Researchers who dove hundreds of times into a sinkhole beneath the murky waters of Florida's Aucilla River have retrieved some of the oldest evidence of humans in the Americas, including stone tools apparently used to butcher a mastodon.

Scientists said on Friday that the tools, animal bones and a mastodon tusk found at the site showed that people already had occupied the American Southeast by 14,550 years ago — 1,500 years earlier than previously known.

The site provided some of the most compelling evidence that humans had spread across the New World earlier than the so-called Clovis people, whom archaeologists for six decades considered the Americas' first people. The Clovis people, recognized for their distinctive spearheads, are known from archaeological evidence about 13,000 years old.