"My conclusion is that this is a deeply disturbed surface," he said. "It's been walked on, driven on, walked on by animals. Sleds carrying building stones have been driven over it.
"It has a lot of indentations, and some of them are what has been interpreted as footprints. We found a huge variety of those of different shapes and depths."
In addition to the 1.3-million-year date, Renne's rock samples also revealed reverse magnetic polarity. The Earth's magnetic field reversed its polarity 790,000 years ago, Renne explained, so the rocks must be older than that.
He says he thinks it's very unlikely that the indentations are footprints but that he was keeping an open mind.
"We can't definitely rule out that these are footprints," he said. "I'm a geologist and not an anthropologist. But if that's true, it would be one of the most remarkable discoveries in centuries."
Gonzalez, of the English research team, responded to Renne's report with a written commentary released to the media.
She stressed that the layer of ash in which the features lie has been difficult to date, because it consists of many different materials that may be of different ages.
Her group used lasers to measure radiation in particles of the ash layer and dated them as being about 40,000 years old.
"[Those dates] now need to be explained in view of the new dates obtained by Renne's group," she wrote.
Gonzalez believes that the new dates spotlight the need for further research on the site by other techniques and by independent groups to establish a reliable timeline.
But even if Renne's ancient dates are correct, Gonzalez says, she isn't ready to rule out the possibility that the features could be footprints.
"Even if we are wrong and the ash is indeed 1.3 million years old, as suggested by Renne et al., that is not automatically a reason to disregard interpretation of the features reported as 'footprints' simply because they are not in agreement with the established models for the settlement of the Americas," she wrote.
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