New findings confirm the age estimate of these footprints at 21,000 to 23,000 years old, much older than the previous date of human presence on the continent.
When did humans first appear in North America? Traditionally, the most widely accepted theory is the Clovis model, which suggests that the first humans came to North America about 13,000 years ago via a land bridge connecting Siberia to Alaska known as Beringia. Archaeological evidence for this theory comes from Clovis-style stone tools found in several locations in North America.
In recent years, however, this view has been challenged by new evidence suggesting that the human presence in North America may be much older. For example, at the Monte Verde site in Chile, evidence was found that humans may have lived in the Americas more than 18,000 years ago. There are also sites in North America, such as Bluefish Caves in Canada and Meadowcroft Rockshelter in the United States, that suggest a similar or even older age.
When fossil footprints were dated to be more than 20,000 years old in 2021, it sparked a global debate that captured the public’s imagination and sparked dissenting commentary across the scientific community about the accuracy of the ages.
“The immediate reaction in some quarters of the archaeological community was that the accuracy of our dating was insufficient to make the extraordinary claim that humans were present in North America during the Last Glacial Maximum. But our specific methodology in this research was worth it,” says Jeff Pigati, a USGS research geologist and co-author of a recently published study confirming the age of the White Sands footprints.
Footprints and carbon dating
The controversy centered on the accuracy of the original ages determined by radiocarbon dating. The age of the White Sands prints was first determined by dating the seeds of the common aquatic plant Ruppia cirrhosa found in the fossilized prints. However, aquatic plants can absorb carbon from carbon atoms dissolved in the water rather than from the surrounding air, which can cause the measured ages to be too old.
“Even when the original work was published, we tested our results against multiple lines of evidence,” said Kathleen Springer, a USGS research geologist and co-lead author of the current Science article. “We were confident in our original age and in the strong geological, hydrological and stratigraphic evidence, but knew that independent chronological control was essential.”
For their follow-up study, the researchers focused on radiocarbon dating of needle pollen because it comes from land plants and therefore avoids the potential problems encountered when dating aquatic plants like Ruppia. The researchers used careful procedures to isolate about 75,000 pollen grains for each sample they dated. More importantly, the pollen samples were taken from exactly the same layers as the original seeds, so a direct comparison was possible. In all cases, the age of the pollen was statistically identical to that of the corresponding seed.
Pollen to the rescue
“The pollen samples also helped us understand the broader environmental context at the time the footprints were formed,” said David Wahl, a USGS research geographer and co-author of the current Science paper. “The pollen in the samples came from plants typically found in cold, wet glacial conditions, in stark contrast to modern beach pollen, which reflects the desert vegetation found there today.”
In addition to the pollen samples, the team used another type of dating, called optically stimulated luminescence, which dates the last time the quartz grains were exposed to sunlight. Using this method, they discovered that the quartz samples collected in the footprint layers had a minimum age of about 21,500 years, confirming the radiocarbon results.
Since three different lines of evidence point to the same approximate age, it is highly unlikely that they are all false or biased, and taken together they provide strong evidence for the age range of the footprints between 21,000 and 23,000 years.
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Independent age estimates resolve controversy over ancient human footprints at White Sands