The conceit of the left is that before Columbus/Europeans arrived on the North American continent, it had thriving Native American civilizations north of the Rio Grande with populations approaching 20 million.
Columbus and other European explorers/settlers ruined those wonderful Native American civilizations.
This is pure wishful thinking by leftist historians who can provide no evidence of any large native populations.
If there were populations that large, most of them would have been mostly in the eastern half of N.A. as the western half's topography was simply too harsh to support large populations except for the west coast, California, Oregon, Washington. But even in those areas there is no evidence of huge NA populations and/or advanced civilizations.
There would have been plenty of evidence in buildings, artifacts, agriculture practices, and graves of dead people to prove their point.
The fact is they can't provide any evidence to prove that there were more than two or three million inhabitants north of the Rio Grande five hundred years ago. It was mostly an empty land with populations gathered together probably no larger than two or three thousand.
Most NAs were hunter gatherers or lived in small communities with small farming practices. It was simply impossible for those people to amass large populations given their modes of living which would require extensive huge agriculture and an infrastructure to support large populations/millions of people in one area like today's large cities/metro areas.
History isn't bunk like Henry Ford claimed, except when leftist historians get done twisting history.