In July, researchers in England claimed the prints proved that humans were in the Americas 40,000 years ago — much earlier than the accepted date of 11,500 years ago. But Paul Renne, director of the Berkeley Geochronology Center and an adjunct professor at University of California-Berkeley, says the prints are about 1.3 million years old. "You're really only left with two possibilities," Renne said. "One is that they are really old hominids — shockingly old — or they're not footprints."
     Earlier this year, a British-Mexican team led by Dr Silvia Gonzalez of Liverpool John Moores University announced that the site at Valsequillo Lake near Puebla in southern Mexico likely contained the oldest evidence of human occupation in the Americas. The researchers hypothesized that early hunters walked across ash freshly deposited near a lake by volcanoes that are still active. The so-called footprints, subsequently covered by more ash and inundated by lake waters, eventually turned to rock. The researchers used several methods to date minerals and fossils from above, below and on the footprint layer itself. Radiocarbon dating was carried out on shells and animal bones in the sequences, and mammoth teeth were dated using a technique called electron spin resonance.
     They obtained dates for lake sediments incorporated into the ash by a technique called optically stimulated luminescence. The results converged on the highly controversial date of 40,000 years. Under the traditional view, the first Americans trekked from Siberia to Alaska across a land bridge that linked these land masses at the end of the last ice age (between about 10,000 and 12,500 years ago).
     But Paul Renne, a geochronologist at Stanford University, and colleagues have now used argon dating and palaeomagnetic analysis to show that the so-called Xalnene basaltic tuff on which the purported footprints were found was in fact far older even than Dr Gonzalez and her team suggested. The results show the tuff is 1.3 million years old. The footprints would therefore predate the first known appearance of Homo sapiens in Africa by more than a million years.
"This casts serious doubt on whether those marks are human footprints," co-author Michael Waters, of Texas A&M University, said.
     Although some scientists have conceded it is possible that archaic humans such as Homo erectus could have made it to the Americas, the possibility is considered remote in the extreme. "If you look at the original work that was presented, there was an optically stimulated luminescence technique. That technique cannot be used to date that sort of material - it should never have been applied," said Dr Waters. The Texas A&M researcher also criticised the use of radiocarbon dating on shells from the sequences: "Freshwater shell is notorious for producing erroneous ages."
     Dr Gonzalez said her team would be submitting a formal scientific response for publication in an academic journal. She added she would not rule out the possibility that her theory was correct without doing further research. "The new finding doesn't necessarily mean that (1.3 million years ago) is the correct date. The results would need to be replicated to make sure that everything makes sense," Gonzalez said. She also said part of the problem in verifying the dates of the deposits in Mexico's Valsaquillo Basin is the amount of different materials in the particles. "But the fact that that is the case doesn't automatically mean that they aren't footprints," Gonzalez said. Her team has funding to do further analysis in the basin for the next three years.
     Dr Waters said he thought the marks were actually left over from quarrying: "The Xalnene tuff is a lithified volcanic ash. The locals go out there and quarry it for building material," he explained. "What you're seeing in the depressions is where the metal tools are diveting into the tuff. Every time it rains, water collects in the depressions, sediments collect in them and they weather out into oddball shapes."

Sources: Associated Press, EurekAlert!, Yahoo! News (30 November 2005), BBC News (1 December 2005)