Well, as a person of science, I believe it is all a matter of sun and natural pigmentation. You might disagree, what with your religious aspect of things, but I believe God has nothing to do with it. Its all the sun and your genetics. When the earliest people walked this Earth, they resided in Africa, which made their skin dark, with the sun being directly overhead. Then they started traveling, and as their skin became lighter, so did their children. However, some people stayed in Africa, and made dark babies for years to come.
That was a very good question, and I hope my paragraph answers it!