Study confirms age of oldest fossil human footprints in North America

Two new lines of evidence support the 21,000 to 23,000-year age estimate of the footprints first described and dated in 2021

The 2021 results began a global conversation that sparked public imagination and incited dissenting commentary throughout the scientific community as to the accuracy of the ages. 

  

“The immediate reaction in some circles of the archeological 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 targeted methodology in this current research really paid off,” said Jeff Pigati, USGS research geologist and co-lead author of a newly published study that confirms the age of the White Sands footprints. 

  

The controversy centered on the accuracy of the original ages, which were obtained by radiocarbon dating. The age of the White Sands footprints was initially determined by dating seeds of the common aquatic plant  Ruppia cirrhosa that were found in the fossilized impressions. But aquatic plants can acquire carbon from dissolved carbon atoms in the water rather than ambient air, which can potentially cause the measured ages to be too old. 

  

“Even as the original work was being published, we were forging ahead to test our results with multiple lines of evidence,” said Kathleen Springer, USGS research geologist and co-lead author on the current Science paper. “We were confident in our original ages, as well as the strong geologic, hydrologic, and stratigraphic evidence, but we knew that independent chronologic control was critical.” 

  

For their follow-up study, the researchers focused on radiocarbon dating of conifer pollen, because it comes from terrestrial plants and therefore avoids potential issues that arise when dating aquatic plants like Ruppia. The researchers used painstaking procedures to isolate approximately 75,000 pollen grains for each sample they dated. Importantly, the pollen samples were collected from the exact same layers as the original seeds, so a direct comparison could be made. In each case, the pollen age was statistically identical to the corresponding seed age. 

 

“Pollen samples also helped us understand the broader environmental context at the time the footprints were made,” said David Wahl, USGS research geographer and a co-author on the current Science article. “The pollen in the samples came from plants typically found in cold and wet glacial conditions, in stark contrast with pollen from the modern playa which reflects the desert vegetation found there today.” 

  

In addition to the pollen samples, the team used a different type of dating called optically stimulated luminescence, which dates the last time quartz grains were exposed to sunlight. Using this method, they found that quartz samples collected within the footprint-bearing layers had a minimum age of ~21,500 years, providing further support to the radiocarbon results. 

 

With three separate lines of evidence pointing to the same approximate age, it is highly unlikely that they are all incorrect or biased and, taken together, provide strong support for the 21,000 to 23,000-year age range for the footprints. 

New radiocarbon (14C) and optically simulated luminescence ages have confirmed the controversial antiquity of the ancient human footprints discovered in White Sands National Park, and reported in a study in 2021. Addressing the widespread criticism of their previous study, researchers report that the independent ages from multiple resolved sources conclusively show that the footprints were left behind between roughly 23,000 and 20,000 years ago, demonstrating that humans were present in southern North America during the Last Glacial Maximum (LGM).

When and how humans first migrated into North America has long been debated and remains poorly understood. Current estimates for the timing of these first occupants range from ~13,000 years ago to more than 20,000 years ago. However, the earliest archaeological evidence for the region’s settlement is sparse and often controversial. In a Science study published in September 2021 (Bennett et al.), researchers reported the discovery of in situ human footprints preserved in an ancient lakebed dating to between ~23,000 and 21,000 years ago in what is now White Sands National Park – findings which suggest nearly 2,000 years of human occupation in North America during the height of the LGM. However, since the study’s publication, the accuracy of the radiocarbon dates has been debated. It’s argued that the ancient seeds from the aquatic plant (Ruppia cirrhosa) that were used to date the surfaces the footprints were embedded in have the potential to be affected by old carbon reservoir effects that could influence the reported radiocarbon ages and make them appear older than they truly are.

Here, Jeffery Pigati, Kathleen Springer, and colleagues report new evidence in the form of multiple independent age estimates of the White Sands footprints, which support their previous study’s claims. “We always knew that we would have to independently evaluate the accuracy of our ages to convince the archaeological community that the peopling of the Americas occurred far earlier than traditionally thought,” said Pigati.

In their new work, Pigati and Springer et al. present calibrated 14C ages of terrestrial pollen collected from the same stratigraphic contexts as the Ruppia seeds. Unlike the seeds, conifer pollen fixes atmospheric carbon and, therefore is not subject to potential old carbon reservoir effects. According to the findings, the resulting calibrated 14C ages range from 23.4 ± 2.5 – 22.6 ± 2.3 thousand years ago. In addition, the authors obtained optically simulated luminescence ages of the sediments from within the footprint-bearing strata, which produced a minimum age of 21.5 ± 1.9 thousand years ago. In both cases, Pigati and Springer et al. show that the resolved dates were statistically indistinguishable from the original calibrated 14C ages of the oldest Ruppia seeds reported previously. In a related Perspective, Bente Phillipson discusses the study and its findings in greater detail.  

“Even as the original work was being published, we were forging ahead to test our results with multiple lines of evidence and independent chronologic techniques,” said Kathleen Springer, co-lead author of the study. “Although we were confident in the original seed ages, we wanted to develop community confidence in them as well. Our new ages, combined with the strong geologic, hydrologic, and stratigraphic evidence, unequivocally support the conclusion that humans were present in North America during the last Glacial Maximum.”