Daron Acemoglu and James Robinson ask the interesting question of Why Africa lacked Centralized States.
Archaeologists have suggested a theory linking the use of bronze to political centralization. Copper and tin are both scarce and need to be traded, their supplies can be monopolized, and so can trade. This seems to have created both the incentive and the opportunity to concentrate power and develop urban centers, for example in Knossos in Crete which was the core of Minoan Greece.
A plausible idea, since the Bronze Age was major era of change. They go on:
Not every part of the world experienced a Bronze Age … in general Sub-Saharan Africa jumped right into the Iron Age without ever passing through this intermediate stage.
In contrast to copper and tin, iron is very widely spread as the great archaeologist Gordon Childe put it “cheap iron democratized agriculture and industry and warfare too”. So the jump to Iron Age technology may have impeded the development of states in Africa by making it more difficult for elite to concentrate and monopolize power.
My response: not a very convincing theory of why Africa remained largely a land of chiefdoms rather than states. In the Americas political centralization proceeded smoothly enough without either a bronze age or an iron age. A Bronze Age was not necessary to put a society on the path to statehood. I think we need a better explanation of why Africa lacked centralized states, as well as the other appurtenances of ancient civilizations.