 This interpretation was challenged in a 2003 review which concluded the features in question could also have arisen by genetic drift.The ages of the earliest positively identified artifacts at the Meadowcroft site are safely within the post-LGM period (13.8k–18.5k cal years BP).Stones described as probable tools, hammerstones and anvils, have been found in southern California, at the Cerutti Mastodon site, that are associated with a mastodon skeleton which appeared to have been processed by humans.