The debate over just how old the oldest fossilised human footprints found in North America really are has been settled, according to new research.
The beautifully-preserved footprints belonging to adults and children are more than 20,000 years old, upending previous theories about when humans first arrived on the continent.
Previous estimates suggest the original inhabitants, known as the Clovis people, arrived about 13,000 years ago.
Now however, two new lines of evidence support the 21,000 to 23,000-year-old estimate of the prints made two years ago – confirming humans were present in North America when ice sheet and glacier coverage on Earth’s surface peaked, known as the Last Glacial Maximum.
The original 2021 results kicked off a global conversation among the science community as to the accuracy of the ages.
Co-lead author of the new study, Dr Jeff Pigati, said the latest findings confirm the age of the White Sands footprints, found in New Mexico.
Dr Pigati, of the US Geological Survey (USGS), said: ‘The immediate reaction in some circles 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 targeted methodology in this current research really paid off.’
The controversy centred 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 fossilised 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 of the new 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 the follow-up study, published in the journal Science, the research team focused on radiocarbon dating of conifer pollen, because it comes from terrestrial plants and therefore avoids potential issues that arise when dating aquatic plants such as Ruppia.
The team used painstaking procedures to isolate around 75,000 pollen grains for each sample they dated.
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 study co-author Dr David Wahl, a USGS research geographer.
‘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 [flat, dried land] which reflects the desert vegetation found there today.’
The USGS team also used a different type of dating called optically stimulated luminescence, which dates the last time quartz grains were exposed to sunlight.
Using that method, they found that quartz samples collected within the footprint-bearing layers had a minimum age of around 21,500 years, providing further support to the radiocarbon results.
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