Millions of years before the White Sands National Monument became a stark landscape of gypsum sand dunes, it was the site of a salty sea. When those waters evaporated about 12,000 years ago, they left behind a dry playa and evidence of a dramatic battle for survival. In a study published in Science Advances on Wednesday, scientists studying footprints on the playa describe two ancient foes locked in a game of predation: humans and giant ground sloths.

The prints described in the study include the first-ever fossilized human footprints found in this desert — and, crucially, the first well-documented human footprints associated with those of an extinct Pleistocene-era ground sloth. Some of the human footprints are actually inside the prints left by the sloths, which the scientists say is likely “extremely rare” evidence of two ancient creatures trapped in a tangle of stalking and possibly hunting. Co-author and Bournemouth University professor Sally Reynolds, Ph.D., tells Inverse that her team was immediately struck by how unique the prints were upon discovery, explaining “we knew then that the site had an important story to tell.”

“We’ve been working on fossil footprints for a long time, but the one thing that you really, really want is to get something about the interaction between one animal and another,” co-author and Bournemouth University professor Matthew Bennett, Ph.D., followed up in a video statement released to press Wednesday. “It’s that behavioral ecology, the way that one thing behaves towards another, that’s really important.”

Ancient footprint
Human footprint inside a sloth track. 

The footprints are a clue in the hunt for the animals that caused the giant ground sloth’s downfall, suggesting that ancient humans were involved. Much of these megafaunas became extinct during the late Pleistocene, which is about the same time that these prints were probably left behind (although Bennett and his team have yet to directly date the prints). Seven to 8 feet tall when standing on their hind legs, giant ground sloths were intimidating creatures quite unlike the relaxed, tree-dwelling sloths we know today. The tracks they left behind are shaped like elongated kidneys complete with claw marks — evidence of a massive creature with a long stride. No one is sure how these formidable animals became extinct, but Bennett says these prints do “give us a better understanding of whether we as humans are guilty or not in that role of extinction.”

The interaction between these human and sloth footprints — over 100 were excavated and detected by both aerial and geophysical surveys — paints a picture of harassment and stalking. The careful placement of the human tracks indicates they were intentionally walking within the sloth tracks, and the paths of the sloth tracks show several circular patterns, complete with sudden heel pivots. The scientists describe the circular patterns as “unusual”; these weren’t left by freely strolling animals but rather by hunted individuals deliberately prodded into compromising positions.

ancient footprints
View of the Alkali Flat at White Sands with excavated footprints in the foreground.

“It suggests that they were following the sloths closely, with little time elapsed,” explains Reynolds. “The failing circles described in the paper indicate that the humans were harassing the sloths and that the sloths were exhibiting defensive behaviors. They clearly viewed the humans as a threat.”

While the sloths may have seen humans as a threat, they were no wilting daisies, either. The researchers write that “sloths would have been formidable prey. Their strong arms and sharp claws gave them a lethal reach and clear advantage in close-quarter encounters.”

That advantage is why the team can’t say definitively whether humans caused the giant ground sloth’s extinction. While it certainly looks like they were hunted by early Americans, it’s too early to say whether or not the humans came out on top. The scientists plan on further excavating White Sands, searching for more evidence of the movement of ancient hunters. Mammoth tracks have been found in this area as well — another example of huge prey that our ancestors decided to tackle despite the giant risk.