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Mexican footprints' origin offers puzzle to researchers

"You're really only left with two possibilities ... One is that they are really old hominids -- shockingly old -- or they're not footprints."

Paul Renne, director of the Berkeley Geochronology Center

Footprints discovered in Mexico are either more than 1 million years older than other evidence of humans in the Western Hemisphere or not footprints at all, according to a new report that was to be published in the journal Nature on Thursday.

In July, researchers in England claimed the prints proved that humans were in the Americas 40,000 years ago -- much earlier than the accepted date of 11,500 years ago.

But Paul Renne, director of the Berkeley Geochronology Center and an adjunct professor at Uni- versity of California-Berkeley, says the prints are about 1.3 million years old.

"You're really only left with two possibilities," Renne said. "One is that they are really old hominids -- shockingly old -- or they're not footprints."

The footprints were first discovered by a team of British scientists in 2003 in an abandoned quarry close to the Cerro Toluquilla volcano in the Valsequillo Basin, near Puebla, Mexico.

The researchers hypothesized that early hunters walked across ash freshly deposited near a lake by volcanoes that are still active.

The so-called footprints, subsequently covered by more ash and inundated by lake waters, eventually turned to rock.

The new study was conducted by geologists at the Berkeley Geochronology Center and the University of California, Berkeley, who were part of an investigative team of geologists and anthropologists from the US and Mexico.

Paleoanthropologist Tim White, professor of integrative biology at UC Berkeley, said he was not surprised at the new finding.

"The evidence [the British team] has provided in their arguments that these are footprints is not sufficient to convince me they are footprints," said White, who did not contribute to the new study.

The oldest accepted human fossil from the Americas is an 11,500-year-old skull. Homo sapiens are not thought to have appeared in Africa until about 160,000 years ago.

Geologist Silvia Gonzalez of Liverpool's John Moores University, who was the leader of the British team, said she would not rule out the possibility that her theory was correct without doing further research.

"The new finding doesn't necessarily mean that [1.3 million years ago] is the correct date. The results would need to be replicated to make sure that everything makes sense," Gonzalez said in an interview on Wednesday.

She also said part of the problem in verifying the dates of the deposits in Mexico's Valsequillo Basin is the amount of different materials in the particles.

"But the fact that that is the case doesn't automatically mean that they aren't footprints," Gonzalez said.

Her team has funding to do further analysis in the basin for the next three years, she said.
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