Traditionally, paleoanthropologists believed that humans arrived in North America around 16,000 to 13,000 years ago. Recently, however, evidence has accumulated supporting a much earlier date. In 2021, fossilized footprints from White Sands National Park in New Mexico, the United States, were dated to between 21,000 and 23,000 years ago, providing key evidence for earlier occupation, although this finding was controversial. In a new study, Dr. Jeffery Pigati of U.S. Geological Survey and colleagues returned to the White Sands footprints and obtained new dates from multiple, highly reliable sources; they, too, resolved dates of 20,000 to 23,000 years ago.
The ancient human footprints found at White Sands National Park in New Mexico, the United States. Image credit: Bennett et al., doi: 10.1126/science.abg7586.
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 about 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 September 2021, paleoanthropologists reported the discovery of 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, New Mexico, the United States.
Their 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,” Dr. Pigati said.
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 Dr. Kathleen Springer, also from U.S. Geological Survey.
“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.”
“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,” Dr. Pigati said.
In their follow-up study, Dr. Pigati, Dr. Springer and their colleagues 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 cirrhosa.
They 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.
According to the findings, the resulting calibrated carbon-14 ages range from 23,400 to 22,600 years ago.
In addition to the pollen samples, the researchers 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.
“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,” Dr. Springer said.
The findings appear in the journal Science.
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Jeffrey S. Pigati et al. 2023. Independent age estimates resolve the controversy of ancient human footprints at White Sands. Science 382: 73-75; doi: 10.1126/science.adh5007
