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Elana Spivack

A Disputed Paper Claimed Humans Came to North America 23,000 Years Ago — It Just Got Backup

— Karen Carr/National Park Service

In January 2020, Jeff Pigati and Kathleen Springer, both research geologists at the U.S. Geological Survey, went to New Mexico’s Tularosa Basin at White Sands National Park to see about some footprints. These weren’t just any footprints; the fossilized tracks represent the oldest human footprints in North America. What’s more, Tularosa Basin, about 20,000 years ago, was in the midst of what’s known as the Last Glacial Maximum (LGM). During this chilly, final part of the Pleistocene Era, the global sea level was about 400 feet lower and glaciers covered 25 percent of Earth’s land. Their mission was to find out just how old these footprints were using different dating techniques of biological markers around them.

They were able to complete one method before the pandemic hit. They’d used radiocarbon dating on fossilized impressions of Ruppia cirrhosa seeds, a type of aquatic ditch grass. These seeds had been in the same stratigraphic layer as the footprints, placing them in the same time period.

Then the pandemic rolled in. They couldn’t return to the Basin to collect samples for the other analyses. They went ahead with publishing their results from the Ruppia seeds in a 2021 paper in the journal Science, concluding that the seeds were between 21 and 23,000 years old; therefore, so were the footprints. Not only does this conclusion put an age to footprint fossils, but it alters the suggested timeline of when humans came to North America. According to the paper, if these footprints were 23,000 years old, then humans had entered North America before the LGM ice sheets formed, challenging the notion that humans have only been on the continent for about 15,000 years.

This paper raised controversy, even drawing a comment also published in Science four months later, noting that Ruppia seed fossils aren’t reliable age markers through radiocarbon dating, which analyzes how much the isotope carbon-14 has decayed in organic matter. Aquatic plants can absorb dissolved carbon from the water, which could inflate their radiocarbon-dated age by thousands of years.

Pigati and Springer knew this, which is why their plan all along had been to include two other dating methods. “We knew even at that point that dating aquatic material could be potentially problematic,” Pigati told Inverse. So, they planned to bolster their analysis with dating techniques on two other, distinct specimens. In January 2022, they finished the work they’d set out to do. Today, they publish another paper in Science, showing the analyses they’d intended to do since 2020. This evidence, too, shows that the footprints are between 21,000 and 23,000 years old.

“The bottom line is that people were in what is now southern New Mexico 23,000 years ago,” Springer tells Inverse. “These people had to have been here before the ice sheets closed.”

The new evidence comes from fossilized pollen and some quartz. The pollen skirts the carbon-absorption issue from Ruppia because it comes from terrestrial trees, and can’t suck up dissolved carbon. Radiocarbon dating of pollen seeds from the same layer of earth as the footprints rendered that they, too, were 21 to 23,000 years old. What’s more, these pollen came from conifers, which thrive in colder climes. While there aren’t any conifers where the Tularosa Basin is now, it would’ve been cold enough tens of thousands of years ago.

As for the quartz, Pigati, Springer, and their team performed what’s called optically stimulated luminescence (OSL). OSL is based on the buildup of luminescence, or light, properties in quartz crystals. It’s possible to actually see how long it’s been since the crystals last saw sunlight. This, too, corroborated the 21 to 23,000 hypothesis.

“That means that there was enough of a population that sustained itself for people to visit this lake edge over and over and over again and leave their thousands of footprints during that period of time,” Springer says. “So what it does establish is: People were here.”

Another U.S.G.S. research geologist, Summer Praetorius, published a paper this past February in the journal Proceedings of the National Academy of Sciences examining the ways that people reached North America to create this population Springer describes. This further research questions how ancient peoples came to current North America via a coastal route when interior continental paths were blocked with ice sheets.

As for Springer and Pigati, they’ll continue dating White Sands and its surrounding area. “Our focus is on White Sands and telling the story of the context of the footprints,” Springer says.

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