Mexican Footprints II
By: Alun Salt
Tags: Archaeology, Geology, Mexico

I know I’ve had some sleep recently because last night I dreamt I was in a sleep-deprivation experiment, but I haven’t had much. Therefore you should also visit the other sites listed at the end for more intelligent comment.

A foot and an ancient footprint. Photo from the Mexican Footprints media section.

The dating of the Mexican footprints is proving to be a problem. This week in Nature the footprints are the subject of a Brief Communication Geochronology: Age of Mexican ash with alleged ‘footprints’. I’ve added two recent press release from Berkeley and Texas A&M on re-dating the prints. They tackle quite a problem, how do you sample an absence of something to get a date?

I printed out the article, which was really clever as I don’t have access to it from home. However I’ve just discovered I left it on the printer, which is really stupid. I’ll try and reconstruct the argument as best I can.



A good footprint is a hole. How do you date it? You could date the material its in but does that make sense? If I go outside now and chip a hole out of some local granite do I make a hole that’s millions of years old? So how about dating it by seeing what fills the hole? That sounds like a good idea except if they’re ancient holes they could still be refilled by modern material.

As far as I can tell the Liverpool team have dated using particles (shell?) that they say are within the soil horizon the prints appear in. Have they dated later infill. Even if they haven’t how reliable is a preservation of the sample? If the sample is contaminated with Carbon-14 then the dates could appear younger than they necessarily are.

The American team have used Argon / Argon dating. They’ve also got extra dating by examining the magnetic polarity of the sediments. The earth’s magnetic field flips over time and these rocks were laid down when the polarity of the earth’s magnetic field was inverted. This would also confirm that the rocks are much older than the 30kyr date claimed by the Liverpool team.

I’m inclined to go with the American interpretation on these prints, but there’s stuff in the American press releases that bothers me. It’s mentioned that the Liverpool team’s results “have yet to publish a peer-reviewed analysis of the footprints”, but they omit to mention that they will appear in a journal, Quaternary Science Reviews in January. Given there are plenty of papers ‘in press’ from September and earlier, I’d guess this has been going through the peer-review process for a while, and the fact they have a date would indicate that it’s been in a while and it passed. The American claim is factually correct, but only for another month. If I was patient enough I could post next month that the American claims about the lack of peer-reviewed publication are “demonstrably false”.

Additionally the two press releases seem to imply human colonisation of the Americas started at 11,500 years ago. Monte Verde would appear to push that back a thousand years. I’m told by people working in the field that (a) there are also pre-Clovis sites in Mexico and (b) Americans aren’t invited to view them. This quote by Tim White, Professor of integrative biology at UC Berkeley, might explain why:

“The evidence Paul has produced by dating basically means that this argument is over, unless indisputable footprints can be found sealed within the ash.”

The argument is over before the paper is published? I don’t know where the samples Liverpool dated are from, because the paper doesn’t appear till January. So how can I or he possibly know that the American re-dating is relevant to that material?

I suppose he could have seen the data going for peer-review. Personally if I saw a paper that had passed peer-review that I could prove wrong, then I’d submit my paper to appear in the same journal in the same or a following volume. I wouldn’t publish in a different journal before the original paper, but I could be doing something wrong I suppose.

I firmly think it’s up to the Liverpool team to prove their case and footprints alone won’t do it. Where are the artefacts? Where are the sites of similar date? This will take time to find. It’s rare that one find overturns orthodox ideas and often consensus is built up on carefully weighing the evidence. On the other hand the press releases from the USA have made me far more interested in reading QSR next month. If the aim of the Nature communication really was “to nip any misrepresentation in the bud,” then it’s failed spectacularly.

Abnormal Interests has also noted this arguing for caution on interpreting the finds as footprints, with which I agree. I’m more inclined to think these are not footprints But I wonder if the rebuttal is as convincing as is claimed. If I had a certain smackdown then I wouldn’t be playing media games. I may change my mind after reading QSR.

Afarensis has the response from Liverpool.

Badgerminor at Orbis Quintus notes that the “footprints” do look like footprints. I’d like to agree with that, but I don’t know how many marks there are at this site. If there are thousands then by sheer chance you’d expect some to look foot-ish so it becomes a self-selecting sample. I’d like to see dates for things from the site that aren’t claimed to be footprints to see if they differ. If the known quarrying marks date from the same period as the footprints using the same technique then you can write off the dating as inaccurate. If the dates differ then you have confirmation that the prints don’t just look different.