New scientific research has uncovered fossilised footprints in New Mexico which are thought to be the oldest human footprints in North America.
The footprints, discovered in White Sands National Park, are estimated by researchers to be between 21,000 and 23,000 years old.
Found at different depths below the surface, the dozen prints were buried within layers of soil and were revealed after researchers dug a trench on the park's western playa.
Scientists from the US Geological Survey used radiocarbon dating to analyse the seeds stuck in the footprints and concluded they could be up to 22,800 years old.
Many scientists had previously thought the earliest appearance of humans in the Americas was 11,000- 13,000 years ago due to stone spears found across North America.
But the authors of the study, published in the journal Science, said fossil footprints are stronger evidence than "cultural artefacts, modified bones, or other more conventional fossils".
Based on the size of the newly discovered footprints, researchers believe that at least some were made by children and teenagers who lived during the last ice age.
"What we present here is evidence of a firm time and location," they said.
David Bustos, the park’s resource program manager, spotted the first footprints in ancient wetlands in 2009. He and others found more in the park over the years. "We knew they were old, but we had no way to date the prints before we discovered some with (seeds) on top," he said on Thursday.
Made of fine silt and clay, the footprints are fragile, so the researchers had to work quickly to gather samples, Mr Bustos added. "The only way we can save them is to record them — to take a lot of photos and make 3D models."
Earlier excavations in White Sands National Park have uncovered fossilised tracks left by a saber-toothed cat, dire wolf, Columbian mammoth and other ice age animals.
The new research was conducted by experts from White Sands national park, the National Park Service, US Geological Survey, Bournemouth University, University of Arizona and Cornell University.