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Ancient human footprints preserved in the ground throughout White Sands National Park in New Mexico are surprisingly old, scientists reported Thursday, dating back about 23,000 years to the Ice Age.
The results, if they stand up to scrutiny, would rejuvenate the scientific debate about how humans first spread across the Americas, implying that they did so at a time when huge glaciers covered much of their way.
Researchers who have argued for such an early arrival hailed the new study as compelling evidence.
“I think this is probably the biggest find about the settlement of America in a hundred 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 it was a dream find.”
For decades, many archaeologists have argued that humans did not spread to North and South America until the end of the last Ice Age. They indicated the oldest known tools, including spear points, scrapers, and needles, dating from around 13,000 years ago. The technology was known as Clovis, named after the town of Clovis, New Mexico, where some of these early instruments saw the light of day.
The age of Clovis tools aligns perfectly with the retreat of the glaciers. This alignment reinforced a scenario in which Siberian hunter-gatherers settled in Alaska during the Ice Age, where they lived for generations until ice-free corridors opened up and allowed them to expand. to the south.
But starting in the 1970s, some archaeologists began to publish older evidence of humanity’s presence in North America. Last year, Dr Ardelean and his colleagues published a report on stone tools in a mountain cave in Mexico dating back 26,000 years.
Other experts have been skeptical of these ancient findings. Ben Potter, an archaeologist at the Center for Arctic Studies at Liaocheng University in China, said some of the supposed tools could in fact be oddly shaped rocks. Dr Potter also questioned some of the dates that scientists have attributed to their findings. If a tool sinks into the underlying sediment, for example, it may look older than it actually is.
“There are unresolved issues with all of them,” Dr Potter said of the allegedly older sites. “None of them are unequivocal.”
The White Sands study now adds a new source of evidence for an early arrival: Instead of tools, researchers found footprints.
The footprints were first discovered in 2009 by David Bustos, the park’s resource program manager. Over the years, he called on an international team of scientists to help make sense of the findings.
Together, they found thousands of human footprints on 80,000 acres of the park. A path was made by someone walking in a straight line for a mile and a half. Another shows a mother putting her baby on the floor. Other pieces were created by children.
“Children tend to be more energetic,” said Sally Reynolds, a paleontologist at the University of Bournemouth in England and co-author of the new study. “They are a lot more playful, jump up and down.”
Mathew Stewart, a zoological archaeologist at the Max Planck Institute for Chemical Ecology in Jena, Germany, who was not involved in the study, said the evidence that humans left the footprints was “unequivocal “.
Footprints formed when people walked on wet, sandy ground by a lake. Later, sediment gently filled the indentations and the ground hardened. But subsequent erosion resurfaced on the footprints. In some cases, the footprints are only visible when 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 heels and toes.
Mammoths, large wolves, camels and other animals have also left footprints. A series of prints showed a giant sloth avoiding a group of people, demonstrating that they were in close company.
“What is fascinating about the study of footprints is that they present snapshots in time,” said Dr. Stewart.
The work of determining the age of the indentations fell to Jeffrey Pigati and Kathleen Springer, two research geologists from the United States Geological Survey.
In 2019, they took to White Sands to get a feel for the site. While skirting some of the footprints, researchers occasionally came across ancient seeds of ditch grass that had grown by the lake. In some places the abundant seeds formed thick blankets.
The researchers took some of the seeds back to their lab and measured the carbon they contained to determine their age. The results were a shock: the grass in the ditches had grown thousands of years before the end of the last ice age.
Dr Pigati and Ms Springer knew these numbers would be controversial. They therefore embarked on a much more ambitious study. “The darts are going to start flying, so we better be ready for them,” Dr Pigati recalled.
Scientists dug a trench near a cluster of human and animal footprints to get a more accurate estimate of their age. On the side of the trench, they could see layer after layer of sediment. Carefully mapping the surrounding soil, they were able to trace the footprints of humans and animals across six layers in the trench, interspersed with eleven seed beds.
The researchers collected ditch grass seeds from each bed and measured their carbon. These measurements confirmed the first results: the oldest footprints at the site – left by an adult human and a mammoth – were located under a seedbed dating back about 22,800 years.
In other words, the people who left the footprints walked White Sands about 10,000 years before the Clovis. The youngest footprints, the researchers estimated, were around 21,130 years old. This meant that people lived or visited the lake regularly for around 2,000 years.
“It’s 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 difficult to refute.
Dr Potter praised the White Sands team for the care given to the new study, saying it was the strongest case ever presented for people in the Americas 16,000 years ago. But he would feel more confident in the extraordinary age of the footprints, he said, if there was any other evidence beyond the grass seeds of the ditches. The seeds might have absorbed older carbon from the lake water, making them appear older than they actually are.
“I would like to see more solid data, and I’m not sure if it’s possible to get more solid data from this particular site,” he said. “If that’s true, then it really has profound implications.”
If humans were well established in New Mexico 23,000 years ago, they must have started to spread from Alaska long before that. “It’s starting to go back in time,” said Dr Reynolds of the University of Bournemouth.
Some researchers have argued that people could have spread across the Americas even when glaciers were at their peak. Instead of traveling to the mainland, they could have moved along the coast. Alternatively, Dr Ardelean and his colleagues proposed that people traveled inland over 32,000 years ago, before Ice Age glaciers reached their maximum extent and blocked this route. .
Dr Gruhn argued that both scenarios remain possible in light of new evidence from White Sands. It would take more work to find earlier sites that favored one over the other. “We have a lot to do,” she says.
Mr. Bustos and his colleagues have scheduled further investigations at White Sands. They want to know more about the behavior of the people who left their marks there. Did they hunt animals around them? Did they live at the lake all the time or did they just visit?
They have to work quickly. The erosion that revealed the footprints will erase them from the landscape in a matter of months or years. Countless footprints are disappearing before scientists even see them.
“It’s a bit heartbreaking,” Bustos said. “We are running to try to document what we can.”
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