Why is skin black in sunny countries and white in a cold climate?

Diana - Interesting question. A lot of people think that it's due to vitamin D production. So, what happens is, you get UVB (ultraviolet B) emitted by the Sun, which 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 - Also, Nina Jablonski, who we had on the programme 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 more prone to having a baby with spina bifida, for example. So black people evolved to be black in order to avoid getting that kind of folate degradation. I asked Nina, 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, 55,000 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!