for National Geographic News
The first Americans may have arrived at least 25,000 years earlier than thought, new methods of dating ancient footprints show.
The analysis centered on 325 indentations found in a quarry in present-day Mexico's Valsequillo Basin.
The footprints were formed when prehistoric people walked in soft, damp volcanic ash along a lakeshore shortly after a volcanic eruption. The ash later hardened into rock, preserving the tracks.
A 2005 study had dated the ash layer to 1.3 million years ago—raising questions as to whether the indentations were even made by humans.
But dating the ash is complicated by the fact that an eruption occurred underwater, said Silvia Gonzalez of Liverpool John Moores University, in the United Kingdom.
A team led by Gonzalez initially found the features in 2003 and claimed they were human footprints.
The gigantic explosion pulverized some of the underlying bedrock, mixing it with the ash, she said at an American Geophysical Union meeting in Florida last week.
When the ash was dated using traditional radioisotope methods, the result was a hybrid of the age of the older bedrock and that of the eruption itself, Gonzales said.
To attempt to date once again the indentations, Gonzalez's team used a new method called optically stimulated luminescence.
This involved collecting samples in the dark, then irradiating them with an atomic reactor.
When ultraviolet light is shone onto the irradiated samples, the resulting fluorescence reveals how long it has been since the rock was last exposed to sunlight—or volcanic heat.
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