My ideas about skin color & diet aren't original. I am pretty sure that I originally read them in Great Human Diasporas by L.L. Cavalli-Sforza. Here's the extract:
In Europe the development of agriculture has lead to the spread of cereals as the primary foodstuff over the last ten thousand years. Unlike meat, particular fish liver, cereals contain no vitamin D. They do, however, contain a precursor taht becomes vitamin D if exposed to the ultraviolet light from the sun's rays absorbed through the skin. Cereal eaters can produce enough vitamin D to survive and grow normally if they are fair-skinned. Therefore, people can inhabit northerly regions where there is less sunlight, and continue to eat coreal products because a fair skin color has been less selected during evolution. In certain northerly regions, however, an even in the far north, some peoples such as the Eskimoes, have always eaten enough vitamin D from fish or meat to make fair skin unnecessary....
