Despite living in the high UV Kalahari, Bushmen (who may be older than other African populatiions) are light brown like most of humanity.

Nina Jablonski’s explanation hold fails at the extremes of pigmentation: North Europeans and West Africans. People who live at similar latitudes to them do not share their pigmentation.

Here is a hypothesis that suggests Darwin was not so wrong afer all.
SEXUAL SELECTION AND HUMAN GEOGRAPHIC VARIATION