There are absolutely no societies built on hunting/gathering, as most of the North American Indian tribes were, that can generate enough food to support forty million or more people. North of the Rio Grande the Indian population probably was in the area of two to five million at most. In central America the Aztecs had cities and the populations there were probably a lot larger. But nowhere near forty million. Leftist historians (or hysterians) at work again.
There is evidence that South America was extensively farmed. An early Spanish explorer who went from the Andes to the Atlantic down the Amazon reported seeing large cities. No one believed him. New evidence seems to show he wasn’t lying.