A giant sloth bone might hold the key to the peopling of the Americas.

When did people get to the Americas? The answer remains a subject of fiery debate.

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 November 19 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 Vizcaíno, has been radiocarbon dated to between 29,000 and 30,000 years old—thousands of years before people were thought to be there. (Also see "Photos: Speared Mastodon Bone Hints at Earlier Americans.")

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

"So, it's strange and unexpected."

What's the controversy?

Giant sloths, saber-toothed cats, oversize armadillos, and other large mammals once roamed the Americas—a diversity that would easily rival an African savannah today. (Watch a saber-toothed cat video.)

But by 11,000 years ago, many of the species had disappeared, likely due to climate change or the arrival of human hunters in the New World. But when exactly humans got here, and how they arrived, remains unknown.

What's new?

In 1997, severe drought forced local farmers to drain a lagoon in Arroyo del Vizcaíno, which exposed a mysterious bed of gigantic bones.

After a series of bureaucratic roadblocks, paleontologists excavated the site in 2011 and 2012, unearthing over a thousand fossils. "From the paleontological point of view, that is absolutely marvelous in itself," Fariña said.

Many of the bones belong to three extinct ground sloth species, mainly Lestodon armatus. Weighing in at up to four tons, the animals "were the size of smallish elephants," he said.

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