When Columbus stepped ashore in 1492, millions of people were already living in America. It wasn't exactly a 'New World,' but an old one whose inhabitants had built a vast infrastructure of cities, orchards, canals, and causeways. But after Columbus set foot in the Americas, an endless wave of explorers, conquistadors, and settlers arrived, and with each of their ships came a Noah's Ark of plants, animals, and disease. Here is an exploration into...