David Bustos heard about the “ghost tracks” when he first went to White Sands National Park in New Mexico to work as a wildlife scientist in 2005. When the ground was wet enough at certain times of the year, the ghostly footprints would appear on the otherwise blank earth, only to disappear again when it dried out.
It wasn’t until over 10 years later, in 2016, that scientists confirmed that the ghost tracks had been made by real people — and it’s only now that some of the ancient footprints at White Sands have been dated as the earliest in North America.
“We’d been suspicious of the age for a while, and so now we finally have that it’s really exciting,” Bustos said. “One of the neat things is that you can see mammoth prints in the layers a meter or so above the human footprints, so that just helps to confirm the whole story.”
The footprints at White Sands were dated by examining the seeds of an aquatic plant that once thrived along the shores of the dried-up lake, Ruppia cirrhosa, commonly known as ditchgrass. According to research published Thursday in the journal Science and co-authored by Bustos, the ancient ditchgrass seeds were found in layers of hard earth both above and below the many human footprints at the site, and they were radiocarbon-dated to determine their age.
The tracks at one location have been revealed as both the earliest known footprints and the oldest firm evidence of humans anywhere in the Americas, showing that people lived there 21,000 to 23,000 years ago — several thousand years earlier than scientists once believed.
“It’s the earliest unequivocal evidence for humans in the Americas,” said the lead author of the study, Matthew Bennett, a professor of environmental and geographic sciences at Bournemouth University in the U.K.
Fossilized human footprints have now been found throughout the east of the national park, where the bed of a “paleo-lake,” which is now dry, supplies the gypsum-rich earth that is eroded by the wind to create the enormous white dunes for which the region is famous.
Any traces of early human habitation had been disputed because they relied on what seemed to be stone tools that might have formed naturally, Bennett said, or on artifacts that might have moved from their original stratigraphic layers. “A footprint is a really good, unequivocal data point,” he said. “That’s the importance of this site — we know they were there.”
The footprints would in turn give greater credibility to other evidence of early humans in the Americas.
“You can now look at the oldest sites and say, ‘We know they were there during the Last Glacial Maximum,’ so maybe some of these oldest sites are also reliable,” he said.
The term “Last Glacial Maximum” is how scientists refer to the height of the last ice age, about 20,000 to 26,000 years ago.
It has long been debated whether humans arrived in the Americas by a northern route from Siberia before or after the Last Glacial Maximum, when vast sheets of ice would have made migration along the Pacific Coast or through western Canada impossible.
The ancient footprints at White Sands answer that question, suggesting that they may have arrived up to 30,000 years ago, thousands of years before the height of the ice age, Bennett said.
White Sands is now mostly a desert, but it was a lush wetland at the time the footprints were made and populated by mammoths, ground sloths, bovids — cattle — and wild camels, as well as by the Stone Age humans who hunted them.
The footprints, which are intermingled with the tracks of the animals, show that people must have lived there for at least 2,000 years, Cornell University archaeologist Thomas Urban, a co-author of the research, said in an email.
“There are multiple footprint layers spanning a significant amount of time, suggesting a sustained human presence in the area during the Last Glacial Maximum, as opposed to a single event,” he said.
Urban developed the noninvasive use of ground-penetrating radar to reveal the footprints beneath the surface and show the researchers the best places to excavate.
Smaller footprints made by teenagers and children outnumber those made by adults, Urban said, possibly because they were involved in tasks that involved simple labor, instead of skilled tasks like hunting.
“Their presence is simply part of ordinary life and should be expected,” he said. Their activities may have ranged from playing to chores, such as gathering food, water and raw materials for their hunter-gatherer community.
Geologist Cynthia Liutkus-Pierce of Appalachian State University in North Carolina, who has studied ancient human footprints in Tanzania and wasn’t involved in the White Sands research, said it was often difficult to date just when fossilized footprints were made, especially when they were pressed into layers of mud — as at White Sands — and not into more easily dated volcanic ash.
It’s “great to see that this team was able to constrain the date of the footprint formation using radiocarbon dates from the [layers] above and below,” she said in an email.
Unlike bones or artifacts, footprints are unique in that they recorded fossilized behavior, and their analysis can yield clues about the print-makers.
“Human footprints give us a glimpse into the lives of our ancestors and, in this case, provide detailed information on their day-to-day activities and social dynamics,” Liutkus-Pierce said.
This story first appeared on NBCNews.com.