Researchers working in Uruguay say they've found evidence in ancient bones that humans were in the area as far back as 30,000 years ago. This contradicts current assumptions humans first populated the Americas approximately 16,000 years ago by walking across the frozen-over Bering Strait, with the Clovis people of North and Central America generally considered the "first Americans." Of more than a thousand bones found in a streambed at the Arroyo del Vizcaino site in Uruguay -- most of which belonged to the now extinct giant sloth -- many were found with deep slash markings indicative of human stone tools, the researchers said. The Arroyo del Vizcaino site has been radiocarbon dated to between 29,000 and 30,000 years ago, the researchers reported in the Proceedings of the Royal Society B: Biological Sciences. "That's pretty old for a site that has evidence of human presence, particularly in South America," study co-author Richard Farina, a paleontologist at Uruguay's Universidad de la Republica, said. "So, it's strange and unexpected." If humans were indeed living in the area as far back as 30,000 years ago, they likely arrived there by floating over from Africa, with prevailing winds that could have carried them to South America directly without the need of paddles or sails, they said. Some experts say they're holding off judgment. "South America played an exceptionally important role in the peopling of the Americas, and I'm pretty sure we have some significant surprises waiting for us," Bonnie Pitblado, an archaeologist at the University of Oklahoma who was not associated with the study, said in an email to the National Geographic Society. "Maybe people killing sloths at [the Arroyo del Vizcaino site] 30,000 years ago is one of them, maybe it's not -- but it certainly isn't going to hurt to have it on our collective radar screen as we continue to contemplate the peopling of the New World."