"Clovis first" is the traditional story about population of the Americas,but there is mounting good evidence for earlier people, and more and more archeologists are agreeing that there are pre-Clovis groups. As to how they got here, the route that is gaining favor is across the Bering land bridge and down the west coast. It helps this notion that so far the earliest human remains currently come from Santa Rosa Island, within Channel Islands National Park. This material dates from 13,000 BP (before present). Being on an island, it is clear that the group possessed some sort of water craft, which strengthens the left coast migration hypothesis.

We are a long way from consensus on how folks got here, to put it mildly. At least we have more data than we did fifty years ago. I think it is fair to say that archeological research has demonstrated that pre-Columbian groups were considerably more sophisticated than they were thought to be 100 years ago or so, about the time that reasonably scientific archaeology began to be practiced.