Footprints in the ash

Footprints left in volcanic ash that fell in central Mexico’s ValsequilloBasin about 40,000 years ago are evidence that humans have inhabited the Americas far longer than previously confirmed, a new study suggests.

Analyses of three-dimensional laser scans of the imprints (example at right) confirm their human origin, says Silvia Gonzalez, a geoarchaeologist at LiverpoolJohnMooresUniversity in England.

Previous finds of human remains elsewhere in the region couldn’t be precisely dated because they were found in layers of mixed gravels that probably incorporated materials of many different ages.

However, a new analysis of the coarse-grained, print-ridden volcanic ash — which would have hardened quickly after it fell, says Gonzalez — strongly suggest the material fell around 40,000 years ago, she and her colleagues reported today in Fort Lauderdale, Fla., at a meeting of the American Geophysical Union.

Excavations at several sites have suggested that humans have inhabited the Western Hemisphere for at least 20,000 years, but results suggesting dates of occupation before 14,000 years ago typically haven’t been confirmed and remain controversial.

Nevertheless, says Gonzalez, recent excavations at a site in Baja California have unearthed a rock shelter containing heaps of shells that have been carbon-dated as 44,000 years old, a finding that bolsters the notion that people lived throughout the region about 40 millennia ago.