I’ve gone down a few rabbit holes recently, and the answer seems to be that we don’t really know.
I had assumed black, because it has been believed that our particular human species, homo sapiens, emerged about 300,000 years ago in East Africa near the equator, where darker skin gives more protection from the sun by blocking UV rays. And this is possible, but seems less certain, for several reasons.
First, our close primate relatives, chimpanzees, often have light skin color. They don’t need dark skin because their body hair provides protection from UV rays. Presumably the last common ancestor of humans and chimps was also hairy, and so perhaps also had light skin. As successive species led to homo sapiens, they lost body hair, and darker skin would have been advantageous.
But evolution doesn’t cause mutations to occur just because they would prospectively be advantageous, so at what point in this evolution did darker skin become dominant? I don’t think we know. And unless we can pull DNA from very ancient hominid skeletons, we may never know.
It’s quite plausible that by the time homo sapiens appeared, “black” skin was dominant. But there are some other reasons we may not know. One is that the San people are said, based on genetic evidence, to be one of the oldest populations in Africa, and while they would normally be classified (based on our standard socially constructed classifications) as black, they are distinctly lighter than many other African ethnic groups (of which there are about 3,000 – genetic diversity in Africa is greater than in the rest of the world).
Notably, the San live in southern Africa, farther away from the equator, just outside the tropic zone. But perhaps their ancestors were darker, migrated there, and evolved lighter skin.
More recently, the oldest homo sapiens fossil was found in Morocco, again, outside the tropics. Perhaps humans actually originated there, and didn’t need as dark a skin color for sun protection. But just because that is the oldest fossil found to date, doesn’t prove homo sapiens began there.
Another recent genetic study of people in Africa suggests that there has been variation in skin tones among humans since before homo sapiens appeared (we were not the first human species, and were just one of several for a large part of our history), and that early homo sapiens themselves may have varied from lighter to darker in skin tones (but not “white” in the European sense, since that seems to be only about 8,000 – 20,000 years old). It’s possible that some darker skin tones evolved from paler ones. But it also may be the case – simultaneously, not in contradiction – that lighter and darker skin tones existed side by side among homo sapiens populations all along.
In the end it doesn’t really matter in any moral sense, of course. But it stands in contrast to two niche theories: one, that white skin, being more recent, is a sign of being “more evolved” (an incoherent concept, since evolution is not teleological), and the other, that black skin marks the “original man,” and is therefore superior (less “devolved,” perhaps, although I haven’t seen anyone use that term).
Skin color really is superficial. It can have beneficial or harmful effects depending on the shade and where one lives (while darker skin helps protect from UV damage in the equatorial region, light skin helps capture more UV radiation to produce vitamin D in higher latitudes). And melanin has a few other biological effects. But that’s all. Nobody’s skin color makes them more or less human, more or less evolved, more or less morally upright, more or less intelligent, more or less worthy.