North America’s Oldest Known Footprints Point to Earlier Human Arrival to the Continent

When picking up a tired toddler on a long walk, or hunting and confronting a giant sloth, the humans walking the wetlands around a shrinking lake were simply going about their lives in prehistoric North America. They could never have imagined that their everyday acts would create an incredible, enduring tableau for us to ponder thousands of years later. Their fossilized ancient footprints found at White Sands National Park humanize them, revealing the actions of their lives in ways that static bones and stone tools cannot.

Footprints can’t tell us who these prehistoric humans were. Yet a new study suggests something very surprising about this group. They were living in what’s now New Mexico likely during the Ice Age some 23,000 years ago—thousands of years earlier than the ancestors of modern Native Americans are generally believed to have arrived on the continent.

The authors of a study published Thursday in Science put the footprints into the spotlight back in 2021 by publishing research that dated ancient seeds, found in layers with the prints, to 21,000 to 23,000 years ago. Now, in an attempt to corroborate those controversial dates, the researchers’ new study has employed two additional dating methods. They have dated tree pollen from Ice Age conifers and quartz grains in sediment. Both lines independently confirm the earlier date range, according to the authors, a conclusion that raises fascinating questions about who these humans were, where they came from and what happened to them.

Even after thousands of years, the striking array of footprints found in New Mexico’s White Sands National Park evokes a strong human connection between those who view them and those who made them. “The incredible stories they tell us could never be told with artifacts or fossil bones alone,” says study co-author Kathleen Springer, a geologist with the U.S. Geological Survey (USGS).

One set of prints appears to have been made by a woman and a toddler who intermittently walked on its own and then was picked up and carried. At some places the child’s little prints disappear even as the woman’s broaden in the mud under the burden of the youngster’s extra weight.

Other tracks tell the story of a group of ancient hunters apparently stalking a giant sloth. Their prints follow the animal’s prints and at times appear inside the sloth’s own, as though they stepped in its tracks as they trailed it. There is no evidence to tell us how either humans or the sloth ultimately fared at the end of the hunt.

The prints are located in the Tularosa Basin, a desert area home to the world’s largest stretch of gypsum sand dunes, which cover some 275 square miles. But tens of thousands of years ago, during the last Ice Age, the ecosystem was dramatically different. Then, the basin was home to prairie-like grasslands, stands of conifer trees and a large body of water known as Lake Otero.

The lake and its lush surrounding vegetation attracted not only humans but also many now-extinct species such as plant-eating ancient camels, mammoths and ground sloths. Predators, like American lions and dire wolves, enjoyed good living here as well.

But when the climate shifted, a centuries-long drought began to rapidly shrink the lake, and the shorelines became surrounded by soft wetlands. Humans and animals frequented this area, and when it dried up, many thousands of their footprints became fossilized.

Some prints have been visible on the surface since at least the 1930s, but researchers uncovered a vast treasure trove of them by delving below the surface, carefully digging trenches in the park’s gypsum sands. The ground is terrible for bone preservation; gypsum eats away at bone fossils. But the same sands perfectly preserved footprints of the species that once called this landscape home.

The USGS team first dated the footprints by sampling ancient ditchgrass seeds (Ruppia cirrhosa) from the levels just above and below them. Using radiocarbon dating, the team reached the surprising conclusion that humans had walked these shores roughly 21,130 to 22,860 years ago—thousands of years before most theories based on the archaeological record suggest that humans arrived in North America.

Some experts questioned the initial dating. The seeds were dated by measuring their levels of carbon isotopes. Carbon decays through the centuries at a constant rate, so scientists can pinpoint ages by calculating how much carbon was in the environment when the plant was alive. The problem is that the White Sands seeds are from aquatic plants, which when alive could absorb carbon from very old sources in the lake that had not been exposed to the atmosphere for thousands of years, like groundwater leaching through rocks. This carbon reservoir effect can make plants that absorbed such carbon seem older than they really are.

For the new study to corroborate the results, the team first turned to pollen evidence from land plants—conifer trees—that was collected from the same layers as the original seeds, just above and below the prints in several different sets of tracks.

“Dating pollen is really hard,” says co-author Jeff Pigati, a geologist with the USGS. But Pigati and colleagues did so with a yearlong effort that involved four different labs across the country.

Each sample originally consisted of a two-pound sediment block, which was chemically melted away to produce a cubic centimeter of material that was mostly pollen, but mixed with other minerals and organics. Next, a micro-sieve isolated the pollen by grain size. Later, the scientists used laser light to separate the pollen cells from other things that remained by identifying the pollen cells’ unique fluorescent properties. The arduous process produced about 75,000 grains of pure pollen for dating. “The sample is big enough that you can see it, but that’s about it,” Pigati says.

In a second step, the scientists also dated the quartz needles found amid the vast haystack of White Sands’ gypsum dunes. “Harrison Gray spent a tremendous amount of time and effort hunting quartz grains, isolating, extracting and cleaning up quartz grains,” says Pigati. Researchers then figured out when the quartz grains were last exposed to daylight. Three different quartz samples produced date estimates that were indistinguishable from both the carbon-14 ages of the pollen and the radiocarbon dating ages of the seeds.

“I think it’s a very cool find and I’m impressed,” says Torben Rick, the curator of North American archaeology at the Smithsonian’s National Museum of Natural History. “I’m not an expert on pollen radiocarbon dating, but a well-respected team is doing everything they can to shore up the chronology here, and I do think these new dates skew it back toward being in the vicinity of 21,000 or 23,000 years old.”

Rick adds that further scrutiny of the dating results and methods is inevitable, and that’s a welcome part of the scientific process. “But we should also start thinking that this is looking close to being about as valid as you can get with this site,” he says. “So, now: How do we think about when these people might have gotten here and the routes they might have used to get here?”

For many years, archaeologists believed that the first Americans arrived on the continent some 13,000 years ago, when the last ice age was dwindling to a close and retreating walls of ice made it possible to cross over a thawing land bridge from Siberia to Alaska. But over the last half-century, many sites have provided evidence to challenge that theory. Migrants could have arrived earlier, 15,000 to 16,000 years ago, or perhaps more than 20,000 years ago, most likely by following routes along the Pacific Coast. Still, the bulk of archaeological evidence relates to a migration after the Last Glacial Maximum, which ended around 19,000 years ago.

Some genetic studies support this timeline, showing that the ancestral lineage of Native Americans split off perhaps 20,000 years ago, either in Beringia or Asia, and that group began populating the Americas as the Ice Age weakened 14,000 to 17,000 years ago, eventually splitting into different Native American lineages whose migrations can be traced across two continents.

But some genetic modeling studies suggest that humans might have populated the Americas as long as 30,000 years ago. That would mean that humans didn’t just enter America after the Last Glacial Maximum, when melting ice would have made migrations possible, but they also entered at an earlier time. If so, the migrants may have been small groups, because those same models suggest that American populations didn’t become large until about 15,000 years ago. Archaeological evidence from after that time shows more people populating the Americas. More controversial sites may (or may not) offer evidence to support the theory humans were in the Americas prior to that—like butchered bones from Bluefish Caves, in the northern Yukon, or the stone points from a central Mexican cave that some researchers believe may be 26,500 years old.

Michael Waters, an archaeologist who directs the Center for the Study of the First Americans at Texas A&M University, isn’t convinced that the footprints are as old as the suggested dates.

Waters suspects that the seed ages are unreliable. He notes that several studies suggest they could be off by 7,000 or 8,000 years. He believes that pollen dating is difficult and similarly fraught with potential errors.

“I really remain skeptical,” says Waters, who also cites genetic evidence. If the footprints are actually 21,000 or 23,000 years old, he notes, they were made by humans who were in New Mexico before the ancestral Native American population diverged from Beringian populations and entered the Americas. In his view, that raises two possibilities. One is that an unknown group of people lived in New Mexico 22,000 years ago but ultimately died out and had nothing to do with the later peopling of the Americas. The other is that something is wrong with the efforts to date the site.

“I applaud their efforts to try to resolve this issue,” he continues, “but my suspicion is that the ages that have been generated so far are older than what they truly are.”

Bente Philippsen, who leads the National Laboratory for Age Determination at the Norwegian University of Science and Technology, believes that although each of the three dating methods has its limitations, together they strongly indicate that humans did create the footprints during the Last Glacial Maximum. “It is highly unlikely that different methods applied to different materials will always result in systematic errors of the same direction and same order of magnitude,” she wrote in an essay for Science.

Rick points out that even if one believes the dates are off by 5,000 years—“a pretty extreme view,” in his opinion—footprints that are 16,000 years old would still change many existing perceptions about how and when people came to the Americas. When would such a journey have started? “Because any date here isn’t the oldest date,” he says. “They didn’t just come out of northeast Asia, into the Americas, and sprint directly to White Sands, New Mexico.”

USGS’s Springer says the study can only raise intriguing questions about who these people were, how they ended up at White Sands and what eventually happened to them. But, she notes, many more clues are likely awaiting discovery.

“The Tularosa Basin is huge, White Sands National Park is huge, and this trench is just one little spot in a very large area,” she says. “There are lots and lots of spots, and there are many thousands of prints just ripe for exploration.”

Brian Handwerk | READ MORE

Brian Handwerk is a science correspondent based in Amherst, New Hampshire.