Hmmm, do you suppose it was the environmental warming combined with the decline of ferocious animals like dire wolves, sabre-toothed tigers, etc. that allowed humans to move into these new territories? But that's probably a politically incorrect view.
If you read the book in Comment #10, you will see that a cataclysmic event probably killed off most of the top predators and their large food animals as well as most of the people in the thriving Clovis culture. Actually, there was a thousand year cooling called the Younger Dryas, and then the warming was reestablished allowing various animals to repopulate, or be hunted as the case might be.