John Green teaches you about the changes wrought by contact between the Old World and the New. John does this by exploring the totally awesome history book The Columbian Exchange by Alfred Crosby, Jr. After Columbus "discovered" the Americas, European conquerors, traders, and settlers brought all manner of changes to the formerly isolated continents. Disease and invasive plant and animal species remade the New World, usually in negative ways. While native people, plants, and animals were being displaced in the Americas, the rest of the world was benefitting from American imports, especially foods like maize, tomatoes, potatoes, pineapple, blueberries, sweet potatoes, and manioc. Was the Columbian Exchange a net positive? It's debatable. So debate.