Footprints suggest humans arrived earlier in Americas
Ancient human footprints preserved in the ground across the White Sands National Park in New Mexico are astonishingly old, scientists reported 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.
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, dating back about 13,000 years.
The age of the tools lined up neatly with the retreat of the glaciers. That alignment bolstered a scenario in which Siberian huntergatherers moved into Alaska during the ice age, where they lived for generations until ice-free corridors opened and allowed them to expand southward.
Other experts have been skeptical of such ancient finds. Ben Potter, an archaeologist at the Arctic Studies Center at Liaocheng University in China, said some of these supposed tools might actually be oddly shaped rocks.
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.
The footprints were formed when people strode over ground on the margin of a lake. Later, sediments 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. Ground-penetrating radar can reveal their structure, including heels and toes.
The work of determining the age of the prints fell to Jeffrey Pigati and Kathleen Springer, two research geologists at the U.S. Geological Survey.
In 2019, they went to White Sands to get a feel for the site. Walking around some of the footprints, the researchers sometimes came across ancient seeds of ditch grass that had grown by the lake. In some spots, the abundant seeds
formed thick blankets.
The researchers brought some of the seeds back to their lab and measured the carbon in them to determine their age. The shocking results: The ditch grass had grown thousands of years before the end of the last ice age.
Pigati and Springer knew those numbers would be controversial. So they embarked on a far more ambitious study.
The scientists dug a trench near one cluster of human and animal footprints to get a tighter estimate of their age. On the side of the trench, they could see layer after layer of sediment. They were able to trace the footprints of humans and animals to six layers in the trench, interspersed with 11 seed beds.
The researchers collected ditch grass seeds from each bed and measured their carbon. These measurements confirmed the initial results: The oldest footprints at the site — left by an adult human and a mammoth — were located below a seed bed dating back about 22,800 years.