The footprints found in 2021 caused a sensation. They were fossil marks of human feet discovered in White Sands National Park, in New Mexico. But when the researchers presented their first dates, the results caused suspicion. Experts stated that the footprints had been made between 23,000 and 21,000 years ago, which clashed head-on with ancient theories about the population of the American continent.
“The immediate reaction in some circles of the archaeological community was that the precision of our dating was insufficient to make the extraordinary claim that humans were present in North America during the Last Glacial Maximum (about 20,000 years ago). But our specific methodology in this current investigation really paid off,” says Jeff Pigati of the United States Geological Survey (USGS) and co-lead author of a newly published study confirming the age of the White Sands markings.
The debate focused especially on the accuracy of the original dates, which were obtained by radiocarbon dating. The age of the New Mexico footprints was determined by seeds of the common aquatic plant Ruppia cirrhosa found in the fossilized impressions.
But aquatic plants can acquire carbon from atoms dissolved in water, potentially making measured ages older. That's why they devised a less problematic method: they dated coniferous pollen, because it comes from terrestrial plants and therefore avoids the difficulties of aquatic plants.
The archaeologists used isolated approximately 75,000 grains of fossilized pollen, a powerful scientific tool, for each sample they dated. These grains are very small, typically about 0.005 millimeters in diameter, so many are needed.
They were all collected from exactly the same layers as the original seeds, so a direct comparison could be made. In each case, pollen age was statistically identical to the corresponding seed age of Ruppia cirrhosa.
"The pollen samples also helped us understand the broader environmental context at the time the footprints were left," said David Wahl, a USGS research geographer and co-author of the paper published in the journal Science. "The pollen came from plants normally found in cold, wet, glacial conditions, in stark contrast to what is currently present reflecting the desert vegetation found there today," he adds.
In addition to the pollen samples, the team used a different type of dating called optically stimulated luminescence, which dates the last time the quartz grains were exposed to sunlight. Using this method, they found that quartz samples collected within the footprint-bearing layers were at least 21,500 years old, further supporting the radiocarbon results.
In the 20th century, archaeologists believed that humans reached the interior of North America about 14,000 years ago, coinciding with the formation of an ice-free corridor between two immense ice sheets straddling what is now Canada and northern Canada. USA.
According to this idea, the corridor, created by melting ice at the end of the last Ice Age, allowed humans to travel from Alaska to the heart of North America. But, little by little, this hypothesis is falling apart. In recent decades, for example, evidence of human existence as far back as 16,000 years ago has already appeared.
Fossil footprints discovered in New Mexico were made by a group of people passing by an ancient lake near what is now White Sands. The discovery added 7,000 years to the record of humans on the continent, rewriting American prehistory.
“If humans were in America at the height of the last Ice Age, either the ice posed few barriers to their passage or humans had been there much longer. Perhaps they had arrived on the continent during a previous period of melting,” the researchers question.
