Prehistoric 'footprints' falsified by science

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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.

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.

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