Enlarge Quarternary Science Reviews The volcanic ash layer thought to have human footprints.

Human footprints frozen in time, lodged in volcanic ash in a Mexican valley, seemed poised to rock history.

In the current Journal of Human Evolution, a study tells the story of how they didn't — and how science checks out extraordinary claims.

"The timing and origin of the earliest human colonization of the Americas has been the subject of great debate over the last 100 years and is still a matter of heated discussion today," begins the study. Hiking on the dried bed of Mexico's Valsequillo Lake in the summer of 2003, an archeology team made a discovery they suspected would open a new chapter in the debate.

Crisscrossing the lakebed, they saw tracks, an ash field littered with hundreds of impressions that resembled footprints from adults and children, " along with birds, cats, dogs and species with cloven feet," as Nature magazine later reported. The team led by geoarchaeologist Silvia Gonzalez of the United Kingdom's Liverpool John Moores University, suspected the track's makers had fled an ancient eruption of the looming Cerro Toluquilla volcano, leaving their tracks in the now-famous "Xalnene Ash."

But how long ago did they flee? In 2005, Gonzalez and colleagues announced at the UK's Royal Society's Summer Science Exhibition that "optical stimulated luminescence" results, where ash from the site was baked and then examined for the kind of light it emits as a signal to the last time it melted, gave a result of 40,000 years ago.

"Accounting for the origin of these footprints would require a complete rethink on the timing, route and origin of the first colonisation of the Americas," said a Royal Society statement announcing the find. People only emigrated to the America's about 13,000 years ago, suggests the conventional picture painted by archaeological records. These " Clovis" people, named for the New Mexico town where the oldest dated site for their distinctive tools ands arrowheads were discovered, populated the continent in as little as two centuries after crossing the Bering Strait.

A handful of sites, notably a suspected hearth in Chile's Monte Verde ruins suggest some people arrived a bit earlier, perhaps 15,000 years ago. But 40,000-year-old footprints in Mexico would suggest that prehistoric modern humans, who are thought to have left Africa as recently 60,000 years ago, raced across Asia and colonized the New World remarkably fast.

A debate erupted. In December of 2005, a team led by geochronologist Paul Renne of the University of California, Berkeley, reported in Naturethat the trackway ash layer dated to 1.3 million years ago, according to analysis of radioactive Argon elements in the rock. If the ash dated to 1.3 million years, that meant the footprints in it couldn't have been made by modern humans, who have only been around for about 200,000 years, tops, as indicated by bones and tools. "I never thought they were tracks," Renne says now. "I've seen them and they really don't have the left-and-right pattern of footsteps. They only look like tracks if you see them in the right light." Quarry marks and recent foot traffic from people who today live nearby more likely explained the impressions, Renne and others suggested.

A number of papers flew back and forth, some supporting the Argon results and one confirming the younger luminescence date. But in the latest turn, the Journal of Human Evolution paper led by Darren Mark of the Scottish Universities Environmental Research Centre, and co-authored by Gonzalez, concedes the fight, replicating the Argon results from Renne's lab.

"Dr. Gonzalez and colleagues from Liverpool John Moores University have accepted that the age of the Xalnene Ash is approximately 1.3 (million years)," Mark says, by e-mail. He adds the finding, "casts considerable doubt on the interpretation that the markings in the Xalnene Ash are hominid footprints."

Could they be footprints of some human precursor "hominid" species? Archaeologists have looked for signs of older human species, such as homo erectus, which was living in Asia more than a million years ago, but have seen no signs of them in the New World, Mark says. "Considering what we know about the timings of hominid migrations out of Africa up into Europe and Asia, it is highly improbable that hominids could have made it to the America's by 1.3 million years before present."

So, science worked, in this case. Someone makes a claim and scientists check it out. Most of the time, the extraordinary claims fails, but there is no shame in science for trying, and that is how scholars have made some of their biggest advances, from plate tectonics to Einstein's theory of gravity.

"The message here is that extraordinary claims do need extraordinary evidence," Renne says. "The fact that the original claimant is recanting the first observation is the news here. No, that doesn't always happen in science, but it should, and that is to (their) credit."

So, does that rule out the chances that people emigrated into the America's further back than 15,000 years ago? Nope, says Mark, it just means there is no evidence for it. "Although the reporting of the alleged footprints by Dr. Gonzalez and colleagues may have been premature, the idea that hominids had made it into the New World by forty thousand years before present is not a radical supposition," Mark says. "It is always worth remembering: absence of proof is not proof of absence and I am sure within the near future there will be much more debate and controversy surrounding the peopling of the America's."

So stay tuned folks. Those sound like fighting words. The end of this chapter in the debate over the First Americans might just mean a new chapter is about to begin.