The lack of clues left behind by ancient Americans has made it difficult for researchers to pinpoint precisely when humans first arrived on the continent. However, it has always been believed to be about 13,000 years ago, just as the world was thawing from the last ice age. Now, evidence from the Chiquihuite Cave in Zacatecas, Central Mexico, seems to suggest that prehistoric humans may have been living in North America as early as 30,000 years ago.