Footprints left in volcanic ash that fell in central Mexico's Valsequillo Basin 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 Liverpool John Moores University 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.