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Environment

Imprints Are Not Footprints

December 5, 2005 | A version of this story appeared in Volume 83, Issue 49

Earlier this year, a team of U.K. scientists reported that ancient footprint-like indentations near Puebla, Mexico, are 40,000 years old, a result suggesting that humans populated the Americas 30,000 years earlier than previously believed. But a new study refutes that conclusion (Nature, published online Nov. 30, dx.doi.org/10.1038/nature04425). With 40Ar/39Ar dating techniques and magnetic analyses, Paul R. Renne of the Berkeley Geochronology Center and colleagues determined that the age of the rocks containing the imprints are 1.3 million years old. Because it's unlikely humans were there that long ago, a more probable explanation is that the indentations are not actually footprints, the authors say. The initial studies, led by Silvia Gonzales at Liverpool John Moores University, used a variety of methods, including 14C dating; however, many of the tests were performed on sediments covering the imprints. The U.K. group has posted a rebuttal of the new work at www.mexican footprints.co.uk/default.htm and says their results will be published in the January issue of Quaternary Science Reviews.

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