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19 July 2009
Isotope analysis dates ancient Mexican

After years of controversy, a new study shows that the skeleton of Mexico's Tepexpan Man is nearly 5,000 years old and lived on the shores of a tree-lined lake. Nowadays the lake has dried and the forests have given way to an arid landscape of cacti.
     The Tepexpan Man was first discovered in the late 1940s near the remains of mammoths, the extinct relatives of elephants that lived in Europe and North America during the Ice Ages. He was an adult male who died in his late twenties. Due to the close association with the mammoths, the skeleton was thought to be at least 10,000 years old and hailed as the 'oldest known Mexican'. Years later, the bones were dated directly with radiocarbon methods and found to be only 2,000 years old. The Tepexpan Man lost its 'oldest man' status, but there was still work to be done.
     Silvia Gonzalez, professor of geoarchaeology at the Liverpool John Moores University, led a team to reconstruct the environment in which the Tepexpan Man lived and to revise the age of the findings. The team used Uranium isotopes to date the skeleton and the results gave an age of about 4700 years. This means that the Tepexpan Man is still not the oldest Mexican, but the difference from the radiocarbon dating is remarkable. "Back in the 1940s," Gonzalez explains, "archaeologists wanted to protect the skeleton from destruction, but the preservatives they used contaminated the bones:" the 2,000 years old given by the radiocarbon dating are an effect of the contamination. The Uranium dating, Gonzalez argues, is not affected by the preserving chemicals.
Tepexpan site
     The team did more than just dating the skeleton. Gonzalez and her colleagues analysed the sediments and fossils of a 4-metre-long core drilled on the now dry Lake Texcoco, near the site where the Tepexpan Man was first discovered. The sediments tell a tale of environment change. When the Tepexpan Man was alive, the lake was deep and full of fish, with forest-lined shores. Today, the former lake area is in the outskirts of Mexico City, set in an arid landscape riddled with cacti. But the area did not stay the same over the last 20,000 years. The level of Lake Texcoco oscillated numerous times and the type of vegetation on its shores changed accordingly. There were also several important volcanic eruptions which affected the lake. These environmental changes had a large impact on the prehistoric populations living by the lake shore, write the authors on the report published last week in Quaternary Science Reviews.

Source: Planet Earth Online (14 July 2009)

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