Melanin and the equator?

That is exactly what happened in the history of the human race. The human species first appeared in central Africa. That's where most of the early hominid and pre-hominid fossils have been found. Therefore they almost certainly had black skin, to protect them from high levels of UV radiation. Later, as populations gradually emigrated to more temperate climates, where UV radiation was less intense, lighter skinned peoples, with lower levels of skin melanin, gradually developed.