BBC News – Old American theory is ‘speared’ http://www.bbc.co.uk/news/science-environment-15391388

In truth, the “Clovis first” model, which holds to the idea that America’s original human population swept across a land-bridge from Siberia some 13,000 years ago, has looked untenable for some time. A succession of archaeological finds right across the United States and northern Mexico have indicated there was human activity much earlier than this -perhaps as early as 15-16,000 years ago. The mastodon rib, however, really leaves the once cherished model with nowhere to go.

Turns out we aren’t quite sure.