Connected trade routes made the first pandemics possible

As the long-distance trade networks of Africa and Eurasia joined up in the last centuries BCE and the first centuries CE, they produced something nobody intended. A disease appearing in one part of the system could now travel the whole of it, carried in the bodies of merchants, sailors and porters. From this period onward, pandemics are a recurring feature of human history. The routes that moved gold, frankincense, bananas and iron moved pathogens as well, welding the region from the Atlantic to the Pacific into a single zone through which infection could pass. The Antonine plague that struck Rome in the 160s CE, the plague of Cyprian in the 250s and the Justinianic plague of the 540s all belong to this new arrangement. Every increase in connection has carried the same charge. It is worth holding when the Atlantic system is assembled fifteen centuries later, and the ships bringing sugar and captive people bring smallpox with them.

By about 250 BCE, on the radiocarbon evidence Rouse reviews, the Cedrosan Saladoid migration had reached the northern West Indies. The colonists were selective about where they stopped. They favoured high, mountainous islands, which hold rain, carry rivers and support the luxuriant forest their farming needed. Flat, dry islands were passed over. That preference explains a good deal of later Caribbean settlement, including on Hispaniola, whose mountains and well watered valleys made it one of the most heavily populated islands in the region by 1492. This is farming logic applied to an archipelago by people who had crossed open water to reach it. They knew which land would grow their crops and they chose accordingly, across a chain more than a thousand kilometres long. The indigenous history of Haiti begins with decisions of this kind, taken two thousand years before the Spanish arrived and recorded now in charcoal, potsherds and dates.

Latin speakers had a single word family for substances that acted in ways nobody could watch. Uenenum covered a potent herb, a magical influence and a poison, and out of it came a verb meaning to bewitch or to infect, a noun for the act itself, and uenefucus, a word for the person who did such things, used of men and of women. In the late republic the Roman state grew worried enough about those people to pass a law in 81 BCE against causing sudden or unexpected death by concealed means, whether the means were chemical or magical. The law treated the two as one offence. Sixteen centuries later, courts in Martinique and Suriname were still hearing cases in which nobody could say whether the accused had used a plant, a powder or a spell, and the statutes they used descended from this one. The vocabulary carried the ambiguity across two thousand years.

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.