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28 July 2012

Why skin is different colours

Once, long ago, we were white and covered in hair. Then we were black and naked. Then we stayed naked and became chocolate brown, porcelain pale, and every shade inbetween.

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Once, long ago, we were white and covered in hair. Then we were black and naked. Then we stayed naked and became chocolate brown, porcelain pale, and every shade inbetween.

Why are humans so many different hues?

This question has long fascinated Professor Nina Jablonski, world-renowned biological anthropologist from Pennsylvania State Universtity, USA. She related the story of how we got our skin colour, as it is currently best understood by evolutionary scientists, to a rapt audience at the University of Cape Town on Wednesday.

The naked, many-coloured ape
If you look under the fur of our closest living relative, the chimpanzee, you'll find pale skin. And it's likely that the common ancestor chimps and humans shared, before the two species diverged and went their separate evolutionary ways, was also light-skinned and dark-furred.


Our early, forest-dwelling ancestors may have had pale skin covered
by fur, like our modern-day chimpanzee relatives.

But around 2 million to 1.5 million years ago, the early hominids who would eventually become Homo sapiens left the forests for the sun-baked savannah, and started striding around on two legs and developing big, temperature-sensitive brains – two physical changes that put us at risk of over-heating.

We cooled down by progressively losing our fur and gaining the ability to sweat profusely.

As we became naked apes, however, so our skins became exposed to the sun's rays. By around 1.2 million years ago, our ancestors in Africa (from which all members of the current human family most probably come), had acquired permanently dark skins to protect against the damaging effects of ultra-violet radiation in sunlight.

It's well known that high levels of sun exposure can damage the skin's DNA and lead to skin cancer, and for some time, scientists thought this was the evolutionary driver for dark skin, i.e. that those of our ancestors with paler skin would have been more likely to die of melanoma and consequently produce fewer paler-skinned babies.

But skin cancer takes a long time to develop, and usually only kills people once they are past reproductive age and have already produced offspring to carry on their genes.

It wasn't because of skin cancer that we evolved dark skin; it was because of folate.

Folate: essential, and sun-sensitive
Folate is an essential vitamin used to build DNA, and a deficiency in pregnant women can cause babies to be born with severe birth defects, especially of the nervous system. Folate happens to be very sensitive to UV radiation, and when it's carried in the bloodstream in capillaries near the skin's surface, sunlight can destroy it.

So we evolved a natural sunscreen: melanin. This pigment, which gives skin its colour, absorbs and blocks UV radiation. The more melanin you have, the darker your skin tone, and the better you can block UV and keep your folate intact for your baby developing in utero. Our ancestors with darker skins thus produced healthier, darker-skinned babies who were more likley to survive and produce darker-skinned babies in their turn.

For the next million years or so, everyone stayed dark brown.

The great trek from the tropics
Then, roughly 100 000 years ago, some of our ancestors decided to leave home in Africa and move North, and from there progressively spread all over the globe.


Some of our ancestors moved from Africa north to less sunny parts. Some then moved south again to areas like India, Australia and South America. (1-Homo sapiens. 2-Neanderthals. 3-Early hominids.)

The descendants of those who stayed in the intense African sun kept their protective darker skins.

But those who moved North of course met with less and less sunlight the further they moved from the equator. Here there was progressivley less risk of folate being destroyed by sunlight, but a new problem arose: not enough vitamin D.

The “sunshine vitamin”
Vitamin D, essential for the normal formation of bone and a healthy immune system, among other benefits, can be manufactured by our bodies in the presence of sufficient sunlight – specifically, UVB radiation.

As our ancestors moved to less sunny climes, those with darker skins were at a disadvantage because their high amounts of melanin meant they could not absorb enough UVB to produce Vitamin D. Without sufficient of the “sunshine vitamin”, the female pelvic bones form abnormally, often not allowing for successful birth. Immune systems weakened from lack of Vitamin D also meant more, and earlier, deaths from disease.

Thus paler skin became a selective advantage for those further from the sun. Those really far from the sun – at latititudes further than around 52 degrees North and South – became palest, but still needed to get enough Vitamin D from diet, in the form of oily fish or dairy products.

This de-pigmentation of skin is thought to have occurred several times in our evolution. As has re-pigmentation: when early humans who'd moved north and lost melanin moved again back down to sunnier lattitudes they developed darker, UV-protective skins once more. This same mechanism has happened in many different parts of the world.

Modern health implications
Even as recently as a few hundred years ago, most people didn't venture far from their place of birth. But as transportation became more efficient and rapid, more people moved (or were moved, by the slave trade) far from their ancestral homelands.

These days, we can whizz to wildly different latitudes just for a vacation.

There are some serious implications for skin tones evolved over millenia. Pale skin at low latititudes is at serious risk of skin cancer, and requires conscientious protection from the sun, especially in summer.

At high latititudes, there's a risk of Vitamin D deficiency for those with darker skins – but even for those with lighter skins if they spend much of their time indoors, as is the norm now in big cities.

Covering the skin with clothing for warmth also lowers exposure to the little sunshine there is in cold climates. But covering the skin for cultural reasons also has health implications: for example, Islamic women who wear burqas have a higher rate of osteoporosis and other health issues related to vitamin D deficiency.

Olivia Rose-Innes, EnviroHealth Editor, Health24, July 2012

References
Jablonski, N. (2012). Skin Pigmentation: Its evolution and meaning in the modern world. Darwin lecture series, University of Cape Town.

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Preventing disease with folate

 
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