Humans began to spread across North America around 12, 000 years ago, as the climate became warmer  During the same period the large mammals that were once abundant in North America, such as the mastodon, the woolly mammoth, and the saber-toothed tiger, became extinct
Thus, contrary to the myth that humans formerly lived in harmony with the rest of nature, it is clear that even 12, 000 years ago human activity was causing the extinction of animal species
the evidence it cites is consistent with the alternative hypothesis that the large mammals' extinction was a direct result of the same change in climate that allowed humans to spread across North America