With the new method, the dates of the bedrock fragments and the magma from the site in Mexico match—and revealed the date of the eruption itself, Gonzalez said.
"We couldn't possibly do this even five years ago," Gonzalez said.
Just to be sure, her team also used carbon-14 dating to determine the ages of shells in the sediments above and below the ash layer.
"You can't date just one layer," she said. "You have to makes sure the whole stratigraphy makes sense."
The sediments immediately below the ash were between 70,000 and 100,000 years old, Gonzalez's team found.
The sediments immediately above the ash ranged from 9,000 to 40,000 years old. The dates of all three layers therefore suggest the footprints were made about 40,000 years ago, she said.
(Related: "20,000-Year-Old Human Footprints Found in Australia" [August 3, 2006].)
To make sure the footprints were indeed human, Gonzalez's team made 3-D images with a laser scanner, then compared them to human footprints made by volunteers marching up and down a beach in the United Kingdom.
"Now we are quite happy to give the word that these are indeed [human] footprints," she said.
"We know there were adults and children," she added, adding that looking at the prints was touching.
The laser scanning also had an unexpected side benefit: It preserved the data of the Mexico site, which is rapidly disappearing as road builders take stone from the quarry.
"Half the site is gone," Gonzalez said.
Because ancient footprint trails tend to be found in quarries and other vulnerable sites, mining and development are major problems for archaeologists.
"The problem is who is going to be responsible for the preservation of these sites," Gonzalez said.
Other scientists are skeptical of the conclusion that humans arrived 25,000 years earlier than generally thought.
"I am amazed that they are still flogging that dead horse," said Paul Renne, of the University of California, Berkeley's Geochronology Center. Renne led that team that initially dated the Valsequillo Basin strata.
"We are about to publish even more data showing that the rocks are 1.3 million years old and that the 'footprints' are not," he said by e-mail.
Rafael Suárez of the Museo Nacional de Historia Natural y Antropología in Montevideo, Uruguay, is more cautious—but also dubious.
"Very old human occupation of the Americas is possible," he said, "but if there were indeed people here that long ago, what happened to them in the next 25,000 years?"
"In this time, surely the population should have increased, and this would bring the presence of a high quantity of sites 16,000 to 20,000 years old," he said by email.
(Read: "Who Were the First Americans?" [September 3, 2003].)
James Dunbar, an archaeologist at Florida's Bureau of Archaeological Research in Tallahassee cautions that carbon-14 dating in the 40,000-year age range is often suspect, because the samples might include "dead carbon," or carbon not originally from living organisms.
Dunbar is impressed, though, by the digital laser imaging system used in scanning the footprints.
That method, he said by email, is a "very sophisticated way to help see features that are sometimes not detectable to the human eye."
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