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.”