Why are native African people black?
If dark skin absorbs more heat, why aren't native African people white, and Inuit people black?
Diana - Logically, it should be the case that if you have pale skin it's going to reflect more light and more heat and would be suited to warmer, sunnier places. But what is actually happening is people with darker skin have more melanin, more pigment, in their skin and this prevents the short wavelengths of light, the UV, from penetrating deep into the skin. It also means the skin can produce vitamin D in suitable levels.
So if you live in more northern climes where there is less sunlight, so for example in the UK, then it's much better to have pale skin which allows more sunlight to get into your skin and more vitamin D to be produced. Vitamin D is great for strengthening bones. It prevents you from getting diseases like rickets which would be very selective against a population if you were living in northern Europe.
Chris - We interviewed Nina Jablonksi, Professor of Anthropology from Penn State University in the US about this. She also made a good point about this, that UV radiation damages folic acid which you need for the development of the nervous system.
If you get folic acid depleted you get diseases like spina bifida; so, in continents like Africa where there is a lot of UV in the sunlight, if you don't protect yourself with lots of melanin you will deplete your folic acid, leading to an excess of neural tube defects like spina bifida and this would manifest in a cost to reproductive fitness in the population.
So, as there is so much sunlight, Africans can afford to have dark skin and still make enough vitamin D and not lose their folate.
But once you get up to the parts of the latitudes we live in (the UK), where it's miserable all the time, vitamin D becomes the real problem. You need to make enough vitamin D and so you have to have pale skin.
There's so little UV because we hardly ever see the Sun anyway that it doesn't become a problem from the folic acid depletion, neural tube defect perspective.