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 water, beautiful botanical gardens, and immaculately clean streets. The earliest cities in the Western Hemisphere were thriving before the Egyptians built the great pyramids. Native Americans transformed their land so completely that Europeans arrived in a hemisphere already massively "landscaped" by human beings. Pre-Columbian Indians in Mexico developed corn by a breeding process that the journal Science recently described as "man's first, and perhaps the greatest, feat of genetic engineering.