What does Mann suggest was the previous idea of how people came to the Americas?

Historian Charles C. Mann compiled evidence of a far more complex and populous pre-Columbian society.

Explanation: "From southern Maine down to about the Carolinas, you would have seen pretty much the entire coastline lined with farms, cleared land, interior for many miles and densely populated villages generally rounded with wooden walls. And then in the Southeast, you would have seen these priestly chiefdoms, which were centered on these large mounds, thousands and thousands of them, which still exist. And then as you went further down, you would have come across what is often called the Aztec empire and maybe better known as the Triple Alliance 'cause it was a group of three people, which was a very aggressive, expansionistic empire that had one of the world's largest cities as its capital, Tenutchtitlan, which is now Mexico City."