For much of the twentieth century, the dominant model — the Clovis-first hypothesis — held that the first Americans were people associated with the distinctive Clovis stone tool technology, which appeared in the archaeological record at roughly 13,000 years ago.

Under this model, the first Americans had crossed from northeastern Asia into Alaska via the Bering land bridge during the last glacial period, moved south through a corridor between the retreating ice sheets, and spread rapidly across the continent.

This model was clean, well-supported by the available evidence, and widely taught.

It was also increasingly under siege.

Sites across the Americas were producing evidence of human occupation that predated the Clovis horizon.

Monte Verde in Chile had been dated to approximately 14,500 years ago.