- There were quite possibly more people living in the Americas than Europe in 1491 (scientists disagree but even the conservative estimates are higher than those found in most high school textbooks)
- Some Indians were living in harmony with their environment, but many had radically changed their surroundings:e.g., the Great Plains of America, the areas of New England where the Pilgrims and Puritans first settled.
This is fascinating because the idea still is disseminated that Indians were perfect Earth-keepers as oppossed to those greedy Europeans. The evidence gives the lie to these antiquated ideas. The truth is that Indians like any other group of people they had good and bad practices and that they impacted their environment for their benefit. See for instance, Mann's discussion of the rise and fall and rise and fall of the passenger pigeon.
If you would like a slightly more balanced view of the Pre-Columbian Americas this would make a worthwhile read.
What happened to all those "supposed" people you ask. Oh, just your friendly neighborhood lethal microbes brought by European people and livestock.