If modern humans came out of Africa 300,000 years ago, how did everyone who isn’t black get their skin colours?
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There’s a tradeoff between too much sunlight giving you sunburns and skin cancer, and too little sunlight resulting in vitamin D deficiency. The pigments in dark skin absorb much of the sunlight before it can do any of that, which is advantageous when you’re exposed to lots of sunlight, but problematic when you’re not.
As for how, there are many different genes that impact skin color, so there will naturally be variation in skin tone in any population. Of the people that left to higher latitudes, those with lighter skin had more babies. Repeat for thousands of generations.
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