British scientists on Tuesday claimed to have unearthed 40,000 year-old human footprints in central Mexico, challenging previous studies that put the arrival of the first humans in the Americas at about 13,500 years ago.
Scientists Silvia Gonzalez, from Liverpool John Moores University, and Matthew Bennett, of Bournemouth University, found the footprints in an abandoned quarry close to the Cerro Toluquilla volcano in the Valsequillo Basin, near Puebla, south of Mexico City in 2003.
Gonzalez said the footprints were preserved as trace fossils in volcanic ash along what was the shoreline of an ancient volcanic lake.
"Climate variations and the eruption of the Cerro Toluquilla volcano caused lake levels to rise and fall, exposing the Xalnene volcanic ash layer," Gonzalez said.
She said the footprints, which were preserved when water levels rose, were now hard as concrete and had been uncovered without excavation as quarry workers had already removed two to three metres of lake sediment that had been deposited on top of the volcanic ash layer.
The footprints were analysed and dated by a team of international scientists using laser technology.
The findings challenge previously held ideas about the settlement of the Americas.
Scientists have long believed that the first humans came to North America after the last Ice Age ended about 13,500 years ago.