Something happened around the black sea, probably in old Europe, the caucuses, Anatolia or Mesopotamia, and it led to the bronze age. That something had to do with the area serving as the nexus of intercontinental trade. The ‘explosion’ of civilization (meaning technology) radiated from there outward. It is nearly impossible to reach west africa by east-west overland route. So the empires that were forming in west africa were just about at the same level of development as everyone else the same non-linear travel distance from the center of the world. There were empires, but there simply wasn’t enough trade ‘out there on the fringe’. Now we can argue about IQ, yes, but that didn’t stop the south americans.
Europe lagged mesopotamia, egypt, hrappans, and yellow river china for the simple reason that africa lagged europe: lack of sufficient production capital to tax to form armies of imperial conquest. Rivers concentrated production and concentrated production allowed defense and taxation. the rivers of europe, the steppe, the desert, and the tundra are not as productive. That is why it required mediterranean trade in the old world, north sea trade in the medieval world, and atlantic trade in the modern world, to bring europe to economic parity.