And how did the dark skin disappear?
Adaptation; it wasn't necessary anymore.There are some things the body will jettison if it's too resource-expensive and not needed. Take muscle... muscle is the first tissue to be jettisoned by the body if it's not used. Muscle is very hungry tissue and expensive to maintain, especially when not being used and/or when food is scarce. The body will keep only the minimum. Melanin is not needed at high latitudes where there is less sunlight, and the body is covered with more clothing. The body stops producing it.