Right our early ancestors had fair skin, but as you said, were hairy..
as this hair dissapeared depending where they were living those with more or less pigmenet would survibe better, pass on the genes for more, or less pigment and their children would survive better. In places where there was no Selection due to sunlight other factors would decide what genes were passed on.
This is because UV light damages our genes, if we are protected ..dark pigment ( lots of malanin) or hair or sunblock we can bred and pass on our genes, If we are not protected ..fairer skin exposed due to lack of hair in early humans we would not breed as well, die earlier and not pas son our genes..so in places with lots of sun..pale skin genes “died out”
So near the equator if you had more pigment you surviuved better than the pale gene carriers so bred more and spread the dark pigmnet geen more widely.
But light only became a selection after hair ahd gone..before that all were the same as the hair provided protection
here is a paper that describes six main reasons for changes in skin colour which would have occurred depending on geographical lcoations ( how much sun etc) http://www.ncbi.nlm.nih.gov/pubmed/19481954 if you follow the link to full text article that may give you the whole article our UNi does..not sure about schools
Skin coloration depends on the variety and amounts of various forms of a color natural mocleule known as melanin. there are different types of melanin and different combinations give the differences in skin coloration. Melanin protects us from the harmful radiation of the sun and that’s why when we go on holiday and sunbathe our skin goes darker. the skin cells are induced by the sun light to produce more melanin for protection and so we go darker.
In places like Africa because of the very hot climate with almost permanent sunlight the native people have melanin permanently produced in their skins to give them the dark colour. They too though do tend to go slightly lighter or even more darker depending on whether they are exposed or not to more sun. It is a kind of an evolutionary adaptation.