New discoveries could rewrite the history of early Americans — and the 4-ton sloths they lived with

Sloths weren't always slow-moving, furry tree-dwellers. Their prehistoric ancestors were huge - up to 4 tons - and when startled, they brandished immense claws.  

For a long time, scientists believed the first humans to arrive in the Americas soon killed off these giant ground sloths through hunting, along with many other massive animals like mastodons, saber-toothed cats and dire wolves that once roamed North and South America.

But new research from several sites is starting to suggest that people came to the Americas earlier - perhaps far earlier - than once thought. These findings hint at a remarkably different life for these early Americans, one in which they may have spent millennia sharing prehistoric savannas and wetlands with enormous beasts.