Alleged 40,000-Year-Old Human Footprints In Mexico Much, Much Older Than Thought

Contact: Robert Sanders
rsanders@berkeley.edu
510-643-6998
University of California - Berkeley

Alleged 40,000-year-old human footprints in Mexico much, much older than thought

Berkeley -- Alleged footprints of early Americans found in volcanic rock in Mexico are either extremely old - more than 1 million years older than other evidence of human presence in the Western Hemisphere - or not footprints at all, according to a new analysis published this week in Nature.

The study was conducted by geologists at the Berkeley Geochronology Center and the University of California, Berkeley, as part of an investigative team of geologists and anthropologists from the United States and Mexico.

Earlier this year, researchers in England touted these "footprints" as definitive proof that humans were in the Americas much earlier than 11,000 years ago, which is the accepted date for the arrival of humans across a northern land-bridge from Asia.

These scientists, led by geologist Silvia Gonzalez of Liverpool's John Moores University, dated the volcanic rock at 40,000 years old. They hypothesized that early hunters walked across ash freshly deposited near a lake by volcanoes that are still active in the area around Puebla, Mexico. The so-called footprints, subsequently covered by more ash and inundated by lake waters, eventually turned to rock.

But Paul Renne, director of the Berkeley Geochronology Center and an adjunct professor of earth and planetary science at UC Berkeley, and his colleagues in Mexico and at Texas A&M University report in the Dec. 1 issue of Nature a new age for the rock: about 1.3 million years.

"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."

Renne's colleagues are Michael R. Waters, director of the Center for the Study of the First Americans at Texas A&M University; Joaquin Arroyo-Cabrales and Mario Perez-Campa of the Mexican National Institute of Anthropology and History; Patricia Ochoa Castillo of the Mexican National Museum of Anthropology; and UC Berkeley graduate students Joshua M. Feinberg and Kim B. Knight. The Berkeley Geochronology Center, located a block from the UC Berkeley campus, is one of the world's preeminent anthropological dating laboratories.

Paleoanthropologist Tim White, professor of integrative biology at UC Berkeley, is familiar with the "so-called footprints" and knows Renne well, frequently collaborating with him in the dating of million-year-old sediments in an area of Ethiopia where White has excavated numerous fossils of human ancestors. He is not surprised at the new finding.

"The evidence (the British team) has provided in their arguments that these are footprints is not sufficient to convince me they are footprints," said White, who did not contribute to the new work that Renne's group is reporting in Nature. "The evidence Paul has produced by dating basically means that this argument is over, unless indisputable footprints can be found sealed within the ash."

Renne determined the new date using the argon/argon dating technique, which reliably dates rock as young as 2,000 years or as old as 4 billion years. The British-led researchers, however, relied mainly on carbon-14 dates of overlying sediments. Carbon-14 cannot reliably date materials older than about 50,000 years.

The idea for another test that, it turns out, throws more cold water on the footprint hypothesis came to Renne one morning in the shower. Many rocks retain evidence of their orientation at the moment they cool in the form of iron oxide grains magnetized in a direction parallel to the Earth's magnetic field at the time of cooling. Because the Earth's field has repeatedly flipped throughout the planet's history, it is possible to date rock based on its magnetic polarity.

Feinberg found that the rock grains in the volcanic ash had polarity opposite to the Earth's polarity today. Since the last magnetic pole reversal was 790,000 years ago, the rock must be at least that age. Because the Earth's magnetic polarity changes, on average, every 250,000 years, the argon/argon date is consistent with a time between 1.07 and 1.77 million years ago when the Earth's polarity was opposite to that of today.

Moreover, Feinberg found that each individual grain in the rock is magnetized in the same direction, meaning that the rock has not been broken up and reformed since it was deposited. This makes extremely unlikely the possibility that the original ash had been weathered into sand that early humans walked through before the sand was welded into rock again.

"Imagine two-millimeter-wide BBs cemented together where they're touching," Feinberg said. "The paleomagnetic data tell us that these things did not move around at all since they were deposited. They haven't been eroded and redeposited anywhere else. They fell while they were still hot, which raises the question of the validity of the footprints. If they were hot, why would anybody be walking on them?"

The British researchers, funded by the United Kingdom's Natural Environment Research Council, have promoted their hypothesis widely, most prominently at a July 4, 2005, presentation and press conference at the Royal Society's Summer Science Exhibition 2005 in London. The team, which includes Gonzalez as well as Professor David Huddart from John Moores University, also involves scientists from Bournemouth University, the University of Oxford and the Australian National University. They have yet to publish a peer-reviewed analysis of the footprints.

In all, the British team claims to have found 250 footprints - mostly human, but also dog, cat and cloven-hoofed animal prints - in a layer of volcanic ash deposited in a former lake bed now exposed near a reservoir outside Puebla. Its dating techniques returned a date of 40,000 years ago, in contrast to the oldest accepted human fossil from the Americas, an 11,500-year-old skull. This makes the rock "one of the most important areas in the study of early human occupation in the Americas and would support a much earlier human migration than is currently accepted," the team wrote.

One of the team members, Matthew Bennett of Bournemouth, was quoted on a Royal Society Web site as saying, "Accounting for the origin of these footprints would require a complete rethink on the timing, route and origin of the first colonization of the Americas."

Renne, Knight, Waters and the Mexico City archeologists visited the site at the Toluquilla quarry last year while collecting rocks from another anthropological site across the reservoir. Renne noted that the black, basaltic rock is very tough and is mined in slabs for building. Pre-Columbian Mexicans also constructed buildings from the rock, which they called xalnene, meaning "fine sand" in the Nahuatl language. Today, trucks headed toward the quarry routinely drive across the xalnene tuff in which the alleged footprints are found, and the rock itself is pockmarked with many depressions in addition to the alleged footprints.

"They're scattered all over, with no more than two or three in a straight line," which would be expected if someone had walked through the ash, Renne said. If the depressions were footprints, they could not have been made by modern humans, he noted, since even in Africa, Homo sapiens did not appear until about 160,000 years ago. Given the age of the volcanic rock and lacking other evidence of early human ancestors in the Americas 1.3 million years ago, the researchers wrote in their paper, "we consider such a possibility to be extremely remote."

Many paleontologists have withheld judgment on the alleged footprints, awaiting good geological dates, Feinberg said. "With this study, we're trying to nip any misrepresentation in the bud."

### The research was supported by the Center for the Study of the First Americans, the North Star Archaeological Research Program and the Berkeley Geochronology Center.

From Archaeology magazine.

Insider: Fantastic Footprints

Volume 58 Number
5, September/October 2005

(Bournemouth University) Geologist Matthew Bennett of Bournemouth University used 3-D laser scanning to create a cast of what some say is a 40,000-year-old human footprint found in Mexico.

(© Sylvia Gonzalez)

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."

Comments on Renne et al paper in Nature this week from Dr Silvia Gonzalez

Determining the age of the deposits from the Valsequillo Basin has been a challenge for scientists during the last 40 years. The original excavations of Cynthia Irvin Williams and Juan Armenta in the 1960’s and 1970’s, suggested very early human occupation (at about 22,000 years ago) at some archaeological sites in the Basin. Later work lead by Virginia Steen McIntyre in the 1970’s suggested dates of 240,000 and 360,000 for some layers with large animal bones (mastodont, horse and camel) associated with stone tools.
These ages were regarded as impossible and widely rejected by the scientific community as they were regarded “too old” for the existing models for the settlement of the Americas.

The deposits have been difficult to date because they are very heterogeneous and in particular the Xalnene ash, in which the suggested footprints are found, has particles derived from different compositions suggesting perhaps different origins (Figure 1, Figure 2).
The eruption that produced the ash was highly explosive, in association with magma-water interactions and potentially could have incorporated some older rocks into the ash. We have obtained Optically Stimulated Luminescence (OSL) dates of around 40,000 years old from some backed particles incorporated into the ash and now they need to be explained in view of the new dates obtained by Renne’s group.

Figure 1

Figure 2

It is clear that the dates reported by Renne’s group need to be replicated and independently confirmed. This is important because we applied the Ar-Ar method as well and had no good results, and concluded that they were not reliable. Also, it is not clear from where exactly they took their samples and which fraction was dated. We took our samples directly from the footprint horizons.

Even if we are wrong and the Xalnene ash is indeed 1.3 million years old, as suggested by Renne’s et al, that is not automatically a reason to disregard interpretation of the features reported as “footprints”, simple because they are not in agreement with the established models for the settlement of the Americas.

This highlights the desperate need of further dating with different methods on the deposits in the Valsequillo Basin to make sure they make sense between themselves and to be able to establish a reliable chronology, considering the palaeontological and archaeological evidence included in them. This is exactly what we are doing at the moment with a grant from NERC (Natural Environment Research Council), during the next three years.

What we have ahead is a new era in the scientific research of the archaeological evidence found in the Valsequillo Basin and that is what Science is all about.

Our own dating efforts up to now and arguments explaining why we interpret the trace fossils preserved on the ash as human footprints will appear in the January issue of the Quaternary Science Reviews.

The Oldest American?

Footprints from the Past

Right human footprint, showing toe impressions and typical figure of eight shape

Human left footprint showing toe impressions and slight heel impression. Modern size 43 for scale

The discovery of 40,000 years old human footprints in Central Mexico challenges accepted theories on when and how humans first colonised the Americas.

The timing, route and origin of the first colonisation of the Americas remains one of the most contentious topics in human evolution. Experts from many disciplines are searching for the answers to three seemingly straightforward questions:

• From where did the first people come?

• How did they enter the Americas?

• When did they arrive?

Until recently archaeologists thought they had the answers to these questions. Evidence suggested that the Americas had been colonised towards the end of the Pleistocene period by hunter-gatherers migrating from Siberia into Alaska across the Bering Land Bridge, an exposed continental shelf, when sea levels were lower. This is known as the Clovis-First Model.

According to this model the earliest occupation of the Americas began 11,500 years ago.

The discovery of fossilised human footprints in the Valsequillo Basin, Central Mexico challenges this accepted viewpoint and provides new evidence that humans settled in the Americas as early as 40,000 years ago.

Location of Mexico and Mexico City

Location of the modern Valsequillo reservoir and Cerro Toluquilla south-east of Puebla.

Toluquilla quarry and the upper surface of the Xalnene Ash where the footprints are preserved.

The footprints were discovered in the summer of 2003, on the floor of an abandoned quarry by Dr Silvia Gonzalez, Professor David Huddart (Liverpool John Moores University) and Professor Matthew Bennett (Bournemouth University). At the time of the discovery the team were working on dating and mapping the geology of the Valsequillo Basin, Puebla, Mexico.

Dr Gonzalez is one of a growing number of scientists who believes that the first Americans may have arrived by water rather than on foot, island hopping along the Pacific coast. Click here to find out more about her research on the Pacific Coastal Migration Route.

The footprints research was carried out as part of a wider project funded by NERC through the EFCHED programme, entitled ‘Human dispersals and environmental controls during the Late Pleistocene / Early Holocene in Mexico: Implications for the Peopling of the Americas’. The research is also supported by INAH (Instituto Nacional de Antropologia e Historia) in Mexico.

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.