A publication of the Archaeological Institute of America
A team of British and Mexican researchers led by geoarchaeologist Sylvia Gonzalez of Liverpool John Moores University surprised the world this summer with the announcement of their discovery of 40,000-year-old human footprints in an abandoned quarry south of Mexico City. That date would put humans in the Americas some 27,000 years before the earliest widely accepted site, Monte Verde, Chile, which dates to 12,500 years ago.
The team, which discovered the prints on the quarry surface in 2003, believes that the more than 200 human and animal footprints were made in a layer of volcanic ash following an eruption in the Toluquilla Basin 40,000 years ago; following the eruption, the rising waters of a nearby lake covered and preserved the hardened ash layer. On their website, www.mexicanfootprints.co.uk, the team reports they used at least five different techniques to date the deposits, including carbon-dating shells found in the ash as well as direct dating of the deposits using Optically Stimulated Luminescence, which can determine the time elapsed since minerals in soil were exposed to sunlight.
But despite the team's emphasis on using multiple dating methods and careful analysis of the footprints themselves, many scholars are leery of the pronouncements from Gonzalez's team. "As far as the Toluquilla footprints go, the key here is publication in a peer-reviewed journal," says Michael Collins, a Paleoindian expert at the Texas Archaeological Research Laboratory at the University of Texas, Austin. (ARCHAEOLOGY has learned that the team's paper was rejected twice by Nature, one of the world's premier peer-reviewed scientific journals, though it is said to be slated for publication in Quaternary Science Review.) "All the information out there so far is in the news media. This team, which is well-funded, has relied heavily on press releases in the past instead of publishing in refereed journals. That's not the way to do it. You can't do science by press release," says Collins, who also questions the dates obtained by the team by carbon-dating shells, which he says could have been deposited before or after the eruption. "I guess it's possible, but the dating sounds shaky. Again, I'm just going off a press release here, but the footprints are so ambiguous, and the dating is so totally questionable, that I have to say it doesn't deserve the attention it's received by media outlets like the BBC."
The ambiguity of the footprints is also an issue for Paul Renne, a geologist and the lab director at UC Berkeley's Geochronology Center. "If these were really footprints, it would be very exciting. But I visited the site last summer, and what I saw were scuff marks--impressions that could have been made by machinery or grazing animals," he says. "The area has been a dump and quarried for building stone. I saw nothing that I thought was even moderately convincing. Now, maybe I didn't see the optimal stuff--that's a possibility. But the surface is so disturbed that the only thing to do is to excavate fresh material that is unimpeachably undisturbed. There is also some question about whether this deposit is 40,000 years old. It may be as old as 500,000 years, in which case if they were human footprints...that would be something."
Human trackways are nothing new to archaeology. The Sistine Chapel of archaeological footprints are the 3.5 million-year-old Laetoli trackways, discovered in Tanzania in 1976 by a team led by Mary Leakey. Bruce Latimer, director of the Cleveland Museum of Natural History and a specialist in the evolution of hominid locomotion has studied the Laetoli tracks. "A human footprint is unlike any other footprint made by any creature. It is unmistakable. The only thing close is a degraded ostrich print. And my feeling is, if you have to equivocate about it, it's probably not human," he says. "From what I've seen of the Mexican footprints, [the researchers] don't yet have one good print that you can say, 'that's human.' Now take Laetoli--those are clear as can be. You could find those on a modern-day beach. If the footprint images they are releasing are the best they have...I don't think we are looking at human footprints." Latimer adds that "there are no ostriches in Mexico, either."
While issues of dating will have to wait for publication in a scientific journal, there soon may be a way for anyone who has an interest in the footprints to have an up-close look at the site and arrive at their own conclusion. Geologist Matthew Bennett of Bournemouth University, who discovered the tracks together with Gonzalez and David Huddart (also of Liverpool John Moores), is heading up an innovative program of mapping and scanning at the site. "In detective stories you always read about plaster casts being made of footprints," he says. "But we used a 3-D laser scanner to make submillimeter-perfect reconstructions. During fieldwork this coming January we will be doing a 100 percent scan of the entire surface of the quarry to create a complete and perfect record of the footprints. Then it will be put on a Mexican web site and anyone will be able to visit the site in virtual reality for themselves."