In 2021, archaeologists caused a stir when they announced that a set of fossilized human footprints in New Mexico dated to more than 20,000 years ago—thousands of years before humans were thought to have set foot in North America. Now, two more dating methods appear to confirm this age.
For decades, people have believed that the earliest humans in North America were Clovis people, and there is a lot of evidence that they appeared on the North American continent as early as 13,000 years ago. However, some scattered signs of human presence may predate the Clovis people by thousands of years.
In 2021, a controversial study examined human footprints at an archaeological site in White Sands National Park, New Mexico, and found that the footprints were approximately 21,000 to 23,000 years old. If this were true, the timeline of human migration would change dramatically.
However, in order to be a healthy demonstration of how science should work, this study is not without its skeptics. The age of the footprints was initially determined through radiocarbon dating of seeds of a plant called Ruppiacirrhosa preserved in the sediment where the footprints were found. The problem is that this is an aquatic plant, which means the captured carbon came from the water rather than the air, so it's possible to arrive at an age older than the footprints themselves.
So in the new study, the team used two additional methods to date the samples. They collected tens of thousands of pollen grains from the same strata as the original seeds—only this time the pollen came from conifers, a land plant. They conducted radiocarbon dating on the pollen, and sure enough, the age of the pollen was between 21,000 and 23,000 years old.
"Pollen samples also help us understand the broader environmental context in which the footprints were produced," said study co-author David Wahl. "The pollen in the samples came from plants typically found in cold, wet glacial conditions, which contrasts with pollen from modern dunes, which reflects the desert vegetation found there today."
Next, the researchers used another method called photostimulated luminescence, which works by counting the last time the sample was exposed to sunlight. They calculated samples of quartz grains inside the footprints and found similar windows again - they had been buried for at least 21,500 years.
With three different techniques all pointing to the same time frame, the researchers say their original age estimate is now much stronger. The study raises new questions about how exactly the first humans arrived in North America - after all, it was at the height of the last ice age, and huge glaciers are thought to have blocked passage from Siberia to Alaska to Canada.
The research was published in the journal Science.