The Columbian Exchange rewired the planet's biology in one generation

Mann's core framework is that Columbus's voyages did something no prior human contact had done: they reconnected two landmasses whose plants, animals, and microbes had evolved apart since the breakup of ancient supercontinents, then let them collide at the speed of sailing ships. Potatoes and maize crossed to the Old World; wheat, horses, and sugarcane crossed to the New; and pathogens like smallpox moved alongside them, with catastrophic asymmetry since American populations had no comparable immunity to give back.

Mann treats this exchange as ecologically unprecedented in scale — not simply trade, but a wholesale mixing of gene pools across previously separate biospheres, compressed into a handful of centuries rather than the millions of years such separation had taken to build. He calls the resulting blended, homogenized global ecosystem the "Homogenocene," arguing it deserves recognition alongside the more familiar epochs of geological and human history.

The exchange wasn't a one-time event but an ongoing process whose consequences — in agriculture, disease patterns, and biodiversity loss — are still unfolding today.

Takeaway: 1492 didn't just connect two hemispheres politically; it merged them biologically, and we're still living inside the aftermath.