Why people became white, article on LiveScience:
Humans come in a rainbow of hues, from dark chocolate browns to nearly translucent whites.
This full kaleidoscope of skin colors was a relatively recent evolutionary development, according to biologists, occuring alongside the migration of modern humans out of Africa between 100,000 and 50,000 years ago.
The consensus among scientists has always been that lower levels of vitamin D at higher latitudes — where the sun is less intense — caused the lightening effect when modern humans, who began darker-skinned, first migrated north.
But other factors might be at work, a new study suggests. From the varying effects of frostbite to the sexual preferences of early men, a host of theories have been reviewed.
Vitamin D plays an important role in bone growth and the body's natural protection against certain diseases, and the inability to absorb enough in areas of less-powerful sunlight would have decreased life expectancies in our African ancestors. The further north they trekked, the more vitamin D they needed and the lighter they got over the generations, due to natural selection.
This explanation accounts for the world's gradients of skin color traveling south to north, the prevalence of vitamin D deficiency among African immigrants to higher latitudes, as well as the relatively darker skin of Canada's Inuit peoples, who have good levels of vitamin D despite living in the Arctic, due to their diet rich in oily fish.
Rest of article here.