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24 May 2010
Tracks in Mexican ash field are not ancient human footprints

In 2005, a team lead by Silvia Gonzalez from John Moores University in Liverpool, (England) announced that they had found human footprints in a dry lake bed in Mexico, and that the ash had been dated with optically stimulated luminescence to 40,000 BP. The hundreds of impressions in the ash of Valsequillo Lake included adults and children as well as cats, dogs, birds and other large mammals. The scientists concluded that the people and animals had been trying to escape from an eruption of the nearby Cerro Toluquilla volcano.
     The find was extraordinary because the prevailing theory was that humans arrived in the Americas approximately 14,000 BP. This date was based upon artifacts of the Clovis people, found throughout the American Southwest. Clovis, New Mexico (USA) is location of the oldest Clovis site. It was thought that these early emigrants came from Asia and walked over a land bridge across the Bering Strait to North America. According to a statement issued by the Royal Society announcing the Mexico find, "Accounting for the origin of these footprints would require a complete rethink on the timing, route and origin of the first colonisation of the Americas."
     Since modern humans are believed to have left Africa only 60,000 ago, it would have been very remarkable for them to have crossed Asia and reached the Americas in a brief 20,000 years.
     In December, 2005 an article in the journal Nature written by Paul Renne, a geochronologist from the University of California, Berkeley (USA), disputed the findings of Gonzalez' team. Using argon dating, Renne established the age of the ash to be 1.3 million years BP. Modern humans evolved approximately 200,000 years ago, so they could not have left the footprints in the ash, if Renne's date was correct. Renne's article also questioned the identification of the tracks as 'footprints'. "I've seen them and they really don't have the left-and-right pattern of footsteps", he noted. "They only look like tracks if you see them in the right light." He proposed that the impressions were more likely modern quarry marks or the result of recent foot traffic.
     The debate continued for some time, with new dating results published supporting one side or the other. But now, Gonzalez has co-authored a paper with Darren Mark in the Journal of Human Evolution where she concedes that the argon date is most likely correct and that tracks are not evidence of very early human habitation of the Americas. The article states, "Dr. Gonzalez and colleagues from Liverpool John Moores University have accepted that the age of the Xalnene Ash is approximately 1.3 (million years). Considering what we know about the timings of hominid migrations out of Africa up into Europe and Asia, it is highly improbable that hominids could have made it to the America's by 1.3 million years before present."
     This back and forth debate is the normal exchange of science. Scientists make discoveries, propose theories and publish data. Wrong answers and errors are necessary to the process and further the pursuit of knowledge.
     Disproving the dates for the Valsequillo Lake tracks does not prove that humans arrived in the Americas only 14,000 ago. Mark noted that, "Although the reporting of the alleged footprints by Dr. Gonzalez and colleagues may have been premature, the idea that hominids had made it into the New World by forty thousand years before present is not a radical supposition. It is always worth remembering: absence of proof is not proof of absence and I am sure within the near future there will be much more debate and controversy surrounding the peopling of the Americas."

Source: USA Today (20 May 2010)

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