Now, a study published in the journal Science confirms the initial finding with two new lines of evidence: thousands of grains of pollen and an analysis of quartz crystals in the sediments.

“It’s more or less a master class in how you do this,” said Edward Jolie, an anthropological archaeologist at the University of Arizona who has studied the White Sands footprints in the field but was not involved in the new study. “As Carl Sagan said, ‘Extraordinary claims require extraordinary evidence.’ They have some extraordinary evidence.”

Lorena Becerra-Valdivia, a fellow at the Oxford Radiocarbon Accelerator Unit, said that the results support her modeling work, which suggested that people first crossed into present-day North America before 29,000 years ago, possibly traveling via the ocean.

“If anything, early findings like the White Sands footprints should inspire further scientific investigation in what is a dynamic and changing field,” Becerra-Valdivia said.

Some critics who raised concerns about the initial study said that they were encouraged by the follow-up analyses but remained unconvinced.

“I don’t agree that it resolved the issue of the timing, but they have made progress,” said Loren Davis, an anthropologist at Oregon State University. “Knowing the age of this is important, because if these researchers are correct and people are truly in New Mexico at 23,000 years ago, or even 21,000 years ago, it means we have to change our fundamental understanding of some things.”

A snapshot of life in the Pleistocene

Fossil footprints were first seen in New Mexico’s Tularosa Basin in the early 1930s and were initially thought to be evidence of a bigfoot, said David F. Bustos, a resource program manager at White Sands National Park. They turned out to be from a giant ground sloth, a 2,000-pound mammal that went extinct around 10,000 years ago. Researchers also found tracks from trudging mammoths, a dire wolf and other ice age creatures.

Bustos said he first saw fossilized footprints in the basin that looked human in 2009, and a growing team of scientists began to study them. Those tracks brought to life a vivid snapshot of the Pleistocene, the epoch that started 2.6 million years ago and ended 11,700 years ago.

The thousands of footprints found in White Sands are an extraordinary but evanescent record of life around Lake Otero, the body of water that rested inside the basin during the Pleistocene. The ancient tracks are the remnants of complex interactions. Children played. Humans stalked giant sloths. A person walked a mile, carrying a child and placing them down occasionally. But the fossilized prints are slowly being destroyed by erosion — they are so soft they can be cut with a butter knife.

“It was hard to believe that humans could be walking along with the mammoth prints nearby, and that the prints could be of the same age,” Bustos said.

To place these interactions in time, Kathleen Springer and Jeffrey Pigati, scientists with the U.S. Geological Survey who typically spend their time studying the paleoclimate, joined the team of scientists working on the prints.

They began the work in January 2020, taking samples of seeds from an aquatic plant called ditchgrass that was interspersed with the footprints. Using careful geologic studies and radiocarbon dating, they got an astonishing result: The prints were 21,000 to 23,000 years old.

“It was kind of a big deal and earth-shattering and rocked the world of the archaeologists,” Springer said.

For decades, experts believed the first people in the Americas migrated from Siberia across the Bering Strait on a land bridge exposed during the last glacial maximum, sometime between 26,500 and 19,000 years ago. The land bridge was then submerged as ice sheets melted around 13,000 years ago. These people, the thinking went, developed the 13,000-year-old Clovis culture in New Mexico, which was characterized by the use of distinctive stone points.

In recent decades, archaeologists have found evidence that disrupts this “Clovis First” hypothesis. But the thinking that has continued to dominate the field is that people’s travel into the Americas would have been blocked by ice sheets, making much earlier arrivals over land unlikely.

Googling for oldest structure in the Americas leads to heaps of debate

The White Sands footprints, which appear to have been made during the last glacial maximum, would require a radical rethinking of long-standing assumptions about how people got here — one that has been welcomed by some scholars and rejected by others.

For many Indigenous people, the study is simply a confirmation of things they already knew from knowledge passed down over generations, said Kim Pasqual-Charlie, a member of the Pueblo of Acoma who has visited the site many times.

“These are our ancestral footprints,” Pasqual-Charlie said.

Paulette Steeves, an Indigenous archaeologist and a professor at Algoma University, has compiled a database of the evidence for earlier human presence in the Americas. She said that the White Sands find is only one strand in a growing body of evidence that people were in the Americas much earlier than archaeologists long believed.

“Think about the rest of the world [and] how much our understanding of human evolution has grown and been informed due to more archaeological work in the advancement of sciences. However, in the Americas, it has remained static,” Steeves said. “When it comes to adding Indigenous voices and expanding the time frame for Indigenous peoples in the Americas, there is still a lot of racism and bias in American archaeology.”

But there was a known problem with dating a site using ditchgrass seeds. Because ditchgrass is an aquatic plant, it takes in carbon dissolved in the water during photosynthesis. That could include older sources of carbon that can make the seeds look artificially old. Oregon State’s Davis worked with a team that took ditchgrass samples from 1947 and analyzed them using radiocarbon dating. The results suggested the plants were 7,400 years older than they were.

So he and other scientists asked for additional lines of evidence.

“You’re talking about a potential paradigm shift regarding the peopling of North America,” said David Rachal, a geoarchaeology consultant who was also critical of the original study and remains skeptical of the new one. “We have good models to say when people showed up and got on the scene. If it pushes back, it will upset everything we think we know.”

Analyzing ancient pine pollen and crystals of quartz

Springer and Pigati were keenly aware of the shortcomings of ditchgrass as a dating method and had always planned to see whether other streams of evidence supported their initial study.

For the follow-up, they gathered ancient pollen from coniferous trees that was embedded around the footprints. This type of material would not have the same problem as aquatic plants, because trees take carbon from the atmosphere. They also used a technique called “optically stimulated luminescence” to measure the energy built up in crystals of quartz within the White Sands sediments. This method allowed them to calculate the last time the mud that contains the footprints was exposed to sunlight or heat.

The pollen study was an arduous undertaking, requiring scientists in four laboratories scattered across the United States to work together to prepare and analyze the age of 75,000 grains of pollen. Using radiocarbon dating, they found the pollen from conifer trees dated to 22,600 to 23,400 years ago, matching their first results.

Their study of the quartz crystals showed they were 21,500 years old, give or take 2,000 years.

“I think that this study is so far the most convincing evidence of early human presence in the Americas,” said Bente Philippsen, leader of the National Laboratory for Age Determination at Norwegian University of Science and Technology.

Others raised technical questions about the methods and sampling. Rachal said it was possible that the pollen was “reworked,” meaning that older pollen could have gotten into the samples. Davis said he was glad to see the quartz crystal analysis but wanted to see more samples taken from different layers.

Springer and Pigati argued that their evidence is solid, and they will continue to work at the site.

“It didn’t bother me at first, but it’s starting to really nudge at me, I guess you could say,” said Pasqual-Charlie, of the Pueblo of Acoma. “How much more evidence do you need to say: We did exist back then. We’ve been in the Southwest region for a very long time.”

Jolie, of the University of Arizona, said that a debate about the dating methods may continue back and forth for years — and that is part of how science moves forward. But he added that for him, as a scientist and a person with mixed Oglala Lakota and Hodulgee Muscogee ancestry, the site brings to life a crucial period of human history that had been off limits, erased by the passage of time and the changing landscape.

“It’s a fun way to think about a shared common past for a lot of Native people,” Jolie said. “You can visualize little kids splashing in the mud. There’s nothing quite like seeing that little toddler footprint in the sand.”