The Columbian Exchange

In 1492 two halves of the world that had been biologically separate for ten thousand years were reconnected, and the consequences were mostly not the ones anybody at the time was trying to produce. Alfred Crosby named the process the Columbian Exchange in 1972, and the name has stuck because it captures the right thing: this was a biological event with political consequences rather than a political event with biological side effects.

Going east: maize, potatoes, tomatoes, cassava, chillies, beans, squash, cacao, tobacco, vanilla. Going west: wheat, rice, sugar, coffee, bananas, cattle, pigs, sheep, horses, and smallpox, measles, typhus and influenza.

That last clause decided the outcome. The American population declined by somewhere between fifty and ninety per cent in the first century and a half after contact, and epidemic disease accounts for the great majority of it. Smallpox frequently ran ahead of the Europeans themselves, reaching societies through trade networks before any European arrived, so that expeditions inland sometimes found depopulated country and drew the wrong conclusion about how many people had ever lived there. Violence, forced labour and the destruction of food systems made everything worse and are not incidental; the demographic catastrophe is simply not explicable by conquest alone.

Why the asymmetry? The usual answer is domesticated animals. Eurasia had cattle, pigs, sheep, goats and poultry living in close proximity to dense human settlements for thousands of years, and a great many human epidemic diseases came out of that arrangement. The Americas had almost no comparable herd animals: llamas and alpacas, in one region, not living in houses. The consequence was populations with no prior exposure meeting pathogens that had been selecting for transmissibility among humans for millennia.

Go the other way and the story is about calories. The potato grows in poor soil and cool wet climates and yields several times the calories per acre of wheat or rye, which is why it transformed northern European diets and why one econometric study attributes about a quarter of Old World population growth after 1700 to it. Maize did comparable work far from Europe: it became a staple across west and southern Africa and allowed Chinese farmers to cultivate hillsides that would not take rice. Cassava tolerates drought and exhausted soil and now feeds hundreds of millions of people.

Those last two sentences matter because the exchange is usually told as Europe's story, and by population the crops that mattered most were adopted elsewhere. Neither the African nor the Chinese adoption of maize is a European event, and both were larger in human terms than the potato in Ireland.

Which is the other thing the Irish case shows. A crop good enough to double the population you can feed is a crop good enough to become the only thing you grow, and a monoculture of genetically near-identical plants is a standing invitation to a pathogen. The famine is not an argument against the potato. It is the same fact (extraordinary yield) seen from the far side.

There is a smaller point that lands harder than it should. There was no tomato in Italian cooking when Michelangelo was alive. No chilli in an Indian kitchen, no potato in an Irish one. The cuisines that feel most ancient and most national are, in their defining ingredients, a few centuries old. The lesson is not that these traditions are counterfeit. It is that 'timeless' is nearly always wrong, and that a claim of antiquity is itself a historical claim, which means it can be checked.