The latest installment of the Mexican footprints controversy continues. Funnily enough, it actually continues with what should have been the first stage of the issue - the inital arguments by Sylvia Gonzalez et al.
1) a joint UK-Mexican team announce the find of footprints in Mexico dated to around 40,000 years ago. Announcement recieves wide coverage in popular media but is not yet published in the literature.
2) responses come in. Some are sceptical that the observed features are footprints. Paul Renne then go on to do some good old geochronology. They get dates of 1.3 Myr and publish this in Nature. They also then announce results from palaeomagnetic studies that support an old date such as this.
3) The original study published with a footnote at the end providing something of a response to Renne. Some of the original authors have asked to be removed from the paper, though are happy to let their data be used.
Clearly the above order is screwed up. This isn't how such things can happen, particularly not in the notorously er 'difficult' debate on the peopling of the Americas.
Is the Gonzalez response convincing? Well, maybe, sort of, kinda. They maintain that they have found footprints and they also note that (as yet) stratigraphic evidence does not support the dates of Renne et al. They do admit that there are two competing sets of dates that in purely geochronological terms both seem pretty solid. To my mind, the most challenging data is the palaeomag (it is easier to interpret in some ways) and they do note the limited nature of the Renne sample in this regard. Basically they argue for more time to investigate this dating dichotomy, which isn't entirely unreasonable. I tend to side with Renne here, but I'm not ruling out a surprise yet.
In my mind, the debate should have kicked off only after people had read the evidence for the supposed footprints. After all, they are a pretty crucial issue here. It is not out of the realms of possibility that they are both footprints and 1.3 million years old. Renne et al dismiss the notion of footprints on the basis of age and this is simply not good science.
The stratigraphic evidence is also important and has a significant impact upon the validity of Renne's claims. Again, he should have waited to see this laid out, rather than desperately rushing to get a Nature paper published. I happen to believe that Nature should have refused his manuscript; they seem to have gone ahead simply because it is a big story (gasp!). Not acceptable.
Even if this ends in 'defeat' for Gonzalez et al, we should not lose sight that the paradigms surrounding the peopling of the America's require a healthy scepticism and rigorous testing. Just because this may not work out, don't be surprised if we eventually find irrefutible evidence of early occupation.
Anyway, here is the abstract:
Gonzalez, S. et al. (2006) Human footprints in Central Mexico older than 40,000 years. Quaternary Science Reviews, 25, 201-222.
The timing, route and origin of the first colonization to the Americas remains one of the most contentious topics in human evolution. A number of migration routes have been suggested and there are different views as to the antiquity of the earliest human occupation. Some believe that settlement happened as early as 30 ka BP, but most of the currently accepted early sites in North America date to the latest Pleistocene, related to the expansion of the Clovis culture, while the oldest directly radiocarbon dated human remains are 11.5 ka BP. In this context new evidence is presented in this paper, in the form of human footprints preserved in indurated volcanic ash, to suggest that Central Mexico was inhabited as early as over 40 ka BP.
Human and animal footprints have been found within the upper bedding surfaces of the Xalnene volcanic ash layer that outcrops in the Valsequillo Basin, south of Puebla, Mexico. This ash layer was produced by a subaqueous monogenetic volcano erupting within a palaeo-lake, dammed by lava within the Valsequillo Basin during the Pleistocene. The footprints were formed during low stands in lake level along the former shorelines and indicate the presence of humans, deer, canids, big felids, and probably camels and bovids. The footprints were buried by ash and lake sediments as lake levels rose and transgressed across the site. The ash has been dated to at least 40 ka BP by OSL dating of incorporated, baked lake sediments.