Question on Human Evolution?

I've recently had very great interest in human evolution and how we end up looking the way we do. I know that are ancestors originated from Africa and that through migrations to different continents/places we began to differ in looks and genetics. My question is however, if Native Americans came from Asia, how would they be darker than today's Asians? And if it was due to heat then wouldn't they have coiled hair like Africans because of the brain's need to keep cool against harsh condition?