Ancient human footprints preserved in the ground in New Mexico’s White Sands National Park are astonishingly old, scientists reported on Thursday, dating back about 23,000 years to the Ice Age.
The results, if examined closely, would rejuvenate the scientific debate about how humans first spread across the Americas, implying that they did so at a time when massive glaciers covered much of their path.
Researchers who have argued for such an early arrival hailed the new study as solid evidence.
“I think this is probably the biggest discovery about America’s population in 100 years,” said Ciprian Ardelean, an archaeologist at the Autonomous University of Zacatecas in Mexico who was not involved in the work. “I don’t know which gods they prayed to, but this is a dream find.”
For decades, many archaeologists have maintained that humans did not spread across the Americas until the end of the last ice age. They pointed to the oldest known tools, including spearheads, scrapers and needles, which are about 13,000 years old. The technology was known as Clovis, named after the town of Clovis, NM, where some of these first instruments came to light.
The age of the Clovis tools matched neatly with the retreat of the glaciers. That alignment reinforced a scenario where Siberian hunter-gatherers moved to Alaska during the Ice Age, where they lived for generations until ice-free corridors opened and allowed them to expand south.
But beginning in the 1970s, some archaeologists began publishing older evidence of humanity’s presence in North America. Last year, Dr. Ardelean and his colleagues a report of stone tools in a mountain cave in Mexico dating back 26,000 years.
Other experts were skeptical of such ancient finds. Ben Potter, an archaeologist at the Arctic Studies Center at Liaocheng University in China, said some of these hypothesized tools could actually be oddly shaped rocks. dr. Potter also questioned some of the dates scientists have assigned to their finds. For example, if a tool sinks into underlying sediment, it may appear older than it actually is.
“There are unresolved issues with each of them,” said Dr. Potter about the older alleged sites. “None of them are unequivocal.”
The White Sands study now adds a new line of evidence for an early arrival: Instead of tools, the researchers found footprints.
The footprints were first discovered in 2009 by David Bustos, the park’s program manager. Over the years, he has engaged an international team of scientists to help understand the finds.
Together, they have found thousands of human footprints in 80,000 acres of the park. One path was made by someone walking a mile in a straight line. Another shows a mother putting her baby on the floor. Other songs are made by children.
“The kids tend to be more energetic,” said Sally Reynolds, a paleontologist at Bournemouth University in England and a co-author of the new study. “They are much more playful and jump up and down.”
Mathew Stewart, a zooarchaeologist at the Max Planck Institute of Chemical Ecology in Jena, Germany, who was not involved in the study, said the evidence that humans left the footprints was “unambiguous.”
The footprints were formed as people stepped over moist, sandy ground at the edge of a lake. Later on, sediments gently filled the imprints and the ground hardened. But the ensuing erosion resurfaced the prints. In some cases, the impressions are only visible if the ground is unusually wet or dry – otherwise they are invisible to the naked eye. But ground-penetrating radar can reveal their three-dimensional structure, including the heels and toes.
Mammoths, dire wolves, camels and other animals also left footprints. A series of prints showed a giant sloth dodging a group of people, showing they were in close company.
“The fascinating thing about studying footprints is that they represent snapshots in time,” said Dr. stewart.
Jeffrey Pigati and Kathleen Springer, two research geologists with the United States Geological Survey, were tasked with determining the age of the prints.
In 2019, they went to White Sands to get a feel for the site. While walking around some of the footprints, the researchers sometimes came across old seeds of ditch grass that had grown near the lake. In some places, the abundant seeds formed thick blankets.
The researchers returned some of the seeds to their lab and measured the carbon in them to determine their age. The results came as a shock: the ditch grass had grown thousands of years before the end of the last ice age.
dr. Pigati and Mrs. Springer knew those numbers would be controversial. So they embarked on a much more ambitious study. “The darts are going to fly, so we better be prepared,” Dr. Pigati himself.
The scientists dug a trench near a cluster of human and animal footprints to get a better estimate of their ages. On the side of the trench, they could see layer after layer of sediment. By carefully mapping the surrounding soil, they were able to trace human and animal footprints up to six layers in the trench, interspersed with eleven seedbeds.
The researchers collected ditch grass seeds from each bed and measured their carbon. These measurements confirmed initial results: The oldest footprints at the site — left by an adult human and a mammoth — were under a seedbed that was about 22,800 years old.
In other words, the people who left the footprints walked around White Sands for about 10,000 years before the Clovis people. The youngest footprints, the researchers estimate, date from about 21,130 years ago. That meant that people lived or regularly visited the lake for about 2000 years.
“This is a bomb,” said Ruth Gruhn, an archaeologist at the University of Alberta who was not involved in the study. “At first glance, it’s very hard to disprove.”
dr. Potter praised the White Sands team for their care in the new study, saying it is the strongest case yet made for people in America before 16,000 years ago. But he would be more confident in the extraordinary age of the prints, he said, if there were other evidence beyond the ditch grass seeds. The seeds could have absorbed older carbon from the lake water, making them appear older than they actually are.
“I’d like to see stronger data, and I don’t know if it’s possible to get stronger data from this particular site,” he said. “If it’s true, it really has some profound implications.”
If humans were well established in New Mexico 23,000 years ago, they must have spread from Alaska long before that. “That’s starting to turn back the clock,” says Dr. Reynolds of Bournemouth University.
Some researchers have argued that humans could have spread across America even when the glaciers were at their peak. Instead of traveling across the mainland, they could have gone along the coast. As an alternative, Dr. Ardelean and his colleagues proposed that humans traveled inland more than 32,000 years ago, before the ice age glaciers reached their maximum extent and blocked that route.
dr. Gruhn argued that both scenarios remained possible in light of the new evidence from White Sands. It would take more work to find previous sites that favored one over the other. “We have a lot to do,” she said.
Bustos and his colleagues have more studies planned at White Sands. They want to learn more about the behavior of the people who left their footprints there. Did they hunt the animals around them? Did they live permanently by the lake or just visit?
They have to work fast. The erosion that has uncovered the footprints will erase them from the landscape in a few months or years. Numerous footprints disappear before the scientists have even seen them.
“It’s a little heartbreaking,” Mr. Bustos said. “We race to try and document what we can do.”