[dingdong (OP)]

geoffrey mcgilvray  asked the Naked Scientists:
   
Dear Chris,

Why is skin black in a sunny country and white in a cold climate? It can't be a "mistake by nature" so why is it the wrong way around.
                         
Regards,
Geoff

What do you think?

[Bored chemist]

It's the right way round.

The black pigment in the skin protects the underlying skin form the sun's UV light in sunny countries.
In cooler places where there's less sun, not only is there less need for protection, but a small amount of UV getting through helps with making vitamin D.

[dingdong (OP)]

IF YOU ARE IN A SUNNY COUNTRY PEOPLE WEAR WHITE CLOTHING TO REFLECT THE SUN,CARS PAINTED IN DARK COLOURS SOON OXIDISE AND IF YOU PUT YOUR HAND ON THEM YOU BURN.SO HOW DOES BLACK SKIN PROTECT THE BODY WHEN IT GETS HOTTER THAN WHITE.SURELY THE SKIN UNDER THE MELANIN WILL GET HOTTER BUT IF MELANIN BECAME WHITER IN THE SUN WOULD THAT NOT BE A BETTER IDEA .

[chris]

It's nothing to do with reflection, it's actually all to do with absorption - black skin defends against folate deficiency in sunny countries by soaking up UV, which degrades folate. White skin, on the other hand, defends against vitamin D deficiency, which can manifest amongst dark-skinned people living under conditions of insufficient sunlight.

Chris

[SteveFish]

Very light colored skin with a pretty good tan has only a SPF of 3. A dark skinned individual has a SPF of around 15.

[granpa]

If you owned a plantation in the tropics and
you needed to buy some servants/slaves to work on it.
Are you going to buy lighter skinned individuals who
are going to get sun burned and not be able to work
or are you going to buy darker skinned individuals who
wont get sun burned as much?

It looks to me like dark skin evolved at least twice.
Once in Africa and once in India

[Bored chemist]

If you owned a plantation in the tropics and
you needed to buy some servants/slaves to work on it.
Are you going to buy lighter skinned individuals who
are going to get sun burned and not be able to work
or are you going to buy darker skinned individuals who
wont get sun burned as much?

It looks to me like dark skin evolved at least twice.
Once in Africa and once in India

Dark skin was around before humans. How many times it evolved I'm not sure but I think all mammals use melanin as their sunscreen. If so it's convergent evolution or the dark skins evolved before the mammals separated from the rest of the animals.
White skin in humans is a relatively recent invention and presumably evolved somewhere in the North of the European / Asian landmass.

[granpa]

If you owned a plantation in the tropics and
you needed to buy some servants/slaves to work on it.
Are you going to buy lighter skinned individuals who
are going to get sun burned and not be able to work
or are you going to buy darker skinned individuals who
wont get sun burned as much?

It looks to me like dark skin evolved at least twice.
Once in Africa and once in India

Dark skin was around before humans. How many times it evolved I'm not sure but I think all mammals use melanin as their sunscreen. If so it's convergent evolution or the dark skins evolved before the mammals separated from the rest of the animals.
White skin in humans is a relatively recent invention and presumably evolved somewhere in the North of the European / Asian landmass.

That is an assumption.


chimpanzees are born white and become black when they get older.

http://en.wikipedia.org/wiki/Neoteny

[thedoc]

We discussed this question on our  show
 Diana -  Interesting question. A lot of people think that it’s due to vitamin D production. So, what happens is you get UVB emitted by the sun and that goes into your skin, and your skin makes vitamin D. The darker your skin is, the less UVB can penetrate into it, and the less vitamin D you will make. So, obviously, the further north you are, the paler skin you want where you're getting less sunlight. Also, darker skin can give you protection against sunburn, so people like me will know very well that you've got to pack on the factor 50 if you go anywhere near the sunlight!
But there’s also another thing is that skin tone doesn’t vary directly with latitude. It’s not a totally uniform variation that you will get dark people in very, very sunny places and light people in less sunny places. And a lot of people have argued that there’s a huge social part that plays in skin tones, so sexual selection, for example, will determine how pale certain people will be in certain societies.
Chris -  Nina Jablonski, who we had on the program last year, she’s Professor of Anthropology at Penn State University in America, has been looking at this whole question and has shown how folate – folic acid - gets degraded by ultraviolet. If you don't have black skin and you're subject to a lot of ultraviolet then you have low levels of folate and it makes you have Spina bifida for example. So black people evolved to be black in order to avoid getting that kind of folate degradation because I asked her, if you look at the last common ancestor that we and chimpanzees shared – humans and chimps – because chimps have pink skin, what colour was that? And she said they almost certainly had pale skin. So when humans evolved, they had to evolve first to have black skin and then when we moved out of Africa, 155,000 or whatever years ago, people then re-evolved to have white skin for exactly the reason she said. It’s fascinating, isn’t it?
Diana -  Yeah, that’s right. I've heard that too and the very early humans were quite hairy and so, this allowed them to have very pale skin which you know, was covered with nice protective hair.
Chris -  A few people are walking around in Cambridge like that today.
Click to visit the show page for the podcast in which this question is answered. Alternatively, [chapter podcast=2819 track=10.10.03/Naked_Scientists_Show_10.10.03_7303.mp3] listen to the answer now[/chapter] or [download as MP3]