The Columbian Exchange

Disease first, because everything else follows from it

Smallpox, measles, influenza and typhus crossed the Atlantic into populations with no prior exposure. Estimates of the death toll across the Americas over the following century run from a half to over ninety per cent in the worst-hit regions. This was not primarily a weapon; it was an accident of biological history — Afro-Eurasian populations had lived alongside herd animals and dense cities for millennia and carried the resulting immunities.

The consequence drives the whole unit: an empire that had planned to live off indigenous labour suddenly did not have enough of it, and turned to the transatlantic slave trade for a replacement.

Crops move both ways, and populations grow

• To Afro-Eurasia: maize, potatoes, cassava, tomatoes, chilli, cacao. Potatoes and maize yield more calories per hectare than the grains they joined, and they grow on land that was previously marginal. Maize and sweet potato allowed Chinese farmers to cultivate hillsides, and Qing population roughly tripled; cassava did similar work in West and Central Africa.
• To the Americas: wheat, rice, sugar cane, coffee, bananas, and the grape and olive of Mediterranean agriculture.

Animals and landscape

Horses reshaped societies on the Great Plains and the Pampas, making mounted hunting and raiding possible and creating cultures that Europeans later mistook for ancient. Cattle, pigs and sheep multiplied without predators and stripped vegetation. Sugar and later coffee cleared forest on a scale that changed soil and water.

The exchange was uneven: the Americas received crops, animals and diseases; Afro-Eurasia received crops and silver. Only one side lost most of its population.