Evaluating claims for an early peopling of the Americas: experimental design and the Cerutti Mastodon site

Abstract

In a 2017 article, Holen and colleagues reported evidence for a 130 000-year-old archaeological site in California. Acceptance of the site would overturn current understanding of global human migrations. The authors here consider Holen et al.’s conclusions through critical evaluation of their replicative experiments. Drawing on best practice in experimental archaeology, and paying particular attention to the authors’ chain of inference, Magnani et al. suggest that to argue convincingly for an early human presence at the Cerutti Mastodon site, Holen et al. must improve their analogical foundations, test alternative hypotheses, increase experimental control and quantify their results.

Introduction

Current archaeological and genetic evidence suggests that Indigenous peoples in the Americas are descended from those of ancient Siberia, with founding populations separating from ancient North Asians c. 25 000–16 000 years ago (Raghavan et al. 2015; Skoglund & Reich 2016; Moreno-Mayar et al. 2018; Potter et al. 2018). While the arrival and dispersal routes remain contentious (Pedersen et al. 2016; Lesnek et al. 2018), it is probable that, after passing south of the ice sheets, humans quickly dispersed across North and South America (Cinq-Mars 1978; Dillehay et al. 2008; Goebel et al. 2008; Graf et al. 2014). Against this background, recent work by Holen et al. (2017) reports new evidence for stone tool use at the 130 000-year-old Cerutti Mastodon site in California. The authors employ a series of experiments to support their argument that the lithic and faunal remains at the site are anthropogenically modified. Their findings, if accepted, would push the peopling of the Americas back by over 100 000 years, thereby rewriting the history of global human migrations.

Such an exceptional claim has invited heavy criticism. Disputing Holen et al. (2017), scholars have suggested alternative taphonomic explanations for the formation of the Cerutti Mastodon site. These include comparing the Cerutti assemblage to both anthropogenic and non-anthropogenic sites, questioning the absence of features generally associated with the presence of humans, and the possibility that recent disturbances may have altered the remains (Braje et al. 2017; Haynes 2017, 2018; Ferraro et al. 2018). Holen et al. (2018a–c) and their supporters (e.g. Boëda et al. 2017; Gruhn 2018) have responded to these criticisms, contending that alternative arguments fail to explain the features of the Cerutti Mastodon site.

While these previous responses to Holen et al. provide viable criticisms, the experimental design of the project used to support the archaeological claims has not been examined. Rather than reiterate the ongoing debate concerning the archaeological nature of the Cerutti site itself, here we examine the experimental data used to buttress Holen et al.’s argument. We raise questions about the researchers’ analogical foundations, the lack of alternative hypotheses tested, as well as experimental control and the quantification of data. We conclude with recommendations for future experimentation intended to provide more robust evidence, through which Holen et al.’s (2017) claims may be more effectively evaluated by the archaeological community.

Hypothesis testing and analogical reasoning in experimental archaeology

The first step in the development of an archaeological experiment entails hypothesis creation. The method by which one generates ideas should be clearly stated, as the interpretation of an experiment's results is directly dependent on the authors’ underlying assumptions (Domínguez-Rodrigo 2008). The proposed hypothesis must be formulated in a falsifiable cause-and-effect scheme that consequently implies an opposing null hypothesis.

We follow the current best practice for archaeological experimentation proposed by scholars including Lin et al. (2017) and Eren et al. (2016). To the extent that Holen et al.’s logic of hypothesis construction is transparent or reproducible, we find it to be problematic. Instead of setting up the experiment with two opposing hypotheses, the authors have conducted their experiments and summarily confirmed that the observed patterns match those from the Cerutti site. While the authors do acknowledge alternatives, they do not evaluate the other viable explanations for the observed patterns (see also Braje et al. 2017, and the rebuttal by Holen et al. 2018c). In order to suggest that the bones were, in fact, modified by humans, Holen and colleagues must also quantitatively compare lithic and faunal remains—modified via other taphonomic processes—against their experimental dataset.

Beyond hypothesis construction, it is critical to justify the analogues that scaffold the experiment. In their 2017 publication, the authors focus much of their attention on the use of a hafted hammer stone. The origin of this analogy should be explained—beyond a passing reference to late prehistoric Plains peoples (see Holen et al.’s 2017 supplementary information 5, p. 22). The replication of patterns from the La Sena Mammoth site (see also Holen et al.’s 2017 supplementary information 5, p. 22) represents an uncontextualised comparison that appears only tangentially related to the Cerutti Mastodon site—and nearly 100 000 years removed. The experiment should be set up so that the variables are as close as possible to the observation that one is attempting to replicate. In this case, the experiments presented are not only too variable and inconsistent for any meaningful assessment of hypotheses, but are also built on foundations that are insufficiently comparable to the archaeological context in question.

Experimental control

When designing and implementing an experiment, it is important to exert control over variables at a level justified according to the experimental design (for a discussion on experimental validity, see Mesoudi 2011, and for an illustration of its spectrum, see Eren et al. 2016). This ensures that the observed results reflect manipulation of those variables associated with the hypotheses, rather than the interaction of nuisance variables (sensu Lin et al. 2017, but see also a discussion of the role of randomisation in mitigating nuisance variables on page 14 of the same manuscript). Based on a review of Holen et al.’s experimental design, we conclude that there was substantial variation not only in the raw materials used, but also the ways in which the experiments were executed; these variables detract substantially from the arguments made by the authors.

In reporting their study, Holen et al. (2017) list the raw materials and mass of individual tools from the Cerutti site, as well as their experimental hammer stones and anvils. While the individual archaeological hammer stones range in mass from 7.6–14.45kg (without refits; up to 18.25kg with refits), and are made of andesite and pegmatite, the experimental hammer stones range from 1.7–14.7kg and are of andesite or granite. Similar differences characterise the anvils. The variation in raw material, size and mass make it difficult to compare the results of the experiments with confidence. In turn, extrapolating to the archaeological record and the Cerutti assemblage becomes more tenuous.

Acknowledging the rarity of elephant remains with which to conduct such experiments, we recommend that scarce elephant bones not be used to confirm the hypothesis as presented by the authors, but rather function as benchmarks against which more refined experiments may be compared. For instance, after establishing whether or not the different sizes of hammer stones can be used effectively in the processing of large proboscidean bones, it would then be productive to establish a more robust experimental procedure using the bones of cattle or other large mammals.

Holen et al.’s four reported experiments were executed inconsistently. Of these experiments, two were conducted on elephant bones, while the other two were undertaken using cattle and/or kangaroo bones. In the first case, the elephant bones were propped up on a wooden block and struck with a hafted stone. In a second experiment conducted on an elephant bone, a larger hammer stone was used after the failure of another hafted implement. The specifics of the subsequent lines of experimentation, carried out with cattle and kangaroo bones, are less clear, and require more detailed reporting to allow meaningful evaluation or repetition. We suggest future iterations of this experimentation begin by employing both hammers and anvils of approximately the same dimensions, composition and size. These should be randomly assigned to treatment groups, in order to create results that may be compared with one another and with outgroups of assemblages created through the testing of alternative hypotheses.

Once the raw materials used in the experimentation are held constant, it would then be beneficial to ensure that the ways in which the raw materials are made to interact (e.g. hammer stone on bone impacts) are also systematised. In reviewing the highly controlled experiments of Dibble and Rezek (2009), and work by Magnani et al. (2014), we understand that strict laboratory conditions are not always attainable, or desirable, in experimental archaeology. At a minimum, however, we suggest ensuring that the bones be struck and supported as consistently as possible. Ideally, the force would be regular and dealt from similar angles, a set number of times. The reported experiments make no apparent effort to control for any of these variables, casting doubt on the internal validity of the results (sensu Lin et al. 2017).

Quantification of results

The experiments reported by Holen et al. (2017) are entirely qualitative, producing a good initial pilot study and the basis for designing further archaeological experiments, but limited inferential value (Lin et al. 2017: 680). When designing replication experiments, quantification is a basic requirement to enable the comparison of excavated artefacts with experimental results using standard statistical methods (Eren et al. 2016: 108). Quantification allows researchers to assess alternative hypotheses explicitly. Qualitative visual comparisons are insufficient to draw meaningful conclusions regarding differences in the morphological characteristics of bone and stone tool assemblages. Fortunately, there are numerous options for Holen and colleagues to consider. For stone tool morphometrics, for example, Caruana et al. (2014) suggest the use of digital elevation models in ArcGIS to quantify the shape of pitting on stone artefacts ($\displaystyle{{perimeter^2} \over {volume}}$), in order to distinguish anthropogenic traces (whether ancient or modern) from natural characteristics of river cobbles. The authors may further wish to evaluate the morphologies of the bone flakes and impact notches from the Cerutti site, in relation to experimental datasets, employing both dimensional measurements standard in the assessment of anthropogenic bone modifications (Galán et al. 2009), as well as more recently developed geometric morphometric techniques (Yravedra et al. 2018). The use of any or all of these quantitative approaches would better enable the archaeological community to evaluate the authors’ findings.

Finally, quantitative measures depend on assumptions of a sufficiently large experimental sample to ensure an acceptable level of statistical confidence (Drennan 2009: 126). In order to assess the proper sample size needed for an experiment, it is standard across experimental fields for researchers to perform a power analysis: a measure of how well a particular statistical test can distinguish a difference between groups, if indeed there is one (Cohen 1992: 100). After establishing appropriate controls on the raw materials and other experimental parameters, we therefore recommend that the authors employ such an analysis, expanding their experimental sample size appropriately to facilitate meaningful evaluation of their data.

Conclusions

For over a century, anthropological enquiry into the peopling of the Americas has continually expanded to accommodate new archaeological evidence and genetic data. The Cerutti Mastodon site, as reported by Holen and colleagues, joins the ranks of archaeological discoveries that have challenged established narratives about global human migrations. Additional evidence and experiments, however, are required for the interpretation of this site to gain widespread acceptance by the archaeological community, and to identify meaningfully the presence of humans in the Americas over 100 000 years ago. While the genetics of present-day and ancient Native Americans do not indicate any obvious contribution from the authors’ hypothesised early entrants into the Americas (Sankararaman et al. 2016; Browning et al. 2018; Moreno-Mayar et al. 2018), the lack of such a contribution does not refute their contention that Cerutti is an early human archaeological site. There are numerous cases of genetic discontinuity between present-day groups in many parts of the world and the earliest-documented inhabitants of those areas. For example, the oldest incontrovertible modern humans in Northern Eurasia—represented by the ancient genome of the 45 000-year-old Ust’-Ishim femur from West Siberia (Fu et al. 2014)—were not clearly ancestral to any present-day groups. Such may also be the case for the first humans in the Americas.

To be clear, we are not opposed categorically to the demonstration of an early human presence in the Americas. We do, however, expect that significant assertions in support of such a case, including those made by Holen and colleagues, must be built on a solid foundation, following current best practice in the field of experimental archaeology. Central to the argument of Holen et al. is their qualitative dataset derived from experimentation. In effect, the experiments used to support their 2017 article function primarily as a series of pilot studies. While such studies are an important part of the discipline of experimental archaeology, and may suggest preliminary relationships between variables, they are not sufficient in their own right to overturn decades of archaeological research on human migration.

Moving forward, we argue that analysis of the Cerutti site demands a more robust experimental archaeological study, which includes shoring up the analogical reasoning, controlling experimental parameters, increasing sample size and the quantification of results. These constitute the essential first steps that will allow for a more thorough evaluation of the exceptional claims made—as well as the dismissal of the alternative explanations offered—for the Cerutti site. We thank Holen and his co-authors for making extensive supplementary data available for their project, including videos and three-dimensional models, which will prove beneficial for further evaluation of the Cerutti site. Although our piece is a critical assessment, it is the availability of these data that has made it possible to evaluate the authors’ interpretations, to draw conclusions and to make our recommendations.