We know that humans reached North America between 40000 and 16000 years ago...and that's about all we know for certain. Anything in American prehistory before about 10000 years ago remains deeply controversial. But some mastodon bones found in the 1970s recently revealed major clues.
The orthodox view of North American prehistory is that the oldest culture was the Clovis people, named after a series of discoveries in the late 1930s in Clovis, New Mexico. Dating back roughly 13,000 years, the Clovis culture is known from their distinctive tool designs, which they made from bone and ivory. For the last few decades, archaeologists have argued that it was the Clovis culture that migrated from Asia to the Americas, either over the Bering land bridge or by boats.