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
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