Imagine about 60,000 years or so, a small group of these people migrated north, to northern Europe. They had dark skin too, and they would have been the only humans up there. Check in maybe a thousand generations later, and you would find most of the people in the north had lighter skin. So how did that happen? Nobody's skin color adjusted to fit the environment. So how did light skin become more common? Think about the factors that can change allele frequencies: mutation, genetic drift, gene flow, and natural selection. Does that make sense?
