One city in Mexico had 215,000 inhabitants who lived on an island in the middle of a man-made lake. Another, near modern-day St. Louis, attracted thousands of inhabitants before it was destroyed by floods. A city in Peru was surrounded by a wall greater than the one built around Rome.
The Inca empire, he said, was the largest in the world at its height. The Incas created a huge network of roads.
In the Amazon, Indians altered thousands of square miles of wetlands, Mann said, so they could live on mounds of dry ground and travel on raised causeways. They also created vast designs of raised earth whose purposes scientists still don’t understand.
In other parts of the Amazon, he said, researchers have found Indians made the dirt more fertile by mixing it with charcoal and millions of smashed pieces of pottery. Some estimate that 12 percent of the Amazon was transformed for agriculture.