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 argument is most vulnerable to the criticism that ['for early inhabitants of North America the destruction of mastodons, woolly mammoths, and saber-toothed tigers might have had very different significance than the extinction of mammal species does for modern humans', 'in calling the idea that humans once lived in harmony with nature a myth the argument presupposes what it attempts to prove', "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", 'there might have been many other species of animals, besides mastodons, woolly mammoths, and saber-toothed tigers, that became extinct as the result of the spread of humans across North America']