Hominin skeletal part abundances and claims of deliberate disposal of corpses in the Middle Pleistocene

Discussion

An array of unsupervised multivariate statistical tests (k-means, PCA-based CA) and supervised machine learning algorithms (RF, SVM, K-nearest neighbor, decision trees) successfully separate assemblages of undisturbed or minimally disturbed hominin corpses from those that experienced some level of disturbance via cannibalism, secondary interment, and carnivore consumption. These methods also consistently cluster the SH and DC assemblages with the remains of scavenged human corpses, leopard-consumed baboons, and baboons that died naturally within a cave. It is notable, too, that the SH and DC assemblages do not group with El Mirador, which represents a secondary burial.

In other words, the skeletal element abundance data suggest that the SH corpses did not find their way into the cave chamber as complete skeletons and/or that they experienced a substantial level of disturbance after their deposition. While other taphonomic factors may have been at play, we consider the feeding activities of carnivores to be a likely source of this disturbance. Analyses of surface damage on the SH bones reveal that carnivores did, in fact, modify the hominin remains, although the degree of carnivore impact on the final condition of the assemblage is unsettled. To wit, Andrews and Fernández-Jalvo (10) report that >50% of hominin long bone, clavicle, pelvis, sacrum, and rib specimens bear carnivore tooth mark damage. In contrast, Sala et al. (11) find carnivore marks to be much less common, with rates of only 3.7% across the entire skeleton. The damage rates reported by Andrews and Fernández-Jalvo (10) are consistent with—and those of Sala et al. (11) substantially lower than—tooth mark frequencies observed in an assemblage of baboon skeletal remains modified by feeding leopards (12) and in an assemblage of human skeletal remains scavenged by small canids (13), both of which cluster with the SH assemblage based on skeletal element abundances.

It is important to note that even if Sala et al.’s (11) values are closer to representing reality, they still probably underestimate the intensity of carnivore involvement in the formation of the SH assemblage. The prevalence of dry, diagenetic fractures on the SH hominin bones (14) is key here, since such breakage created fragments that did not exist during the corpses’ nutritive phases, periods when carnivore damage was presumably inflicted. This disjunction leads to the artificial depression of damage frequencies relative to actualistic controls (see, for example, ref. 15).

We can gauge the effect of this process on the SH mark frequencies through application of a correction method (16). A diagenetic break produces, at a minimum, two bone fragments. Thus, the number of diagenetically fractured specimens should be divided by two to reproduce, conservatively, the number of specimens present before diagenetic breakage occurred. This corrected value is then added to the number of specimens broken when bones were fresh (i.e., “green”) to arrive at a more realistic denominator for the calculation of mark frequencies. As an example, consider the 587 long bone (i.e., clavicles, humeri, femora, radii, ulnae, tibiae, fibulae, and metapodials) specimens in Sala et al.’s (14) fracture analysis. Between 62.5% and 90.2% of the specimens identified to each element possess the transverse fracture outlines typically (although not exclusively or uniquely) associated with diagenetic breakage (17). Applying these percentages to the 574 long bone specimens examined for tooth marks (11), 439 are expected to be the result of diagenetic breakage and only 135 (574 – 439) the result of breakage by biotic agents during the nutritive phase. Dividing the estimated frequencies of diagenetically fractured specimens per element by two and adding the resulting value to the estimated frequencies of green broken specimens by element produce a final estimate of the frequency of specimens present before the onset of diagenetic fracture. The corrections using this and other correlates of dry breakage are summarized in Table 3. Across all elements, the revised damage frequencies (7.9–9.2%) are several percentage points higher than the noncorrected value (4.9%) and the damage rates rise significantly for specific elements, like the humerus (from 10.1 to 15.6%–19.1%) and femur (from 18.9 to 30.8%–36.0%). Thus, although carnivore damage on the SH bones may not be severe, it certainly is not negligible.

There is agreement that the modifications on the SH bones were probably inflicted by bears and lions (10, 11). But taphonomic studies of bone remains modified by modern bears reveal patterns of skeletal part representation that differ from that of the SH (18), and modern and prehistoric bear damage to the bones of consumed animals (18⇓–20) diverges from that documented on the SH hominin remains (11). There are similar inconsistencies between what is understood about modern lions as taphonomic agents (21) and what is observed in the SH assemblage (10, 11). The application of a taphotype approach, which identifies taxon-specific patterns of furrowing and tooth-marking (22), may help resolve this issue at some point.

As to the DC assemblage, skeletal part data suggest that, similar to the situation at the SH, hominin corpses did not arrive in the chamber as complete skeletons and/or experienced some postdepositional disturbance (see also ref. 23). Dirks et al. (7) are careful to point out that particular bone fragments are excluded from their preliminary element frequency estimates, which means that the DC skeletal part abundances probably underestimate the number of represented elements to a greater degree than do counting methods that consider all identified fragments. Given the collection procedures imposed by the cramped quarters of the cave chamber, it remains unclear how representative the excavated DC assemblage is of the complete deposited assemblage (24). Nevertheless, the recurrent clustering of the DC assemblage with the disturbed and carnivore-consumed samples and, in particular, the naturally accumulated bone sample of cave baboons, is intriguing.

To this point, Val (23) questions the conclusion that the DC hominin remains lack carnivore damage (7, 24) and, in so doing, raises what we consider to be legitimate concerns: (i) only about one-third (559 of a total of 1,550) of the recovered hominin specimens were inspected microscopically for surface modifications; and (ii) surface preservation of the DC bones is generally poor, which may obscure or eliminate original surface modifications, including carnivore damage. Given the positive relationship between the size of a bone specimen and the probability of mark appearance on that specimen (25), it is possible that the subset of DC bones subject to analysis, which includes the larger and more complete specimens (7), is that most likely to preserve marks. However, given the uniqueness of the DC sample and the remarkable behavioral claims attached to it, it seems obvious that the entire assemblage should be analyzed carefully for surface modifications. Even more worrisome is that “few [of the DC bone] specimens preserve a pristine surface morphology” (7). Most surface-quality appraisals correspond instead to the types of poorly preserved bones categorized as grade 3 (“[m]ost of bone surface affected by some degree of erosion…general morphology maintained but detail of parts of surface masked by erosive action”) and grade 4 (“[a]ll of bone surface affected by erosive action…general profile maintained and depth of modification not uniform across whole surface”) in McKinley’s (26) system.

Moreover, under the variable “surface removal” in Dirks et al.’s (7) supplemental data, 553 of the 559 analyzed specimens (98.9%) score as “outer cortical layers gone.” While this description does not necessarily imply the removal of the entirety of a specimen’s cortical layer (24), we think, given the regularity and severity of surface degradation within the DC assemblage, the possibility that evidence of carnivore involvement was eliminated, or at least rendered inconspicuous or unclear, should not yet be dismissed. Even in the absence of direct carnivore involvement, our machine-learning results for an assemblage composed of baboon remains accumulated by natural die-off in a cave demonstrate that an assemblage composed almost exclusively of a single, large-bodied primate with skeletal patterning like that seen in the DC need not necessarily require deliberate disposal by conspecifics.