Each year 500,000 people visit New Mexico's White Sands National Monument to hike and frolic among gypsum dunes. Aside from the tourists, few animals in the region get much larger than a coyote or bobcat. But in an epoch called the Pleistocene, which started 2.5 million years ago and ended about 12,000 years ago, the alkali flats and nearby lakes attracted giants. Mammoths and mastodons walked the playa, as did saber-tooth cats, North American camels and huge, 8,000-pound sloths.
Where these creatures went, ancient humans followed. We know this because the travelers left footprints - physical evidence that people chased the giants. In a western corner of White Sands, scientists recently found a human print inside a ground sloth's paw marks, they report in a new analysis of the park's tracks in the journal Science Advances.
"Thousands and thousands of trackways" crisscross the area, said Vince Santucci, a senior paleontologist with the National Park Service and an author of the new report. The official term for such concentrated pathways is a megatrack. The megatrack in White Sands "is the largest one that we know of in North America."
In 1981, geologists investigated tracks of camels and other four-footed animals at the nearby White Sands Missile Range. It was not until 2011 that researchers began a systematic survey of the megatrack, including drone flights over the sand in 2014. This survey revealed the first collection of human tracks: 27 individual footprints that vanished into a dune.
Santucci and his colleagues measured the stride and the gait to predict where beneath the dune the next print should be and excavated the dune. "And lo and behold, right where we anticipated they would have been, were human footprints," he said.
The tracks at the White Sands flats are remote, and, bordered by the military testing range to the north, mostly protected from human disturbance. The ancient walkers made prints in deposits of lake sediment, which were covered over time by a half-inch layer of sand. The tracks, if exposed to moisture, will crumble shortly after excavation.
"The preservation of the footprints is not the best," said Andrew R.C. Milner, a paleontologist at the St. George Dinosaur Discovery Site museum in Utah. Milner was not a part of this research team but had observed some of the animal tracks at White Sands. "We can definitely see large animals," he said, including the kidney-bean shaped marks of sloth paws.
Humans drove North American ground sloths to extinction around 11,000 years ago. Yet National Park Service paleontologists struggled to determine the age of the human prints using geologic techniques like carbon dating. "The dates are coming up all over the place," Santucci said. In 2016, they invited experts from around the world to help examine the tracks.
Study author Matthew Bennett, a paleontologist at Bournemouth University in Britain, accepted the invitation. The bipedal prints were unmistakably human, he said, with "really good toes, heels and arches."
But Bennett decided to focus on the sloth tracks. He was excavating a sloth trackway when he found what looked like a "Klingon Bird-of-Prey in negative relief." (That's a type of starship, for the non-"Star Trek" fans.) It was, the paleontologists realized, two prints - human and sloth squished together. "Quite a lot of profane language" came next, Bennett said with a chuckle.
Milner said the discovery of the human-within-sloth prints was remarkable. "Having these human tracks that are interacting with Pleistocene megafauna - it's never been seen before." What's more, this suggests a minimum age of the prints. They are at least 11,000 years old, as ancient as the last ground sloth.
A human followed quite literally in the sloth's footsteps. "Given the arid environment, there is quite a narrow amount of time you can walk on that surface to record your footprint," said Matteo Belvedere, a scientist with Switzerland's Paleontology A16 project who specializes in trackways and was not involved with this research.
It was not possible to narrow down the time scale to hours or minutes, he said, but the human probably followed the sloth within a day. This evidence of an interaction between human and extinct giant sloth is "unique in the world," Belvedere said.
The tracks led the scientists to a site that suggests confrontation. Marks in the sand indicate the sloth turned to face approaching humans, scraping the knuckles of its front limbs along the ground as it did so.
Did the Pleistocene encounter have a violent end? Santucci was not convinced. Nor was Milner. "There's no evidence of hunting here," Milner said. "There's no kill site. Maybe ground sloths were fun to harass? Who knows."
Bennett and Belvedere, however, speculated that humans were readying for a kill. Given the position of the claw and knuckle marks, the sloth may have reared up to unleash "the equivalent of a 'Go away!' roar," Bennett said. Another line of toe prints suggests a second person approached the sloth at an angle. Perhaps that person was a hunter about to deliver a surprise lethal blow, Bennett said. (This interpretation, he admitted, leans toward "paleo-poetry.")
"With a lot of time and money you can follow the sloth tracks as long as possible," Belvedere said, to where the animal either escaped or collapsed from a spear. If the environmental conditions were right, as Belvedere suspects, this encounter will have its conclusion written in the sand.