The Columbian Exchange irrevocably homogenized the world's biological landscape. Since Columbus, the number of plant and animal species has continually diminished, and the variation in species from place to place has diminished dramatically. I mean, the first European visitors to the Americas had never seen a tomato or a catfish, Native Americans had never seen a horse. And by making our planet biologically singular, the Columbian Exchange completely remade the populations of animals, particularly humans. And vitally, this cross-pollination also made possible such wonders as contemporary pizza. So we're going to break Columbian Exchange down into four categories. Diseases, boy you're looking good, smallpox, I'm glad you've been eliminated.