Several thousand years ago, a young adult moved barefoot across a muddy landscape. A toddler was balanced on the adult’s hip. There were large animals — mammoths and ground sloths — just over the horizon. It was a perilous journey, and scientists reconstructed it by closely studying an exceptional set of human and animal footprints found recently in the southwestern United States.
“This is an amazing trackway,” said Neil Thomas Roach, an anthropologist at Harvard University, who was not involved in the research, which was published online this month in Quaternary Science Reviews. “We rarely get tracks as well preserved as these are.”
It is one of the most extensive Pleistocene-age trackways found to date, and studying it highlights how ancient sets of fossilized footprints can reveal more than even fossilized bones. It’s rare for bones to reveal behaviors, but tracks can shed a lot of light on animal interactions, said Sally C. Reynolds, a paleoecologist at Bournemouth University in England and an author of the study.
The round-trip journey of the prehistoric young adult and the toddler was spotted in 2017 in White Sands National Park in southern New Mexico. The sequence extends more than a mile and includes at least 427 human prints. The out-and-back journey was probably completed in no more than a few hours, the researchers suggest. (The gypsum sand that records the prints doesn’t hold water well, so the muddy conditions that captured the prints would have been short-lived.)
Most of the human footprints were made by a barefoot adolescent of either sex, or a young adult female with roughly size 6 feet, the team determined. But about every 100 yards or so, a few much smaller human prints suddenly appear within the northbound set of tracks.
“We have many adult tracks, and then every now and again we have these tiny baby tracks,” Dr. Reynolds said.
A toddler-aged child was being carried and periodically placed on the muddy ground as the caregiver readjusted his or her human load, the researchers surmised, based on the three-dimensional digital models they had assembled. There are no toddler footprints within the southbound set of tracks, so the child probably wasn’t carried on that journey.
It’s likely that the child rode on the young person’s left hip. There’s a slight asymmetry between the left and right tracks on the northbound set of tracks. That’s consistent with someone carrying extra weight on that side, Dr. Reynolds said.
She and her collaborators estimated that the young person was moving at just shy of four miles per hour. That’s a good clip: “Imagine running for a bus,” Dr. Reynolds said. “It’s not a stroll.”
The urgency of the journey might have had something to do with the toddler, Dr. Reynolds suggests. “Why else would you travel so fast but encumber yourself with a child?”
There was another reason, however, for making haste over the landscape — the presence of large and potentially dangerous animals. Both a giant sloth and a mammoth ambled across the humans’ path, the trackway reveals. Their prints appear on top of the northbound footsteps but below the southbound ones, meaning that the animals walked by sometime in between the humans’ passage.
The mammoth — most likely a bull, based on the size of its tracks — was apparently uninterested in the humans who had walked by just hours before; its tracks do not indicate any reaction. The giant sloth, on the other hand, stopped and shuffled in a circle when it encountered the human trackway, its prints indicate. The sloth’s response suggests that humans had positioned themselves at the top of the food chain, Dr. Reynolds said.
In the future, Dr. Reynolds and her colleagues hope to better understand the people that inhabited this region. For instance, it’s an open question whether they had migrated seasonally or stayed put in one area throughout the year, Dr. Reynolds said. “We’re trying to assemble these little snapshots of what life was like in the past.”