If you read the accounts from the De Soto march through North America and the descriptions of the populations, then compare that with the next major European movement through the area (which was almost 200 years later) there was a major drop in population between the European accounts. There are recorded stories from Native Americans relating tales of widespread disease and large population losses. These also describe what appears to be small pox. De Soto's Spaniards carried other diseases as well.

Horses also came from the De Soto expedition, and by the time more Europeans came to the area, horses were already a critical part of many of the cultures. Just another indicator of the amount of time that passed.

There is evidence that a culture was in existence across North America that disappeared. Clovis point technology was predominant primarily in Eastern North America, though the first point was found in Clovis NM. Far more Clovis points have been found in the East than anywhere else, and they stop appearing at about the same time across the continent, and after that Folsom point technology takes its place. Clovis technology is also more similar to ancient European technology than to anything else. There is approximately 2,000 years (of more) between when Clovis points were used and when Folsom points were used. There are many theories about this, and I don't know enough about it to speak authoritatively, but it is an interesting point. Maybe the first cultures present in North America were from Europe, they died out and were replaced by migrations from Asia.