Evaluating the claim of 37ky old archaeological material at the Hartley Mammoth locality in New Mexico

Rowe et al. (2022) recently argue that the pattern of breakage and degradation of bone at the Hartley Mammoth locality in northern New Mexico is evidence of human activity 37ky ago. If true, this would make the Hartley locality the oldest solidly dated archaeological site in the Americas by a factor more than two.

The claim

This claim follows a now familiar pattern: Strong early date and geological context, lots of faunal remains, and no obvious lithic tool assemblage. The significant difference, and a welcome one, is that the authors acknowledge right at the start that there are no identifiable stone tools in the assemblage. Instead, they focus on finding other less obvious (for now) and less direct evidence of human activity. That’s exactly the direction these kinds of studies need to take. We need to learn to more reliably read human presence in the past, using a greater diversity of indicators. No question about that.

The basic problem for claims of physical evidence of pre-16k (according to the authors) human activity in the Americas (and pre-15k as far as I am concerned, but I won’t quibble here) is that there are lots of sites that have evidence of the kind also found on old sites in Africa, Europe and Asia, such as broken bones, traces of fire, even broken stones, all of which can be the result of human activity, but all of which can also be caused by other agents such as erosion, floods, etc. None of the pre-16k North American sites so far have a clear and obvious clincher such as an assemblage of stone tools clearly produced by humans, or datable rock art, etc.

Rowe et al. get to the heart of the matter on page 58 of their supplementary material: “Whereas the American sites lacking lithic evidence are often controversial, especially sites older than ~16,000, this pattern of bone breakage is unequivocally recognized as cultural in numerous Old World sites.”

The answer to their implied question, of course, is that those (often) European sites are recognized as cultural not because it’s been established with an certainty that only humans can produce those bone breakage patterns, but rather because those potentially cultural sites are surrounded by sites of similar age that we know are cultural based on other evidence such as curated stone tool assemblages. 

In the absence of other clear evidence of human activity in the region at that time, none of those elements would in themselves be sufficient to establish a site as archaeological in Europe, Africa, Asia, or anywhere else. Because there are clearly archaeological sites that are 30k years old in Europe, a 30ky old mammoth site in that region that lacks stone tools can be uncontroversial as an archaeological site. It probably shouldn’t get a completely free pass, but it can get one, and has in the past. To put it simply, maybe we find those bone breakage patterns in all mammoth sites that are archaeological, but we don’t know that all sites in which we find them are archaeological.

The difference in archaeological status between pre-16k mammoth sites in Europe and the Americas so far comes down to regional archaeological context. The European ones, even if they have no obvious, direct neon signs of human activity, are surrounded by actual archaeological sites. The North American ones, so far, are not.

I am perfectly open to the idea that there are other elements than curated stone tool assemblages that could be diagnostic of human activity, and I hope we find them. This kind of study is exactly what we need to accomplish that. Let’s examine it.

The evidence

Rowe et al. analyze bone breakage and accumulation patterns. They use mainly Computed Tomography (CT) to suggest that the breaks and punctures on the bones are not the result of compression from overlying soil, or from carnivore and scavenger activity.

They note the presence of bone flaking and the degree of fragmentation of bone which suggests intentional breakage for heating, boiling and grease rendering. They compare the assemblage to similar ones mainly in northern Europe that are considered archaeological because of these patterns, and despite the absence of obvious curated stone tool assemblages. 

One of the paper’s main strengths, the use of CT for detailed analysis of bone breakage and puncture, is also one of its temporary weaknesses, as there is not much comparative material out there, especially from similar but clearly archaeological mammoth butchering sites. But that should be remedied over time as the methods are used more widely.

From the experimental work and the CT results reported in supplementary materials, it is clear that the bones were extensively modified by strong, sudden impacts. The question remains, as always, whether humans caused those impacts.

The extensive breakage is consistently described as systematic, but the authors also emphasize that the material was buried in a single event, shortly after death, which suggests a catastrophic event. As we know from the eolith debate, humans are not the only and necessary source of systemic patterning in nature.

The paper discusses and reasonably rules out trampling, heavy machinery, etc as potential sources for the patterning. The rejection of the idea that carnivores and scavengers could have produced the observed bone damage is less categorical but still fairly convincing. 

However, the authors don’t seem to address whether perhaps a landslide could have broken the bones and piled them up in what they refer to, throughout the paper, as a landslide trough. If they did address this in the paper, I missed it on first reading, and I would certainly like to hear their thoughts on it.

The strongest and most intriguing evidence presented is not from the mammoth bones at all, but from possibly burned vertebrate microfauna, including fish teeth and scales found in the soil. I will say that without some sort of animal agent, human or other, the fish bones cemented into calcified aggregates are harder to explain than the broken mammoth bones. 

The predator-other-than-human also requires deposition of the bones after consumption of the prey, and then a separate natural fire event. On the other hand, the human predator explanation requires the presence of humans where we have no previous physical evidence of their presence. It’s getting difficult to decide which is the simpler explanation. That one is a very interesting lead, and I would like to see more work done on this.

The conclusion

In brief, we have another solidly dated locality with at the very least some really interesting paleontological remains. They are studied in great detail, using innovative techniques, and the paper is full of great information. The question of whether the site is archaeological remains open for me at this point. It’s a definite possible.

References

Rowe TB, Stafford TW Jr, Fisher DC, Enghild JJ, Quigg JM, Ketcham RA, Sagebiel JC, Hanna R and Colbert MW (2022) Human Occupation of the North American Colorado Plateau ∼37,000 Years Ago. Front. Ecol. Evol. 10:903795. doi: 10.3389/fevo.2022.903795