Sometime between 10,000 and 15,000 years ago, a giant ground sloth—a hairy, lumbering beast that weighed as much as a metric ton—wandered along a lakeshore in what is now southern New Mexico. Then, something spooked it. It reared up on its hind legs, swatted at its attackers, and then was lost to the ages.
The dramatic scene has been preserved for millennia in the first-ever trackways, or sets of fossilized footprints, that may chronicle human hunters in action against big prey. In some cases, the human prints lie inside those of the sloths, indicating hot pursuit. The find adds credence to the idea that, in the waning days of the last ice age, people may have played a large role in driving these creatures—as well as other giant mammals like mammoths and mastodons—to extinction.
“This is a fascinating find,” says Jordana Meyer, a conservation ecologist at Stanford University in Palo Alto, California, who wasn’t involved in the new study. She has worked extensively with native big-game trackers in Africa, and she says it’s plausible that the human-and-sloth trackways “show hunters in action.”
What was once a bucolic lakeside landscape is now a vast salt flat that’s part of White Sands National Monument. Limited access to the area, as well as its largely secret location, has helped keep the trackways there intact.
In the new study, Matthew Bennett, a sedimentologist at Bournemouth University in Poole, U.K., and his colleagues focused on the footprints and trackways made by giant ground sloths. When a sloth trackway isn’t near any other footprints, it generally takes a straight or slightly curving path. But when human footprints are also nearby, a sloth’s path sometimes takes sharp turns or indicates the animals reared up on their hind legs. This may have freed the creatures’ strong forelimbs—tipped with sharp claws—for defense, allowing them to swivel to and fro. In the process, they would have created smeared-out tracks and arc-shaped scuff marks that the researchers call “flailing circles,” the latter of which were reported for the first time today in Science Advances.
“That’s a radical change from just walking,” says Anthony Martin, a vertebrate paleontologist at Emory University in Atlanta who specializes in trace fossils such as footprints. The sloths “were obviously reacting to something in their environment,” very possibly humans, says Martin, who wasn’t involved in the new study.
The clincher, however, may be trackways at the site that include human footprints inside the larger footprints of the giant ground sloths. In those instances, says Bennett, the humans had to have been purposefully extending their stride to place their feet inside the sloths’ footprints. In these trackways, individual footprints are between 80 and 110 centimeters apart, but a normal human stride—based on the length of the footprint left behind—should measure only 60 centimeters or so.
Researchers have long (and hotly) debated the possible causes of late-ice-age extinctions of giant sloths, mammoths, and other large beasts. The new find, says Bennett, suggests that although factors like climate change and disease may have played a role, humans—armed with stone-tipped spears—may also have been a major factor, as they infiltrated North America after crossing a land bridge from Asia some 15,000 to 25,000 years ago.
But not all scientists are convinced the New Mexico trackways chronicle an ancient hunt. Bennett and his team “conclusively provide a really nice demonstration that big animals and people were frequenting the same places at the same time,” says Anthony Barnosky, a vertebrate paleontologist at Stanford and a colleague of Meyer who wasn’t involved in the study. However, ancient humans may have simply happened upon sets of fresh giant sloth tracks and then decided to walk in the same footprints, he says. “People visiting the beach might do the same sort of thing.”