Genetic mutation, people living in different areas of the world (some closer to the equator/more exposed to sun) interracial breeding, a mix of those.
What really boggles my mind is how man began, and how there were some people in north america, others in africa, others in europe. Its crazy to think how they got from place to place. Maybe the same evolution occurred somewhat simultaneously in different regions?
Then again, you have to think of Pangea. In that case it couldnt have been too hard to originate from one spot and expand into separate civilizations from there.
What a wide open topic. I love this kind of stuff. So many things to consider, so many questions to answer, and sadly so many things we dont know. Its awesome to speculate though.