Most scientists agree that humans began arriving in the Americas between 13,000 and 15,000 years ago, and the Clovis people of North and Central America are generally considered the "first Americans." But new fossil evidence from a streambed in southern Uruguay could challenge such theories.

Results published in Proceedings of the Royal Society B suggest the presence at the site of human hunters who may have killed giant sloths and other megafauna. That itself isn't odd, but the site, called Arroyo del Vizcaino, has been radiocarbon dated to between 29,000 and 30,000 years old—thousands of years before people were thought to be there.

"That's pretty old for a site that has evidence of human presence, particularly in South America," said study co-author Richard Farina, a paleontologist at Uruguay's Universidad de la Republica....