Human fossil footprints

In southern New Mexico lies the Tularosa Basin, an oblong bowl that’s home to a natural sandbox: White Sands National Park.

In 2005, David Bustos came to White Sands to work as a National Park Service biologist. The following year, he took notice of odd animal footprints that would appear only when the ground was wet. (The first of these footprints was reported in the 1930s.) The markings, which would disappear as the earth dried up, became known as ghost tracks.

Then in 2009, Bustos came across tracks he thought belonged to ancient humans walking alongside mammoths. But few scientists agreed, he says, rebutting with “No, no, those are the prints of camels that were slipping in the mud,” referring to now-extinct North American camels. And the tracks would disappear with changes in the weather. Heavy rainfall drowned them; periods of drought dried them up. It would take more than a decade—and cooperative weather—for scientists to agree that the tracks belonged to humans. 

Then, says U.S. Geological Survey (USGS) Research Geologist Kathleen Springer, “What everyone was asking is, ‘How old are these things, really?’”

In September 2021, Bustos, Springer, and a group of other researchers published their findings in the journal Science: The team identified 61 human tracks within multiple levels of earth belowground, and between layers of these tracks were seeds from an ancient aquatic plant. Analyses of these seeds—and the footprints they were embedded within—are now challenging scientists’ understanding of when and how people arrived in the Americas.

Archaeologist Joe Watkins, who was not involved in the study, believes the findings have potentially massive implications. “I think, outside the next big migration of going to the Moon or Mars, the migration to the Americas was the last great migration humanity had,” he says. Upending the timeline of this journey could have implications for both science and Native American cultures, Watkins, who is a member of the Choctaw Nation of Oklahoma, believes.

Visiting the tracks

At first light on a cold January morning in 2020, Bustos set out with Springer and her USGS colleague Jeff Pigati on all-terrain vehicles to visit White Sands’ ancient tracks. The team wore parkas to keep warm amid a prevailing wind. Springer recalls the hour-and-a-half journey through the dunes being like a “Disney ride.” It was the “funnest thing in the world,” she says. “Just being out there is surreal.”

Eventually, after passing through a large swath of national park that borders the site of the first atomic bomb test, Springer, Pigati, and Bustos fell upon thousands of tracks. They belonged to packs of dire wolves, giant sloths, 13-foot-tall mammoths, and other animals. And, Springer says, “there’s a person track right smack dab in the middle of them.”

“When Dave showed us these things and we had our eyes trained on them, we were almost afraid to step down,” Pigati says. “It looks like a painting in some places.”

Bustos recalls the conditions that day being perfect for identifying tracks. The trio saw human footprints going underneath the sediment in places, illustrating that there was more than a surface layer of them. “We were seeing seed layers popping out all over the place,” Bustos says. To determine how old the footprints were, Springer and Pigati’s team would collect these ancient seeds from different layers of footprints. The overwhelming evidence of ancient plant life excited Bustos.

About 36,000 to 19,000 years ago, lakes filled the Tularosa Basin. The largest of these was Lake Otero, which fluctuated in size and activity based on rainfall. Plants like spiral ditchgrass (Ruppia cirrhosa), whose seeds are now embedded in layers of earth, grew along Otero’s edges. About 23,000 years ago, the southwestern U.S. became drier and the lake shrank. This exposed a large area of land that was once underwater, whose clays and silts became overtopped by gypsum sand and silt. People began walking around these surfaces, which “were kind of squishy in some areas,” Springer explains. And the people weren’t alone: The impressions from mammoths, ground sloths, and camels, sit alongside—and sometimes right on top of—human prints. 

“This can only happen if they are in the area at the same time,” Pigati said in an email. “Exactly how they may have interacted in the vicinity of our [research site] is unknown, but there is definitive evidence that humans were harassing or stalking a giant ground sloth on the west side of the basin.”

In many other cases, interactions between humans and animals seem peaceful. “What’s interesting to me,” Bustos says, “is it’s not just a mammoth and a hunter. It’s families. You see a baby mammoth turning around and spinning. We have the juvenile and adult megafauna prints with the babies and adult [humans].”

Having about 10 hours of daylight, the team got to work “trenching” the human footprints, Springer says. With ground-penetrating radar, they were able to determine that footprints were present within layers of sediment. They then dug a trench to view the layers in cross-section. If Springer and Pigati’s cross-section was a sandwich, the footprints would be the filling and the dateable material around the footprints would be the bread. 

Dating tracks only at the surface would yield inaccurate results, Pigati explains. “You can look in every direction and there are trackways everywhere in every horizon,” he says. But if you calculate the age of these footprints on the surface layer of earth only, you’ll just find that they’re younger than the ground they’re upon, he says. By looking at layers of sediment below the surface, the team set out to document how many layers contained footprints and find datable material above and below each layer. Each of these layers would contain footprints from a unique time period, the team posited. “The footprints are imprinted at the top of a given layer of sediment—they don’t really pass through the layers,” Pigati wrote in an email.

As the sun rose higher in the sky, the white sand reflected a near-blinding light, and the day quickly grew warm. The researchers shedded their parkas and worked in t-shirts until the January sun fizzled and it was time to ride back through the dunes, chunks of sediment layers in hand.

The team later extracted 40 to 60 ditchgrass seeds from every level of footprints in every sample. In a USGS lab in Golden, Colorado, they cleaned each set of seeds with a chemical treatment and began combusting them.  

The combustion process, automated by a machine in the lab, didn’t obliterate the seeds but turned them into a gas. From this gas, a machine turned the once-seeds back into solids: graphite pellets about the size of a pencil tip. This technology of making “graphite targets” dates back to the 80s, Pigati says. “Every lab does it the same way in the world.”

The team then took these pellets, tapped them down into cartridges, and sent them off to a lab to measure the amount of a radioactive isotope of carbon, called carbon-14, they contained. Then they waited for results.

What the tracks mean

Months later, they came. The seeds revealed that the footprints they were embedded within are between 21,000 and 23,000 years old—thousands of years older than what scientists generally consider to be the earliest evidence of people in the Americas.

“For forever, people thought Clovis were the first people to cross over [the Bering Strait] about 13,000 years ago,” Pigati says, referring to the commonly-held view among archaeologists. As the story goes, ice sheets in what is now Canada blocked passage between what is now Alaska and the rest of the Americas. Once these ice sheets began retreating, people came south through an ice-free corridor, Pigati explains. His team’s findings now challenge this belief. Carbon dating of the seeds within the White Sands footprints suggest that people were in the Americas while ice sheets still covered much of northern North America. “A lot earlier” than previously thought, Pigati says. 

“When we got the final dates back, it was very…” Springer pauses over Zoom, choosing her words carefully. “The arrows were going to start flying. We knew we needed an [evidence-supported] explanation why these findings were really robust.”

Archaeologist Joe Watkins says that if the dating of the seeds, and therefore footprints, is correct, “it puts us at 20% older than the dates we have from the rest of America.” Therefore, ages of other important archaeological sites like Meadowcroft Rockshelter in Pennsylvania, where scientists have found 16,000-year-old evidence of human life, may need to be reconsidered. (In 2014, Nikhil Swaminathan wrote for Archaeology Magazinethat Meadowcroft Rockshelter is “the longest-occupied site in the Americas.”)

Watkins first heard about the White Sands tracks years ago while working in Washington D.C. as the chief of the Tribal Relations and American Cultures Program of the National Park Service. But he didn’t see them until after he’d retired, visiting White Sands during a NOVA shoot for the film “Ice Age Footprints.” Seeing human footprints intermingling with those of ground sloths was overwhelming and “tremendously exhilarating,” he says.

But Watkins cautions that, with no other aspects of culture present but footprints, scientists like him can only speculate how ancient humans were using the land. “Were they just walking? Were they hunting? Did they have a camp nearby?” he asks. Archaeology may not hold all the answers to these questions: “I do think it’s important for American Indians to be involved so they can offer their insights so we can better understand who these people were,” Watkins says, reflecting on his own Choctaw roots.

As Native American identity is tied to the land, Watkins believes, “the Native peoples will say we’ve always been here,” he says. “I usually say [the first Americans] weren’t Native Americans. They weren’t Siberians. They were whatever they called themselves.” Combining this philosophy with science is a way to integrate belief systems, he says.

Watkins recites his people’s story about two stiff-legged bears that meet in a black-loam prairie and have a battle. Only one survives; it becomes the last of its species. “Some Choctaw interpret ‘stiff-legged bear’ as being a mammoth,” Watkins says. If humans did once walk alongside mammoths in New Mexico’s Tularosa Basin 20,000 years ago, he adds, there’s a chance these people gave birth “to any number of tribal people across the U.S.” 

Racing against time

Bustos, Springer, Pigati, and fellow researchers visited the ghost tracks again in January and April 2022. They extended their trench, identifying more footprints from potentially before and after the original set they excavated. To bolster their findings, Bustos explains, the team won’t rely solely on dating seeds. They’ll date pollen, too. “This work is not done,” Pigati says. “We have a window from 21,000 to 23,000 [years]. We don’t know if people were here before then.” 

But the clock is ticking. Erosion in White Sands is exposing and destroying the tracks, Springer says. Though the Tularosa Basin has been eroding for nearly 12,000 years, climate change is likely exacerbating erosion today, she explains. “The layers, they’re finite,” Bustos adds. “We’re losing thousands of tracks to erosion. It’s a race to document them before they’re gone.”

Already, some tracks reveal clues about ancient daily life in the Tularosa Basin.

“There’s something about the children,” Bustos says. “You see these toddlers running and jumping, and the story really comes alive.” Watkins agrees. “When we look at the tracks of children playing in perhaps puddled water surrounded by ground sloths,” he says, “it takes us back to childhood memories of children playing in puddles all around the world.”