Fossil "Footprints" Stir Debate About Earliest Americans

Brian Handwerk
for National Geographic News
December 1, 2005
In July a team of English researchers reported the discovery of human footprints in Mexico that appeared to be 30,000 years older than when most scientists believe humans arrived in the Americas.

Researchers commonly accept that humans came to the Americas some 11,500 years ago. But new dating of the Mexican find suggests that the features are in fact 1.3 million years old.

If the new dates are correct, the footprints could be among the most incredible hominid traces ever discovered—or, more likely, not footprints at all.

"One-point-three million years is a lot older than I expected," said Paul Renne, director of the Berkeley Geochronology Center at University of California, Berkeley.

"I repeated the experiment nine times using different samples, usually single chunks of this basaltic rock, and they all gave the same unambiguous results."

Silvia Gonzalez, a geoarchaeologist at England's Liverpool John Moores University, and a team of colleagues first discovered the features in 2003. The researchers found them embedded in basaltic lava on the floor of an abandoned quarry near Puebla in central Mexico.

The team identified the indentations as footprints and dated them at 40,000 years old.

But if Renne's new dates are correct, the prints may be those of an incredibly ancient hominid, made well over a million years before the Americas are believed to have been inhabited.

Or they may simply be indentations in the rock.

Renne reports his new dating results in this week's Nature.

The First American?

Renne visited the site in June 2004 and noted that it shows the effects of many impacts over time.

"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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