The commonly accepted theory for the original human settlers in the US is that 15,000 years ago they crossed over from Asia to America near the Bering Sea that had a land bridge then or at most required a short boat ride.
Hence I was intrigued by this news article (via Machines Like Us) that there is some evidence that the first humans here may consist of a people known as Solutreans who came from Europe 20,000 years ago by paddling in boats that hugged the then-existing ice-caps along the northern Atlantic coastline.
Part of the evidence for this claim comes from the remains of a 22,000 year-old mastodon that was found in 1970 near the Chesapeake Bay that had an eight-inch long sharp tapered stone blade still in it. This was presumably the weapon that had been used to kill the animal. This, along with other tools that had been found along the eastern coast of the US, match those used in Europe around the same time.
Of course, there is a lot more work to be done and much more evidence needed to overthrow the strongly entrenched Bering Sea migration theory. For example, there seems to be little or no evidence of human settlements that show European influence along the eastern seaboard from that time, so that it may be the case that even if the very first people did arrive by that route, they quickly died out.