The flange welded double bell demands four sheets and two welds

In the first centuries CE African smiths circulated a new body of forging technique, spreading it along the rivers of the Congo basin. Jan Vansina called the result Africa's second iron age. One product shows what the technique could do. The flange welded double bell was made by forging four flat sheets of iron, reheating each one, hammering it over a rounded form until it curved into a half bell with a flat flange standing out all round the curve, then welding the flanges of each pair together in the fire, then forging a curved iron handle and welding that to the tops of both bells so the instrument held as one piece. Every stage demands temperature control, symmetry and a sound weld. Smiths in the equatorial rainforest were still producing these bells in the nineteenth century, eighteen hundred years after the method appeared, which says the knowledge was taught, held and handed down without interruption across that whole span.

Around the middle of the fifth century CE, people speaking Arawakan languages arrived on Aruba, Curacao and Bonaire, the islands lying off the coast of Venezuela, and brought pottery making with them. Their arrival marks the start of the Ceramic age on those islands. Somebody was already there. The earlier inhabitants belonged to the Archaic age, made a simpler set of tools, and are physically distinguishable in the skeletal record by the shape of the skull. They were displaced. Rouse records the sequence in the flat language of archaeology, and the flatness is worth noticing, because the displacement of one population by another is a thing that happens to people. These three islands sit where the South American mainland meets the Caribbean, which makes them a gauge for movements that later reached the whole chain of the Antilles. What happened here in the fifth century had happened elsewhere before and would happen again, long before any European ship arrived to start the count.

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.