Below the Rio Grande was different because the Mayans and the Aztecs had actual cities and other organized manifestations of cultures with sizable populations. Not true in America above Mexico.
A few million at which time? The Indians greatly outnumbered the whites along the east coast for many years, and the effect of disease among indians in direct contact with whites is part of the folklore. Cahokia is the only place we know about that had a large contentration of people, and that was about 100,000. The mound culture was about on the same level as the pre-historic Briton cultures, and evidence is that like these early Europeans trade occured over thousands of miles. Now of course, all the nonsense about the indians being in harmony with nature has no more basis than their adaptation to local conditions and their religious beliefs, which put so much stress on dreams. War was as much a feature of tribal lives as it was in the old world. The truth lies somewhere in the middle, because Europeans who read about the indians through reports from the News World were inclined to sensationalize and let their imaginations run wild. The notion that the Indians were somehow connected with the Lost Tribes of Israel was rife up until the 19th century linguist finally put paid to the notion that Indian languages had any connection to ancient Hebrew. IAC, can any such phantasies be any more bizarre than those of Rousseau, and his notions of the purity of the primitive life?