Maybe the most intriguing idea in the entire book is that the Americas were ten times more populated at the time of European contact than they ended up becoming in the 1500's (when most eye-witness accounts were written).
The "noble savages" we think of were remnants of a disease holocaust that wiped out 90% of the population. It's almost as if we're trying to understand European civilization by observing concentration camp survivors.
The recurring pattern: small group of Europeans makes contact with a native group. They report a land teeming with people and trade. A hundred years go by, and almost nobody is left. Those remaining live in small bands eeking out a subsistence living.
The culprit: smallpox and a host of other diseases that jumped species in Europe due to co-habitation with animals. The lack of such practices (or animal domestication in general) in the Americas meant that the disease transfer was destined to be one-way.
This dynamic may explain why there's only one genus of Homo left alive today. Whenever two groups contacted each other, one inevitably succumbs to the diseases carried by the other.