HUMAN footprints discovered beside an ancient Mexican lake have been dated to 40,000 years ago. If the finding survives the controversy it鈥檚 bound to stir up, it means that humans must have moved into the New World at least 30,000 years earlier than previously thought.
鈥淚f true, this would completely change our view of how and when the Americas were first colonised,鈥� says Chris Stringer, head of human origins at the Natural History Museum in London. But like several US experts, he is reserving judgement until the dates can be independently confirmed.
The discovery was made by an international team led by Silvia Gonzalez, a geoarchaeologist at Liverpool John Moores University in the UK. She found the fossilised footprints in 2003 in a quarry near the city of Puebla, 100 kilometres south-east of Mexico City. 鈥淚 walked one metre and started to see them,鈥� Gonzalez says. 鈥淚t felt like a thunderbolt.鈥�
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In just two days, Gonzalez and her colleagues found hundreds of human and animal footprints preserved in a layer of ash from a nearby volcano. The footprints were made along the shore of a lake and were submerged after the water level rose, preserving them under sediments.
鈥淭hey are unmistakably human footprints,鈥� says team member Matthew Bennett at Bournemouth University in the UK. 鈥淭hey meet all the criteria that were set up after the Laetoli prints were found [in Tanzania in 1976].鈥� The sizes suggests around a third of them were made by children.
鈥淭his will be incredibly controversial. We have done a year of solid work to make sure it鈥檚 accurate and reproducible鈥�
But when were they made? It has taken the team two years, using a panoply of high-tech dating techniques, to determine that the prints are around 40,000 years old. The key date came from shells in the lake sediments, which the team carbon-dated to 38,000 years ago. Sand grains baked into the ash and dated using optically stimulated luminescence corroborated the finding.
The researchers also used argon-argon, uranium series and electron spin resonance techniques to date the layers. 鈥淭he footprints are clearly older than 38,000 years,鈥� says team member Tom Higham of the carbon-dating lab at the University of Oxford.
The conventional view is that humans arrived in the Americas via Beringia around 11,000 years ago, when a landbridge opened up. There have been claims about earlier waves of settlers, who must have made the crossing by water, based mainly on sites with signs of habitation dated up to 40,000 years ago, but these claims have drawn intense criticism.
Gonzalez and her team expect the same. 鈥淭his will be incredibly controversial, there鈥檚 no doubt of that,鈥� Higham says. They invite other researchers to scrutinise their findings, due to be published in the journal Quaternary Science Review. 鈥淲e have done a year of solid work to make sure it鈥檚 accurate and reproducible,鈥� Higham says.
How people got to Mexico 40 millennia ago is a matter for speculation. Bennett suspects that they migrated along the Pacific coasts of Asia and North America. But when it comes to the dates and footprints, he says: 鈥淭hose are not speculation at all.鈥�
