( n in the Americas and would support a m...)Alleged 40,000-year-old human footprints in Mexico much, much older than thought

One of the team members, Matthew Bennett of Bournemouth, was quoted on a Royal Society Web site as saying, "Accounting for the origin of these footprints would require a complete rethink on the timing, route and origin of the first colonization of the Americas."

Renne, Knight, Waters and the Mexico City archeologists visited the site at the Toluquilla quarry last year while collecting rocks from another anthropological site across the reservoir. Renne noted that the black, basaltic rock is very tough and is mined in slabs for building. Pre-Columbian Mexicans also constructed buildings from the rock, which they called xalnene, meaning "fine sand" in the Nahuatl language. Today, trucks headed toward the quarry routinely drive across the xalnene tuff in which the alleged footprints are found, and the rock itself is pockmarked with many depressions in addition to the alleged footprints.

"They're scattered all over, with no more than two or three in a straight line," which would be expected if someone had walked through the ash, Renne said. If the depressions were footprints, they could not have been made by modern humans, he noted, since even in Africa, Homo sapiens did not appear until about 160,000 years ago. Given the age of the volcanic rock and lacking other evidence of early human ancestors in the Americas 1.3 million years ago, the researchers wrote in their paper, "we consider such a possibility to be extremely remote."

Many paleontologists have withheld judgment on the alleged footprints, awaiting good geological dates, Feinberg said. "With this study, we're trying to nip any misrepresentation in the bud."