Human footprints found in New Mexico are about 23,000 years old, a study reported, suggesting that people may have arrived long before the Ice Age’s glaciers melted.
By Carl Zimmer
Ancient human footprints preserved in the ground across the White Sands National Park in New Mexico are astonishingly old, scientists reported on Thursday, dating back about 23,000 years to the Ice Age.
The results, if they hold 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 massive glaciers covered much of their path.
Researchers who have argued for such an early arrival hailed the new study as firm proof.
“I think this is probably the biggest discovery about the peopling of America in a hundred years,” said Ciprian Ardelean, an archaeologist at Autonomous University of Zacatecas in Mexico who was not involved in the work. “I don’t know what gods they prayed to, but this is a dream find.”
For decades, many archaeologists have maintained that humans spread across North and South America only at the end of the last ice age. They pointed to the oldest known tools, including spear tips, scrapers and needles, dating back about 13,000 years. The technology was known as Clovis, named for the town of Clovis, N.M., where some of these first instruments came to light.
The age of the Clovis tools lined up neatly with the retreat of the glaciers. That alignment bolstered a scenario in which Siberian hunter-gatherers moved into Alaska during the Ice Age, where they lived for generations until ice-free corridors opened and allowed them to expand southward.
But starting in the 1970s, some archaeologists began publishing older evidence of humanity’s presence in North America. Last year, Dr. Ardelean and his colleagues published a report of stone tools in a mountain cave in Mexico dating back 26,000 years.
Other experts have been skeptical of such ancient finds. Ben Potter, an archaeologist at the Arctic Studies Center at Liaocheng University in China, said that some of these supposed tools might actually be oddly shaped rocks. Dr. Potter also questioned some of the dates scientists have assigned to their finds. If a tool sinks into underlying sediment, for example, it may appear to be older than it really is.
“There are unresolved issues with every single one of them,” Dr. Potter said of the older purported sites. “None of them are unequivocal.”
The study at White Sands now adds a new line of evidence for an early arrival: Instead of tools, the researchers have found footprints.
The footprints were first discovered in 2009 by David Bustos, the park’s resource program manager. Over the years, he has brought in an international team of scientists to help make sense of the finds.
Together, they have found thousands of human footprints across 80,000 acres of the park. One path was made by someone walking in a straight line for a mile and a half. Another shows a mother setting her baby down on the ground. Other tracks were made by children.
“The children tend to be more energetic,” said Sally Reynolds, a paleontologist at Bournemouth University in England and a co-author of the new study. “They’re a lot more playful, jumping up and down.”
Mathew Stewart, a zooarchaeologist at the Max Planck Institute for Chemical Ecology in Jena, Germany, who was not involved in the study, said that the evidence that humans had left the footprints was “unequivocal.”
The footprints were formed when people strode over damp, sandy ground on the margin of a lake. Later, sediments gently filled in the prints, and the ground hardened. But subsequent erosion resurfaced the prints. In some cases, the impressions 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 the heels and toes.
Mammoths, dire wolves, camels and other animals left footprints as well. One set of prints showed a giant sloth avoiding a group of people, demonstrating that they were in close company.