The discovery of a footprint in Mexico suggests that humans were on the
continent more than 25,000 years earlier than believed.

BY CATHERINE McALOON
Associated Press

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 González, 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.

González 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,''
González 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
already had removed 6.5 to 9.8 feet 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 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.

'The existence of 40,000-year-old human footprints in Mexico means that the
`Clovis First' model of human occupation can no longer be accepted as the first
evidence of human presence in the Americas,'' said David Huddart, a professor at
Liverpool John Moores University and a collaborator on the discovery.

González 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.