Sloths weren't always slow-moving, furry tree-dwellers. Their prehistoric ancestors were huge — up to 4 tons (3.6 metric tons) — and when startled, they brandished immense claws.Video above: ...

But new discoveries suggest that "humans were existing alongside these animals for at least 10,000 years, without making them go extinct."Some of the most tantalizing clues come from an archaeological ...