Mann shows how a new generation of researchers equipped with novel scientific techniques have come to previously unheard-of conclusions about the Americas before the arrival of the Europeans: In 1491 there were probably more people living in the Americas than in Europe. Certain cities -- such as Tenochtitlán, the Aztec capital -- were greater in population than any European city. Tenochtitlán, unlike any capital in Europe at that time, had running...