According to some Native Americans I've spoken to about the matter, they didn't all come here from "somewhere else". Nope. Some of them did, some of them were always here. Depends on which tribes you are talking about, and in which part of the country.

This is a matter where the medicine people disagree with the conventional science community, and I believe the medicine people have got it right. The science community has become enamoured of one particular theory (which is partially correct) and tried to lump everything under that one theory. Typical.