By the 1990s, the cracks were visible and widening.

Monte Verde had demonstrated pre-Clovis presence in South America.

Several North American sites were producing evidence that proponents argued predated the Clovis horizon, though many of these claims remained contested.

The ice-free corridor route that Clovis people were supposed to have used to enter North America was being questioned on ecological grounds — when the corridor first opened, it may not have been sufficiently rich in resources to have supported a southward human migration, which meant that any people who were in the Americas before Clovis would have needed a different route.

The coastal migration hypothesis stepped into this growing uncertainty.