West African gold crosses the Sahara by the third century

By the first three centuries CE, traders on the trans-Saharan routes were carrying a new commodity north: gold from the mines and river deposits of West Africa. It changed the standing of Africa in world economic history. In later centuries the same metal backed the dinar of the Islamic world and the florin of Renaissance Italy, which means monetary systems in places whose people never saw a West African miner rested on West African labour. The other consequence was currency itself. As trade grew in scale and complexity, regions began producing their first coinages, tokens carrying value separately from any particular commodity being exchanged. Money is a repeated answer to a shared problem, and Africa's commercial networks arrived at it as independently as anyone else did. Gold moving north across the desert is also the start of a pattern that runs through this whole timeline, in which African production ends up denominated in somebody else's accounts.

Textbooks teach a sequence for metals in which copper comes first and iron follows long after, because that is how it went in Europe and the Middle East. South of the Sahel the order runs the other way. Iron smelting was established across two thirds of Africa before copper was worked at all, and knowing how to hold a furnace at heat is what led people to recognise copper ores and smelt them. The clearest case is the belt of copper rich country running from central Zambia into the southern Congo. Bantu speakers of the Sabi group began working those ores no later than the fourth century CE, several centuries after iron reached them. Copper and copper goods then moved out along the Zambezi watershed and up the rivers into the Congo basin. From about the eighth century a further set of Bantu communities spread through the same region, their speech ancestral to the Luba dialects heard there now, and the older Sabi language fell out of use while the mines kept growing.

By around the third century CE the Malagasy speaking community on the East African coast had lived among Bantu speaking neighbours long enough to have married into them. Some of them then sailed south, crossed the Mozambique Channel, and settled Madagascar. What they carried was a working farm system assembled out of two hemispheres: African livestock and African grain beside rice, bananas and chickens from Island South Asia, together with the practices that belonged to each. Madagascar has been that combination ever since. Its people speak an Austronesian language whose nearest relatives are in Borneo. They raise cattle bred in Africa and poultry domesticated in Southeast Asia. They grow African sorghum in the same country as Asian rice. Their ancestry is Bantu and Austronesian together. The island is the clearest surviving record of how far the ancient Indian Ocean trade reached and what it did on arrival, which was to move people, crops, animals and languages until two ends of the world were permanently joined in one place.