Question:
Why did humans turn in different colours?
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Best Answer - Chosen by Asker:
A current theory on skin color difference that makes a lot of sense actually inlcudes several issues, making it more complex than a simple answer.
Several answers have already addressed the potential advantage of retaining dark skin (and more melanin) as a way to block harmful UV rays and reduce the potential for cancer. Just think about all the warnings these days to use sunblock everytime you go out in the sun, and then think about early humans (and modern folks) in Africa with little protection from the sun as they go about their everyday activities - dark skin is definitely a lifesaver.
But why then did folks that moved out of africa and into more northen climates develop lighter skin? what is the advantage? It appears that it actually relates to fetal development in the mothers womb. All humans need Vitamin D, and the need for it is greatly increased in pregnant woman. Just take a look at the milk you drink every day and you will see that most of it is fortified with Vitamin D (big letters on the carton).
Humans do have the ability to produce our own Vitamin D, but it is dependant on ultra-violet radiation absorbed through the skin. With the amount of time we spend indoors and clothed these days, it is difficult to naturally create as much as we need, thus the need for it in Milk and other fortified foods.
In the tropics, there is plenty of UV radiation, and enough still passes through the melanin in dark skin for humans to create our own Vitamin D naturally. However, as you move away from the equator the amount of direct sunlight, and UV radiation decreases - decreasing the ability to absorb UV rays and make our own. Dark skin compounds the situation by decreasing even more the amount that gets through. As a result, a woman with lighter colored skin will be able to produce more vitamin D in low UV areas than a woman with darker skin and is likely to be more successful at carrying babies to term. This is balanced by the lower need for high melanin content to prevent skin cancer - it is not removed completely - but there is a balancing act between the two issues.
Over several millenium this trend adds up to a ligthening of skin color as humans moved farther north. Recent satellite data from NASA has been helpful in providing more strength to this theory. Satellite images of the atmosphere has identified areas where the earth (for many complex reasons) seems to have higher and lower density ozone levels. Ozone helps to reduce UV penetration of the atmosphere and skin color seems to correlate well with the various ozone levels identfied.