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| Tuesday, 17 February 2004 |
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Why are there so many shades of human skin? by Derrick Schokman A friend sent me this hypothetical letter from a coloured person to a white person: "Dear white fella, The dermatological context of this letter prompted me to enquire why there are so many different shades of skin between black and white among the indigenous peoples of the world. It cannot be a random distribution of skin colour because it is quite obvious that people near the equator are darker complexioned than those closer to the poles. There has to be a rational explanation. There is. Current knowledge indicates that variations in skin colour like most of our physical attributes, can be explained as an adaptation to environment through natural selection. Skin cancer When humans gradually evolved from primates they lost body hair and developed melanin in the skin that gave them a dark complexion. Melanin protected them from the damaging effect of the sun's UV rays which could cause skin cancer. When humans began to venture out of Africa and the tropics into cooler environments, they found they did not need so much of this 'sun screen', and accordingly their dark skins containing high concentrations of melanin gave way to less melanin and lighter skin. That was what biologists and anthropologists believed at first. But since all skin cancers typically arise in later life after the first reproductive years, it did not seem likely that they could have exerted enough evolutionary pressure to account for differences in skin colour. There had to be another reason. Folate More recent epidemiological and physiological evidence has suggested that dark skin also acts as a protection against the destruction of the body's stores of folate by the sun's UV rays. Folate is essential for the synthesis of DNA in dividing cells and anything that involves rapid cell proliferation, as for example in spermatogenesis. In other words folate is essential for fertility and foetal development. Although UV rays are generally damaging they have one positive benefit in initiating the formation of vitamin D in the body via shorter wavelength UVB radiation. Vitamin D is necessary for the normal development of the skeleton and the maintenance of a healthy immune system. Dark skinned people living in the tropics receive enough UV radiation during the year to allow UVB to penetrate the skin and make vitamin D. This is not the case outside the tropics where there is less radiation and a dark skin can block whatever sunlight is necessary for catalysing vitamin D. Accordingly humans such areas in evolutionary time have adjusted to having lighter skins, which would enable them to make enough vitamin D and yet protect their stores of folate. This is why people living in the tropics generally have dark skins, whereas people in the subtropics and temperate regions are lighter skinned but have the ability to tan. And those living near the poles tend to be very light skinned and burn easily. Migration There are exceptions. Technological development has turned our world into a global village in the sense that people are now able to migrate easily to different countries with different levels of UV radiation, causing health problems that did not exist before. For instance light skinned northern Europeans who bask in the sun in Florida or on the northern Australian coast increasingly pay the price in the form of premature ageing of the skin and skin cancers (basal and squamous carcinoma). Conversely dark skinned South Asians or Africans settling in Northern UK, northern Europe or NE USA can develop rickets and diseases related to vitamin D deficiency because of a lack of UV radiation. These are insidious problems, but scientists say that we are seeing more of them today. Women The potential to synthesise vitamin D in relation to human skin colour could also explain why women in all populations are generally lighter skinned than men. It could be argued that this has resulted from a process of sexual selection over evolutionary time when men have shown a preference for lighter skinned women. That, say scientists, may be a part of the story, but more importantly women could have a greater need for calcium throughout their productive lives, especially during pregnancy and lactation. They therefore need to be lighter skinned than men in order to allow enough sunlight to penetrate the skin and make the vitamin D which, in turn, is necessary to produce calcium for maternal and foetal bones. What it all boils down to ultimately is that throughout the world human skin colour has evolved to be dark enough to prevent sunlight from destroying nutrient folate, but light enough to foster vitamin D production. It is a reasonable explanation. |
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