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Original Skin. Maryrose Cuskelly
Читать онлайн.Название Original Skin
Год выпуска 0
isbn 9781582438856
Автор произведения Maryrose Cuskelly
Жанр Биология
Издательство Ingram
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 theory. In 2001, an Australian biologist, James Mackintosh, showed in his PhD that melanin acts as an antimicrobial agent in insects. Melanin is quite a ‘sticky’ molecule, to the extent that it can clog up micro-organisms so that they find it difficult to proliferate. Mackintosh went on to suggest that this may be why, in humans, individuals with dark skin are less likely to suffer from serious skin diseases.
During the Vietnam War, when American soldiers were slogging through the hot, humid region of the Mekong Delta, white-skinned soldiers were three times more likely to develop tropical ulcers than their black-skinned comrades. So the enhanced capacity of black skin to protect against disease might be an additional reason why people in the tropics developed the capacity to produce more melanin. This might also help to explain why large amounts of melanin are found in areas such as the throat, the nasal passages, and the genitals—areas of the body that are rarely exposed to sunlight. So, rather than correlating with latitude or exposure to the sun, dark skin may be linked to temperature and humidity. But what about those theory-bending Inuit? They still seem to be the exception.
Putting all this information together, the colour of our skin probably results from a combination of various evolutionary responses to sunlight, vitamin D, folate, and the antimicrobial properties of melanin. Skin colour has nothing to do with intelligence, integrity, or temperament, but is determined by a handful of genes. So can racism (based on skin colour, at least) finally be relegated to the past as a shameful and erroneous belief that we, as a species, have finally grown out of? Perhaps eventually it will be taken out of our hands altogether. In 2006, The Age newspaper reported on the predictions of Dr Oliver Curry of the Darwin@LSE research centre at the London School of Economics, who said that by the year 3006, most humans, due to our increasing interconnectedness, will have a similar brown skin tone.
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