They probably came here by ship.
There are rumors of Egyptian artifacts in the Grand Canyon, specifically in some caves located in a mountain called Isis Temple. There are rumors that Roman weapons, armor, and other goods have been found around Tucson, though I have only read this somewhere and haven't yet found the article itself.
I think we tend to short-change ancient peoples. I believe they were a lot more advanced than we give them credit for.
Yhwhsman
The orthodoxy that the new world was peopled in ancient times solely via the Bering Land Bridge is under serious reconsideration, as new archeological, linguist, and DNA evidence is being examined.
Evidence now suggests that prehistoric man followed the coastal waters from both the Atlantic and the Pacific Oceans when their levels were much more shallow and the continental shelf more exposed.
Evidence for exploration from other peoples in historic times includes the Polynesians, Chinese, Japanese, Cathaginians, Phonecians, Norse, Irish, Welsh, Africans and many others.
Watch out. This interest can become as addicting as Free Republic.