their vitamin D needs, while protecting themselves from folate depletion, sunburn, and skin cancer.
As populations slowly migrated north, the dark-skinned ancestors of what are now the fair-skinned northern Europeans found themselves at risk of developing vitamin D deficiency during the long, dark winters. Not only did they have to cover up more of their skin in order to keep warm, there were also less hours of sunshine in which to absorb UV rays to do the vitamin D conversion. Those with less melanin, and so able to convert more vitamin D, would have had greater reproductive success in the northern climes than those with darker skin. As a result, these populations would have gradually evolved to have lighter coloured skin.
Of course, for every theory there are exceptions, and in this case it is the Inuit people of the Arctic regions and Tibetans. Brown of skin and yet living up near the North Pole—where in mid-winter the sun barely rises above the horizon—how do the Inuit fit into Jablonksi’s theory? In evolutionary terms, the Inuit have only been living in the Arctic regions for a relatively short time: 6000 to 10,000 years. They also traditionally have a diet that is rich in vitamin D and calcium—all that fish and seal blubber. So give them another few thousand years and the Inuit will probably be much lighter skinned than they are now
As for the Tibetans, their skin is lighter than would be predicted by Jablonski’s theory. However, they, too, like the Inuit, have lived in their current location for less than 10,000 years and, because of low temperatures experienced on the Tibetan Plateau, they need to wear sufficient clothing to survive. Their lighter skin allows them to absorb more UV light for vitamin D conversion than if their skin was dark.
The process of tanning is one way that humans cope with the competing benefits and risks of exposure to the sun. In summer, darker, tanned skin provides some protection against sunburn and folate depletion; then, in winter, the tan fades to allow for more absorption of sunshine for vitamin D conversion.
So it would appear that the evolution of the different colour of human skin is the result of a complex balancing act to allow for the absorption of UV to produce vitamin D, the maintenance of sufficient amounts of folate to ensure viable sperm and prevent neural-tube defects, and the protection of the skin against cancers. Across all populations, men tend to have darker skin than women of the same racial groups. Jablonski ascribes this to the increased need women have for vitamin D when pregnant and breastfeeding.
But just when you think the whole skin colour thing has been sewn up, along comes another
