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Old “footprints” stomped out?

In 2003, British researchers working in the Valsequillo Basin in southern Mexico found encased in volcanic rock what appeared to be 270 individual footprints of dogs, big cats, cloven-hoofed animals and humans of all ages. Dating of the rock and associated artifacts suggested the site was about 40,000 years old, thus pushing back the date of humans’ first arrival in the Americas by some 30,000 years (see Geotimes, September 2005). New dates from the same site, however, are changing the story: Either the footprints are far older than previously thought — 1.3 million years older, to be more exact — or, more likely, they are not footprints at all.

This dating debate leaves open one of the biggest questions in American archaeology — when people first colonized the Americas.
Paul Renne of the Berkeley Geochronology Center in Berkeley, Calif., Michael Waters of Texas A&M University in College Station and colleagues collected and analyzed nine samples from the volcanic tuff in which the footprint-like markings were found. They used argon isotopic dating, which reveals the actual age of the crystals in the rock, and paleomagnetic dating, which reveals where Earth’s poles were when the lava hardened at the surface.

The isotopic dating placed the tuff at approximately 1.3 million years old, and the paleomagnetic dating revealed that the tuff was between 1.07 and 1.77 million years old, they reported in the Dec. 1 Nature. The dates “are quite definitive — the results show excellent consistency at both the intra- and inter-sample levels, and there’s really no logical way to interpret our data differently,” Renne says.

But if the markings are human footprints, the obvious implication that they are 1.3 million years old “does not make anthropological sense,” Waters says. The hominid living at that time was Homo erectus, which researchers know was in Africa and parts of Europe and Asia, “but there is absolutely no anthropological evidence of H. erectus in North America,” he says. “I think we can pretty definitely rule out” the imprints as footprints from a hominid, much less a modern human, especially considering the first known appearance of modern humans is not until 195,000 years ago in Africa.

Thus, Waters and Renne say they do not even think the markings are footprints at all. The markings have a huge range of sizes, shapes and depths, and British researchers “have focused only on a subset and ignored the rest,” Renne says. Waters suggests that the marks are from the picking, breaking and prying up of slabs of the tuff for use in local building materials: The site where all of the “footprints” have been found is an old quarry. The divot marks weathered over time, forming deeper depressions in some areas than others, he says.

However, Silvia Gonzalez of Liverpool John Moores University and Matthew Bennett of Bournemouth University in the United Kingdom, two of the original researchers, “do not necessarily accept the validity of the new dates,” Bennett says, suggesting that another laboratory needs to replicate the date results before they can be accepted. Using different techniques, Bennett, Gonzalez and colleagues have dated the ash layer to 38,000 years before present (as determined by radiocarbon dating).

They plan to excavate in the quarry this year to try to find some “untouched prints,” Gonzalez says. Renne says that the finding of such markings in “freshly exposed, pristine surfaces” would be about the only way to make a case that they are actually footprints.

This dating debate leaves open one of the biggest questions in American archaeology — when people first colonized the Americas. Well-accepted dates place people in the Americas about 11,500 radiocarbon years before present (about 13,000 years ago).

Answering that question is “a matter of hard work — reevaluating old sites and finding new ones. The peopling of the Americas was probably not a single colonizing event, like a prehistoric Mayflower, but was rather a process with people likely trickling into the Americas over a long period of time,” Waters says.

Megan Sever

"Footprints push back American migration," Geotimes, September 2005
Mexican Footprints Web site

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