I wonder how they explain human skin variation. What I have read is that all humans can trace their ancestry back to Africa. Our ancestors evolved to produce high levels of melanin (black skin) to protect them fro the tropical sun. As people migrated north they encountered less sun, colder climes, and so had to bundle up. This decrease in sun exposure led to a decrease in vitamin D production, which occurs in the skin. Vitamin D deficiency leads to a disease called rickets, in which the bones are weakened and the child becomes severely bow-legged. People with rickets in prehistory tended to die before reproducing. There was strong evolutionary advantage to having lighter skin where there was less sunlight. Those that could absorb more sunlight with less exposed skin tended to live longer and to reproduce. Any genetic mutations that decreased melanin production were favored by the environment and passed through a line of descendants to modern peoples.