ancient footprints—not!

Ancient footprints embedded in Mexican rock that were supposed to rewrite history books, tripling how long humans had been in the Americas, are probably not footprints after all.

In the summer, I reported on a group of British researchers announced they had found footprints in Mexico that appeared to have been pressed into the ground alongside a lakeshore, and then the rock solidified and preserved the prints until now. They enlisted the help of one of the world's top labs for dating ancient materials, at Oxford University. Dating a variety of materials trapped in the rock layer with the footprints and in the layers above and below it, suggested the prints were laid down 40,000 years ago.

If true, this would be a bombshell because most researchers believe people didn't arrive in the Americas until around 13,000 years ago, plus or minus a couple thousand years depending on which expert you talk to. But 40,000 years is way beyond the pale.

But a new study published this week in Nature dated the rock directly, rather than teeth and tiny balls of carbon trapped inside the rock, as the first group had done. They new study came up with a date of 1.3 million years old, before the rise of anatomically modern humans. So they conclude that the indentations are very unlikely to be footprints. (Read more on the new findings in this National Geographic news story.)

I'm inclined to believe the second study, if only because it eliminates this one finding that doesn't agree with all the others. Also, the attitude of the researchers who discovered the supposed prints made me uneasy. The lead researcher, Silvia Gonzalez, described how when she saw the imprints, "it felt like a thunderbolt." She was clearly convinced they were footprints from the beginning, and she and her colleagues expressed no doubt in their findings.

At a press conference announcing their findings, they said everyone who had seen the indentations thought they were real human footprints. I kind of rudely raised my hand and blurted out, "I don't mean to shoot you down, but I spoke to one researcher who has been to the site and does not think they're footprints."

Also, they said they had shown their data on the prints to several footprint experts who agreed they were human footprints. I saw their photos and virtual casts they made by laser scanning the prints and then making plastic reproductions of them, and I would say, in my layman's opinion, they were less than entirely convincing.

And finally, Gonzalez and her colleagues have not yet published their work in a peer-reviewed journal, the standard seal of approval for quality scientific work. With work on the first Americans though, it's complicated because scientists have strongly held views and so work that doesn't fit in with the rest of the evidence is likely to be disregarded, no matter how good the work is.

But I look forward to seeing what Gonzalez and her colleagues find. I'm sure these latest findings have not swayed them much, if at all. Also, they already got a grant to do more excavations to look for more prints, so I'm sure they'll go ahead with it. I was planning to try to go down to Mexico this spring to see the dig and report on it, and when I first read of the new study, I thought, "There goes my vacation!" But maybe I could still report on this. Whether I do or not, I don't think we've heard the last on these prints.