Early illiterate stone age migrants from Asia, marching across the frozen Bering Strait during the last ice age. Committed genocide against the Neanderthals, and then began fighting among themselves.
Too busy killing each other to invent the wheel or even to begin the Bronze Age. Stone age.
They are no more "indigenous" than wild horses or wild hogs, neither of which arrived before Columbus.
One theory I read was there used to be an empire like what one would find in medieval Europe that comprised what would the USA and maybe Canada. However something bad happened which completely destroyed it. This led to Tribes and the like that explorers would discover.
