Evaluating the claim of Ice Age archaeological material at Chiquihuite Cave, Mexico

The latest claim of surprisingly early archaeological material in the Americas comes from Chiquihuite Cave, Mexico. The paper (Ardelean et al 2020) contains a wealth of excellent information and analysis on (non-human) ancient DNA, paleo-environment, and local geology, but unfortunately, the archaeology component is very underdeveloped. Simply put, the paper as presented does not provide nearly the information necessary to evaluate its main claim that people left archaeological traces at Chiquihuite 26k years ago.

That doesn’t mean they didn’t. It just means this paper doesn’t show it. I want to avoid the sin rightly pointed out by my colleague Ruth Gruhn (2020), which I too often see committed, of ignoring surprisingly early archaeological claims in the Americas simply because they are “much too old to be real”. So let’s look at what would make this paper more complete.

There is only one line of archaeological evidence

Like many other very early claimed archaeological sites in the Americas, such as Pedra Furada, the claim of archaeological status for Chiquihuite is based entirely on lithics, or worked stone material. There is no other line of evidence. The authors looked for ancient DNA and found plenty, from juniper tree to bear and everything in between, but no humans.

The site plan in figure 1a shows two “burned features” and an “altar”. Unless I missed something, these are mentioned nowhere in the main text or in the supplemental materials, and no other details about them are given, so I have no idea whether they are likely to be structured features made by people, such as hearths, or whether they are just concentrations of burned material, which can sometimes be found in caves and require no human participation.

I would love to find out more about those features, and I am surprised to see them on the site plan and not in the text. I am also a little surprised that the review process in Nature didn’t pick this up. As far as I can see, there are no other archaeological materials mentioned in the paper.

The problem with archaeological claims based solely on lithics is that it can be very difficult or impossible to tell whether particular chunks of rocks were worked by humans or by geological processes alone. In the absence of other evidence, archaeologists tend to use two main indicators to decide whether an object is archaeological or geological.

Certain patterns indicate human work with a high degree of confidence, although of course, we can’t totally, absolutely rule out geological production. But short of an immaculate conception of the Acheulean hand-axe, if you will, a hand-axe is archaeological even in the absence of other lines of evidence.

Figure 1: Acheulean hand-axe (http://www.sciencebuzz.org/museum/object/2004_10_acheulean_handaxe)

The other major indicator is provenance of the raw material. If the material is exogenous to the area, that is if it originates far away (the farther the better), and if there is no good geological mechanism that could have brought it to its current location, like glacial transport, for example, even minimally or unworked material could be accepted as archaeological with a high degree of confidence.

Unfortunately for Chiquihuite, there are no highly curated stone tools. The authors describe the assemblage as “expedient”, which means that it is mostly made of lightly retouched or modified raw material. The material is local limestone, with no very obviously exogenous elements. This is complicated by the fact that the cave is limestone, and that it shows episodes of roof collapse, which tend to produce lots of broken rocks that can look like the work of humans.

All these factors conspire to lower the level of confidence that the assemblage is archaeological. Working with the information available in the paper, it could be archaeological, but it just as easily could be geological.

The fact that the assemblage is limestone is not a problem in itself. Limestone isn’t readily thought of as a good stone tool-making material, but humans have worked it at various times and in various places. The fact that the limestone is local, in a limestone cave subject to roof collapse is more of an issue. Just one flake of a material with a geological origin a hundred kilometres away would substantially change the picture.

I wish I could agree with the authors that “The recurrent techno-morphological combinations in the assemblage reveal evident intentionality and systematization in the creation of human-made artefacts, as well as evident standardization at the level of individual pieces.”

Even if there were highly convincing, highly curated stone tools or imported materials at Chiquihuite, there remains the problem that there is no easy way to tell where they come from in the stratigraphy. The critical layer is the one labelled 1206 in figure 1b, which is dated in the 15k Cal BP range. Any substantial find in that layer or below it would be very good evidence of older than expected material.

Unfortunately, because there is a slope in the stratigraphy, and because the lithics are only reported by fairly large squares, it seems impossible to know which objects were found above or below layer 1206 in many cases. I wish there was a figure that shows just the objects found below 1206. The few that can be definitely located below 1206 with the information provided in the paper are not encouraging.

The Eolith problem, again

The problem of telling stone tools apart from geologically worked rocks, when they are not obviously highly curated tools, and when they are found in the general vicinity of their geological source is so serious, that an academic debate raged on this question for about a hundred years from the second half of the nineteenth to the mid twentieth century.

As archaeologists and geologists started recognizing older and older sites and human ancestors, based on worked stone and bone, human fossils, and features such as structured hearths, they kept looking at still older geological deposits to find the oldest evidence of human existence. In oldest deposits, the other lines of evidence gradually petered out, and they were left only with what looked like, or could have been, worked stones, or lithics.

By the early twentieth century, there was a spirited controversy about whether these were archaeological at all. Some argued that they were the work of vanished human ancestors, some that they were the result of geological processes alone. Others concluded that there was no way of telling the difference between the two, and that we had to remain agnostic about the problem of the antiquity of Man, as it was called back then. This was the Eolith debate. Eoliths were these earliest claimed, but uncertain stone tools. Ellen (2011) provides a fascinating account well worth reading.

This is where we’re at with Chiquihuite and early claims in the Americas in general. Unfortunately, not very much has changed since Hayward wrote in 1913 that “it is doubtful, in my opinion, if it can be proved by experiments whether Nature or Man has applied the force [to the lithic material], as the same forces are used by both Nature and Man, the only difference being the intensity with which the force has been used and the way that force has been applied.”

Barnes (1939) did provide some useful experimental results that show that at least for the European flints that were at the heart of the original eolith debate, human work produces assemblages that are statistically different from geological ones in distribution of edge angle. But even that result is of limited value, because if up to 80% of a geological assemblage has much more obtuse edge angles that those produced by humans, that still leaves 20% of a geological assemblage in which archaeologists can find nice, convincing-looking, “stone tools”.

Figure 2: Figure 5 from Barnes (1939)

Without a statistical sense of how the identified assemblage varies from the matrix from which it is selected, and how it varies from other nearby geological assemblages, it is impossible to say how likely it is to represent the work of humans. Pretty much any pile of rocks will contain at least some material that could pass as archaeological.

The question, as always, is not whether there were humans in the Americas 30k years ago, in the middle of the last ice age. The question is whether this particular paper presents convincing archaeological evidence that there were humans at Chiquihuite cave at that time. While this looks like a really solid ancient DNA (aDNA) and geology paper, it seems to have little to do with ice age humans in the Americas. I strongly encourage Ardelean et al. to fill in the blanks in the archaeology section of this otherwise very interesting and valuable contribution.

References

Ardelean CF et al. 2020. Evidence of human occupation in Mexico around the Last Glacial Maximum Nature 584:87–92. https://www.nature.com/articles/s41586-020-2509-0.pdf

Barnes AS 1939. The Differences between Natural and Human Flaking on Prehistoric Flint Implements, American Anthropologist 41:99-112. https://www.jstor.org/stable/pdf/661725.pdf

Ellen RF 2011. The Eolith Debate, Evolutionist Anthropology and the Oxford Connection Between 1880 and 1940, History and Anthropology 22:277:306 https://www.tandfonline.com/doi/full/10.1080/02757206.2011.595007

Gruhn R 2020. Evidence grows that peopling of the Americas began more than 20,000 years ago, Nature News and Views, 22 July 2020. https://www.nature.com/articles/d41586-020-02137-3

Hayward FN 1913, The problem of the Eoliths, Proceedings of the Prehistoric Society of East Anglia 1 (31):347-359.