LONDON — British scientists claimed on Tuesday 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 as hard as concrete and had been uncovered without excavation after quarry workers removed 6 to 10 feet (2 to 3 meters) of lake sediment that had been deposited on top of the volcanic ash layer.
The footprints were analyzed 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. According to that theory, they crossed a land bridge from Asia into what is now Alaska and spread quickly across the continent.
The theory is supported by the stone tools they left behind — all less than 13,500 years old. Their tool technology was named "Clovis" for the New Mexico town where it was first described.
Gonzalez said the findings supported a theory that the first colonies may have arrived by water, using the Pacific coast migration route, rather than by foot.
"We think there were several migration waves into the Americas at different times by different human groups," she said.
The findings are being exhibited as part of a summer exhibition at London's prestigious scientific academy, the Royal Society.
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