Human footprints inside giant sloth footprints date 10,000 years old.

"Last April, Matthew Bennett was lying on a white salt flat in New Mexico, uncovering fossilized footprints that had been preserved in the white rock. The print belonged to a ground sloth—a bulky animal, whose large feet and curved claws left apostrophe-shaped impressions wherever it walked. There were many such tracks around, but Bennett found one that was very different.

Inside the outline of the sloth’s 20-inch-long foot was a human footprint.

He looked at the next track in the series and found the same thing—a human footprint, perfectly nestled inside a sloth one. There were at least 10 of these, all in a row. “It slowly dawned on me what was happening,” he says. Thousands of years ago, a ground sloth had walked along this site, and a person had followed it, carefully matching its every step. “There was a lot of profanity [from me],” Bennett adds. “That’s what geologists do when we discover something.”

If the sloth had been following the person, its larger footprint would have annihilated the smaller one. If the person had been walking in the footsteps of a sloth that had long passed, their feet would have squished into any water or sediment that had collected in the old tracks, creating a distinctive pattern. Bennett and his colleagues found no such pattern. All the evidence was consistent with someone keeping pace with an animal that was ahead of them.

This is just the starting point of unlocking an archive that will tell us something really interesting about how early human pioneers, around 11,000 years ago, were interacting with this ice age fauna, Bennett says." Read full story.