I'm no expert, but this is how I understand it: dark skin color is an evolutionary response to the hot and sunny climates where the human race, by most accounts, originated (where the hair went that covers all the rest of the primates is a whole different story). The melanin protected against sun damage, and there was more than enough exposure to allow the sunlight to make essential vitamin D. I see no logical reason to think that our earliest ancestors in Africa were not at least as "black" as the people who live there now. Maybe more so.

The caucasian strain evolved as humanity moved north, and had to cover themselves to protect themselves from the cold. With minimum exposure to the sun for Vitamin D and other things, the heavy melanin level of the skin became more of a liability than an asset. Bingo: pasty white guys like me.

Of course, now that white people have moved off the glaciers and back into sunny climes, it's skin cancer city.