Stepping back. One of many apparent human footprints in Mexico that researchers have dated to around 40,000 years ago.

Ancient Human Footprints Found in Mexico

LONDON--Humans left their footprints on the shores of an ancient Mexican lake more than 25,000 years before people were thought to have colonized the Americas, according to a new report. But some experts are skeptical that the 40,000 year-old impressions, which would represent the oldest evidence of humans in the Americas, are of feet at all.

"The jury's still out on whether these are footprints," says archeologist Tom Dillehay of the University of Kentucky in Lexington. But he adds, "It would be wrong for scientists to dismiss these outright."

Geochronologist Silvia Gonzalez of Liverpool John Moore University in the U.K. stumbled upon the prints in an abandoned quarry along what had been the shore of an ancient lake in the Valsequillo basin in central Mexico. "It felt like a thunderbolt," she says. Gonzalez believes the prints may have been created when humans walked across a layer of fresh ash deposited by a nearby volcano. Some of the impressions show clear toe prints, she says, and a few form tracks with several steps along a line. Radiocarbon and electron spin resonance dating of nearby sediments places the prints at about 40,000 years old.

The findings push back the date that humans arrived in the Americas, says Gonzalez, whose team reported its results at a press conference here yesterday. This would not rule out a journey across the Bering Strait and through an ice-free corridor into present-day Canada, she says, as earth scientists believe a corridor was traversable several times over the past 50,000 years. But Gonzalez supports a different migration route along the Pacific coast, an idea which is gaining more adherents.

The team's dating results look good but are preliminary, says Henry Schwarcz, a geologist who specializes in dating methods at McMaster University in Ontario, Canada. Doubts center more on the interpretation of the impressions. "I have really serious reservations about whether these are footprints," says geoarcheologist Mike Waters of Texas A&M University in College Station, who excavates another site in the same basin. Waters believes all of the impressions are likely artifacts of quarrying and erosion. But he says the general area is promising for signs of early humans. "It's worthy of further investigation," he says.

Gonzalez says she and colleagues will try to convince skeptics. "I know we are in for a fight, but we are not afraid." Last week, her team secured a $372,000 grant from the U.K. Natural Environment Research Council to do excavations on the site, which may help quell doubts.

Related sites
Research team's site on the footprints
Royal Society Summer Exhibition site on the footprints