The Columbian Exchange

For roughly ten thousand years, the Eastern and Western Hemispheres evolved in biological isolation. The Atlantic Ocean was a wall. On one side, wheat and barley, cattle and horses, influenza and plague. On the other, maize and potatoes, llamas and turkeys, syphilis and nothing that could kill on a continental scale. Each hemisphere developed its own ecosystems, its own domesticated species, its own disease environments. Then, in 1492, the wall came down.

What followed was not primarily a story of conquest, though conquest was part of it. It was a story of biology. Seeds traveled in the pockets of sailors. Rats stowed away in ship holds. Viruses crossed the ocean inside human bodies. Weeds arrived as stowaways in livestock feed. Within two centuries, the biological makeup of every continent on Earth had been permanently altered. No war, no empire, no ideology has ever reshaped the planet as thoroughly as the movement of organisms between hemispheres after Columbus's first voyage.

Columbus brought seventeen ships on his second voyage in 1493. They carried roughly 1,200 men, along with seeds, cuttings, and livestock that would transform the Americas. Wheat, barley, sugar cane, and citrus fruits. Horses, cattle, pigs, sheep, goats, and chickens. Every one of these species was alien to the Western Hemisphere.