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.

Human footprint inside a sloth track. (Matthew Bennett / Bournemouth University)

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.

“It really does look like they were contemporaneous,” says Anthony Martin from Emory University, who specializes in tracks and other so-called trace fossils. “This is a common problem we have with dinosaur tracks: We have something that looks like following behavior, but could have been offset by days or weeks. Here, the humans maybe had the sloth in sight.”

Ground sloths were not slow-moving slackers like the sloths we know today. They were well-armed and potentially dangerous animals, which ranged from bear-sized to elephant-sized. Those that lived in New Mexico were on the smaller end, but they were still substantial beasts with meter-long strides. A human would have had to stretch to walk in its footsteps. What possessed them?

Bennett thinks the pursuer was trying to provoke the sloth—and if he’s right, it clearly worked. At the end of the overlapping tracks, the team found a very different series of sloth prints, indicative of pivoting feet and scraping claws. The animal was rearing up onto its hind legs, and swinging its claws around.

Meanwhile, another set of human footprints approaches from the opposite direction. These are daintier, with impressions made by raised toes. It seems that while the sloth was flailing, someone else tip-toed up to it from the back. That’s a hunt, Bennett says. “The strategy was all about stalking to distract and irritate the animal, and get it to turn its back on someone approaching from the blind side.”

That’s just one possible interpretation, but it’s consistent with other tracks from the region. Sloth trackways usually go in a straight or gently curving line. But when human tracks are around, the sloth paths change direction sharply. The sloths seemed to be carrying out evasive maneuvers, and Bennett’s team also found many other sets of circular tracks indicative of upright, pivoting animals. They call these “flailing circles”—a term that makes me feel more connected to ground sloths than I have ever felt before.

Still, this scenario doesn’t explain why the pursuing human was stepping in the sloth’s footprints. There’s something almost playful about that, and I ask Bennett whether the tracks could have been made by, y’know, a bunch of teenage kids harassing the sloths for kicks.

“It’s really difficult to rule that out,” he says. “But I think that’s highly unlikely. These were fearsome animals. They had claws like Wolverine. I wouldn’t have wanted to go head-to-head with one. It would be a very silly risk to take.” I feel like this underestimates the willingness of teenagers to take silly risks, but I also take his point.

“I’m prone to agree with their interpretation about a possible hunt,” says Richard Fariña from the University of the Republic in Uruguay; it fits with the idea that humans drove the megafauna of the Americas to extinction. Ricardo Melchor from the National University of La Pampa isn’t fully convinced, either. Hunting is the simplest explanation, but “the human footprints are not well preserved,” he says. And why would people hunt a sloth near a lake—a flat landscape “where the animals can escape easily?”

The only other idea that makes sense, says Martin, is “that they were tracking an animal as practice for a future hunt, purposely stepping in the tracks to get a sense of its movements. The only alternative I can see is that they were being scientists!”

Tracks at White Sands National Monument. (Matthew Bennett / Bournemouth University)

Bennett’s team found the tracks in New Mexico’s White Sands National Monument. After the last Ice Age, the lake that used to be there dried out. The wind swept its bed away, creating the largest expanse of white gypsum sand dunes in the world, and leaving behind vast salt flats. They are hot and blindingly white. Bennett is there when I call him. “I’ve got my hat on and I’m slathered in sunscreen,” he says. “I can see the shadowy outlines of the tracks about 20 yards away—human, mammoth, sloth. There are more tracks here than I’ve seen in any other site. It’s the greatest concentration of tracks in the Americas—and perhaps in the world.”

Despite their vast number, the tracks can be hard to spot. They can be shallow, and their visibility depends on subtle moisture conditions; David Bustos, the resource manager for White Sands, calls them ghost tracks. On several occasions, he brought visitors to see them, and no one could. When he invited Bennett over from the University of Bournemouth, Bennett was skeptical. It was only when he saw the tracks himself, he says, that he realized “White Sands hides its secrets well.”

Bennett, Bustos, and their colleagues identified the tracks using geophysical techniques that look for subtle changes in the magnetic properties of the ground. They also used a far more low-tech method: They took pictures from the air by strapping cameras to tall poles. In that way, the team has discovered hundreds of thousands of tracks from mammoths, mastodons, ground sloths, camels, bison, dire wolves, humans, and more.

“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.