Discovery of Human Footprints Pushes Back Date of Earliest Humans in Americas

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.