Footprint claims get stamped on
Age data knock first impressions of marks in Mexican rock.
Marks that were hailed as the earliest traces of humans in the Americas may not be what they seem. A dating study puts the age of the volcanic ash in which the indentations were found at 1.3 million years, which casts fatal doubt on the theory that they are footprints.
Researchers began investigating the site, at Valsequillo Lake near Puebla in southern Mexico, after the discovery of the footprint-like impressions was made public in July (see " Ancient 'footprints' found in Mexico"). Thomas Higham of the University of Oxford, UK, calculated that they were 40,000 years old, making them the oldest evidence of human occupation of the New World.
Berkeley Geochronology Center
Renne and his colleagues used two dating techniques, one examining the ratios of chemical isotopes in the ash and another looking for magnetic signals from the sediments. They publish their results online in Nature1.
"I'm totally unconvinced by the argument that they are footprints," says Renne. But he adds that he cannot be absolutely certain, because of the tiny possibility that they were made by an earlier relative of humans.
"It is conceivable there were hominids in the New World 1.3 million years ago," says Renne. "That would make it the find of the century in archaeology."
The oldest Homo sapiens fossils are African and date to 160,000 years ago. The oldest evidence of humans in the Americas is at Monte Verde in Chile, where occupation is dated to about 14,500 years ago.
The original footprint claim was made by Silvia Gonzalez, a Mexican geoarchaeologist at Liverpool John Moores University, UK. The 40,000-year date was based on radiocarbon analysis of shells in the layer above the ash at the nearby Toloquilla quarry.
Neither Gonzalez nor any other member of her team has commented on Renne's report, saying that they will respond with a scientific analysis after reviewing the data.
Gonzalez says she will return to the site early next year to try to find incontrovertible evidence of footprints protected in ash layers. She has been given US$370,000 by Britain's Natural Environment Research Council to pursue her work.
Tim White, a palaeoanthropologist at the University of California, Berkeley, says that Renne's report indicates "the argument is over" on the markings, "unless indisputable footprints can be found sealed within the ash". Gonzalez's previous evidence "is not sufficient to convince me they are footprints", he adds.
- Renne P .R., et al. Nature, doi:10.1038/nature04425 (2005).