The key points are both dark and pale skin are evolutionary traits once we lost hair. If you sweat you probably want light skin under fur.

We had fur at one point, and our still-furred cousins have pink skin, so our ancestors probably did as well.

It's generally believed to be the case that dark skin evolved with hairlessness, and was lost by some branches of the tree during the glacial period.

Planet was colder, and around equator you actually have quite habitable places even now, deserts are further from equator. But you get tons of light whole year in very stable pattern.