Evaluating the claim of 16k year old remains at Cooper’s Ferry: A comment on Davis et al 2019

Sixteen thousand years seems to be the new target for earliest archaeological remains in the Americas. The team at Gault recently made the claim, along with the team at the associated Friedkin site. Now a team working at Cooper’s Ferry in Idaho have joined in.

Let me start with my standard disclaimer: I have no problem with the idea that there were people in the Americas 16 thousand years ago (kya). I think there most likely were. I would not be surprised, some day, to see solid evidence in support of that claim. In fact, I would not be shocked if solid evidence came up of an inter-glacial human presence.

However, as a Western trained, academic archaeologist, what I look for are clear, well-dated archaeological remains of that age. I have not yet seen any that are older than about 14ky, and that includes Cooper’s Ferry.

The claim

Davis et al. claim to have found archaeological remains that date to between 16 560 and 15 280 calibrated years before present at Cooper’s Ferry in Idaho, a location known as Nipéhe by the Niimiipuu people.  That age range is for the start date of what the authors call layer LU3 in the site’s stratigraphy, as given by a Bayesian age model (we will come back to this). The dates on bone and charcoal in LU3 fall between about 8kya and about 16kya. The oldest date in the series (on bone) is between 16 255 ya and 15 795 ya. So 16kya is really an absolute oldest age, if we conclude that layer LU3 has archaeological material at its very bottom. As we will see, this might be an issue.

Evaluating the claim

First, the good news. LU3 definitely has stone tools, and it is definitely very old. The most curated tools are stemmed points. The excavation appears extremely careful and well documented.

Debitage vs tools

The debitage found below 411.60 masl, which corresponds to a date of about 14kya, is at the heart of the claim of a 16kya archaeological presence. In fact, it is the only claimed evidence: “The LU3 deposit between 411.55 metres above sea level (masl) and the lower contact with LU2 contained 43 pieces of debitage, 20 bone fragments, and a piece of charcoal excavated in situ below the stratigraphic position of OxA-38,051 (15,945 to 15,335 cal yr B.P.) (Fig. 2). These lower materials are objects discarded during repeated periods of human occupation at Cooper’s Ferry, which Bayesian modeling suggests began at 16,560 to 15,280 cal yr B.P. (95.4% confidence) (Fig. 3).”

Table S3 of the supplemental materials shows that while about 20% of remains classified as debitage in LU3 were found below 411.60 meters above sea level masl, all bifaces, blades, and clearly identifiable tools are found above this depth, except the stemmed points found within pits excavated into the site by the original inhabitants (see below).

Table 1 of the article shows that the oldest dates around the critical elevation of 411.60 masl are between 14kya and 15kya. In other words, there is a small portion of debitage found near the deepest and oldest dates, but the tools are all clustered higher up in the layer, and clustered with younger dates.

The stemmed points, which in some ways are the stars of the paper, and which are the focus of the analysis and of the comparative section on Upper Paleolithic Asia, are in fact not connected with the early date, nor do the authors suggest that they are. This is important to keep in mind, the paper and some of its coverage doesn’t necessarily make that clear to a casual observer.

As far as I can tell, the only claimed artefacts which are below 411.60 masl and are not within pits, are identified as modified flakes, a notoriously difficult category that often veers into the eolithic, as an inspection of Figure 6 of the supplementary materials will show.

The debitage is “made primarily from local cryptocrystalline silicate”, with some “fine-grained igneous toolstone”. It isn’t unusual to find these materials, in those proportions, on an old terrace at a bend in a river. Before I rely on debitage alone as an archaeological marker, I would need to know much more about what sorts of rocks naturally occur at those depths in other similar spots in the region. It may be that the proportion of eoliths at Cooper’s Ferry grows as one goes down through the stratigraphy.

This is a pattern that I see often in these claims of very early archaeological material in the Americas. There is definitely archaeological material present, but as one goes down through the stratigraphy, it becomes less and less definite. The easily identifiable tools tend to disappear from the sequence at much more recent dates than the earliest claimed. This seems to be the case here.

The pits

Another, lesser issue, is the presence of large (up to 1 meter in diameter) pits in the stratigraphy. These were dug by the original inhabitants of the site into its old surface, sometimes to depths of half a meter, and filled with remains such as debitage, charcoal and bone.

Figure 2 of the article, which is an excellent stratigraphic chart, shows that three of those pits go clear through the LU3 layer and into the underlying LU2 and LU1 at the bottom of the stratigraphic sequence. Figure S15 of the supplemental materials shows a very clear profile of one such pit and its underlying matrix.

While there is a good correlation between dates and depth for the rest of the layer, the dates on material within these pits are very mixed. There are dates at 7kya and 13kya near the top of one, and a date of 11kya near its bottom.

Almost all of the really diagnostic tools found on the site were recovered within these pits. The pits also suggest that there was significant reworking of the stratigraphy by the inhabitants, not all of which might be obvious today.

The Bayesian model

The authors use a Bayesian age model to estimate the maximum age of layer LU3. This is not an issue in itself. Bayesian modeling can be very useful in archaeology. Bayesian models for radiometric dates take into account not just the physical measurements on samples and their calibration, but also prior beliefs and known information about the samples and the context. “Prior beliefs (archaeological information) applicable to any given dated sample can be absolute (e.g., historical accounts) or relative (e.g., the stratigraphic context)” (quoted from the supplemental material).

Unfortunately, the authors are not very specific about the set of priors they use to develop their Bayesian model. All we know is that “For Cooper’s Ferry, the Bayesian age model was created with OxCal 4.3 using chronometric data in main text Table 1 and known stratigraphic information.” That is quite thin when the outcome is so stunning.

The bottom line

There is certainly some very old archaeological material at Nipéhe. The stemmed point fragment outside the pit feature at the surface of LU3, in particular, makes a strong case that the site is at least 13k years old. The lack of diagnostic material below 411.60 masl, combined with the level of disturbance within and around the pits, along with the fact that the most diagnostic finds occur within the pits, however, casts serious doubt on the 16ky claim.

Personally, I would have presented the site very differently from the way it is done in the Science article. Essentially, the authors are saying: We have a site that has 16ky old material, but we won’t focus on that older material. The site also has 13ky old stemmed points, so we will focus on comparing those stemmed points with Asian Upper Paleolithic remains, because we think they must be related, since there is also 16ky old material.

I think it would have been better to say: We have a great site with some amazing stemmed points that are at least 13ky old, and some really interesting pit features. The site also has bits and pieces that we think are archaeological, and that we think might be as old as 16ky, and we are looking into that. Stay tuned.

Again, I won’t be surprised if future work at the site uncovers stemmed points or other clear artefacts in securely dated contexts older than 15k years. In fact, I hope it does. But at the moment, as is the case for Gault, Friedkin, and Monte Verde, I am stuck at about 14k.

Reference

Davis LG, DB Madsen, L Becerra-Valdivi, T Higham, DA Sisson, SM Skinner, D Stueber, AJ Nyers, A Keen-Zebert, C Neudorf, M Cheyney, M Izuho, F Iizuka, SR Burns, CW Epps, SC Willis, I Buvit 2019. Late Upper Paleolithic occupation at Cooper’s Ferry, Idaho, USA, ~16,000 years ago, Science 365:891-897.