Neanderthals and early modern humans show similar levels of cranial injuries, study finds

A team of University of Tübingen researchers has shown that Neanderthals sustained similar levels of head injuries to the earliest anatomically modern humans in Eurasia. This result contradicts previous views that Neanderthals were characterized by exceptionally high rates of trauma. This post is dedicated to this very interesting research.

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Injuries are part of everyday life, from a scratch on the skin to a broken bone to a fatal trauma. Although many injuries are accidental, others can arise as a consequence of an individual’s or a group’s behaviour, activity or social norms — characteristics that tell us about societies and the inherent tensions and risks within and between different groups.

Writing in Nature, Beier et al. provide evidence that challenges the long-standing view that Neanderthal populations experienced a level of traumatic injuries that was significantly higher than that of humans. The result calls into question claims that the behaviour and technologies of Neanderthals exposed them to particularly high levels of risk and danger.

Reports of injuries and deaths are constantly in the news. As well as being drawn to read the stories of individuals, such information is of interest because of what it tells us about our societies. However, to fully understand what might determine the current degree of violence and injuries, we also need to look back at the past and identify the causal underpinnings. But how far back should we look? Arguably, right back to the evolutionary origins of processes that shape behavioural, social and cognitive tendencies and abilities.

Anthropologists study skeletal remains to reconstruct aspects of ancient lives, building an ‘osteobiography’ that casts light on part of the life history of an individual. Skeletons preserve — in the form of holes, misshapen surfaces, bone misalignments and secondary fractures radiating out from a point of impact — a signature of the traumas that resulted in fractured, cut or perforated bones, even if the injuries subsequently healed.

Traumatic lesions have been frequently identified in Neanderthal fossils, particularly in the head and neck, leading to the view that higher levels of skeletal injury occurred in Neanderthal populations than in human populations. However, this is not so, say Beier and colleagues. The authors assessed published descriptions of Neanderthal and modern human fossil skulls found in Eurasia from approximately 80,000 to 20,000 years ago. Comparing the number of injured and non-injured Neanderthal and human skulls, the authors report similar levels of head trauma in both groups.

The power of Beier and colleagues’ analyses lies in their study design. Instead of comparing Neanderthal data with those of more-recent or living human populations, as previous studies have done the authors based their comparisons on humans who not only shared aspects of their environment with Neanderthals, but whose fossil record also has a similar level of preservation. Beier et al. analysed data for 114 Neanderthal skulls and 90 human skulls. 

They gathered the data for 14 skull bones, and obtained information that ranged from 1 bone in poorly preserved fossils to data for all 14 bones per individual for well-preserved ones. In total, the authors recorded trauma incidence in 295 Neanderthal bones and 541 human bones. They also collected other information, such as the percentage of each of the 14 bones that was preserved for each individual, as well as details including sex, age at death and the fossil’s geographic location.

Beier et al. ran two sets of statistical analyses — one based on the presence or absence of trauma in each of the skull bones, the other on individual fossil skulls as a whole — to test whether there were any statistically significant differences between the prevalence of trauma in the Neanderthal and human fossils. The authors also assessed whether trauma prevalence was linked to sex or age, taking into account fossil preservation, geographic location and possible interaction effects between the different variables. The two analyses gave similar results.

The more complete the fossils are, the more likely they are to have preserved evidence of injuries. This might seem obvious, but is an issue often ignored in such studies. Beier et al. offer a way to deal with this type of bias in the available material. Once the authors take into account the extent of fossil preservation, the predicted prevalence of trauma in Neanderthals and humans is almost the same.

Both Neanderthal and human males had a much greater incidence of trauma than did the females of their respective species. This pattern remains the same for humans today. One final intriguing result is that, although traumatic injuries were present across all of the age ranges studied, Neanderthals that had trauma to the head were more likely to have died under the age of 30 than the humans were. The authors interpret this result as evidence that, compared with humans, Neanderthals either had more injuries when they were young or were more likely to have died after being injured.

Beier and colleagues’ study does not invalidate previous estimates of trauma among Neanderthals. Instead, it provides a new framework for interpreting these data by showing that the level of Neanderthal trauma was not uniquely high relative to that of early humans in Eurasia. This implies that Neanderthal trauma does not require its own special explanations, and that risk and danger were as much a part of the life of Neanderthals as they were of our own evolutionary past. The result adds to growing evidence that Neanderthals had much in common with early human groups. However, the finding that Neanderthals might have experienced trauma at a younger age than humans, or that they had a greater risk of death after injury, is fascinating, and might be a key insight into why our species had such a demo-graphic advantage over Neanderthals.

Is this the final word on the subject of Neanderthal trauma? The answer is no. Beier and colleagues assessed only skull trauma. What if Neanderthals accumulated more injuries to their bodies than did humans? There are data suggesting that this might be the case. Furthermore, although the authors’ analyses demonstrate the power of a well-designed study based on large samples, the data they used were recorded by many researchers and at varying levels of detail, raising the possibility of methodological biases.

Lastly, the causes of the injuries could provide some elusive insights into behaviour, activities or social norms in the past. From the shape, location and extent of traumatic inju-ries in skeletons, and characteristics such as the sharpness of fracture edges or the degree to which injuries had healed, it is sometimes possible to establish the most likely cause of a trauma— for example, whether the injury probably arose as a consequence of a hunting accident, interpersonal violence or inter-group conflict. Moreover, surviving severe trauma might indicate that the injured person was cared for by members of their society. Establishing the likelihood of each of these scenarios among Neanderthals and early modern humans will no doubt continue to challenge scientists for many years to come.

(Source: “The not-so-dangerous lives of Neanderthals”, by Marta Mirazon Lahr)

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Neanderthals are commonly depicted as leading dangerous lives and permanently struggling for survival. This view largely relies on the high incidences of trauma that have been reported and have variously been attributed to violent social behaviour highly mobile hunter-gatherer lifestyles or attacks by carnivores5. The described Neanderthal pattern of predominantly cranial injuries is further thought to reflect violent encounters with large prey mammals, resulting from the use of close-range hunting weapons. These interpretations directly shape our understanding of Neanderthal lifestyles, health and hunting abilities, yet mainly rest on descriptive, case-based evidence. Quantitative, population-level studies of traumatic injuries are rare. Here we reassess the hypothesis of higher cranial trauma prevalence among Neanderthals using a population-level approach-accounting for preservation bias and other contextual data-and an exhaustive fossil database. We show that Neanderthals and early Upper Palaeolithic anatomically modern humans exhibit similar overall incidences of cranial trauma, which are higher for males in both taxa, consistent with patterns shown by later populations of modern humans. Beyond these similarities, we observed species-specific, age-related variation in trauma prevalence, suggesting that there were differences in the timing of injuries during life or that there was a differential mortality risk of trauma survivors in the two groups. Finally, our results highlight the importance of preservation bias in studies of trauma prevalence.

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Neanderthals are commonly depicted as robust hominins who led stressful, dangerous lives. Traumatic injuries, considered to be common among remains of adult Neanderthals, are a major piece of evidence supporting this hypothesis: not only are Neanderthals proposed to suffer from a high prevalence of trauma, but they are also thought to exhibit more traumatic injuries than early modern humans. Explanations for this include violent social behaviour, a highly mobile hunter-gatherer in glacial environments and attacks by carnivores. Moreover, Neanderthals are thought to show unusually high levels of head and neck injuries, attributed to their hypothesized reliance on close-range hunting leading to confrontations with large prey mammals. These interpretations have important implications for reconstructions of Neanderthal palaeobiology and behaviour, and have shaped the prevailing perception of the species. However, they are largely based on anecdotal evidence, because trauma among Palaeolithic humans is often reported on a descriptive, case-by-case basis. The few systematic, quantitative studies that have been conducted to date have yielded contradictory results, but question the prevailing view of ‘the highly traumatized Neanderthal’.

Current research into Palaeolithic trauma suffer from several limitations. Most previous work assessed the proportional distribution of lesions throughout the body in injured Neanderthal skeletons, comparing the derived ratios to those of recent humans. Such approaches provide insights into individual life histories, but -because they focus exclusively on the injured- cannot elucidate population-level trauma prevalence. The latter requires an examination of both injured and uninjured individuals. Furthermore, contextual factors such as age at death, sex and skeletal preservation are rarely accounted for in these approaches. These variables can markedly affect trauma prevalence variation and lesion visibility in the fossil record, and should thus be integral to population-level analyses. Moreover, Neanderthals are routinely compared to recent humans -clinical or forensic samples, rodeo riders and Holocene hunter-gatherers or nomads- but only rarely to Upper Palaeolithic modern humans. However, the latter are the most appropriate comparative sample, sharing similar environments and comparable mobile hunter-gatherer lifestyles. Finally, small sample sizes have hampered the validity of the statistical inferences of most previous research.

Here we assess the hypothesis of higher prevalence of cranial trauma among Neanderthals relative to Upper Palaeolithic modern humans using a population-level comparison, including contextual data and using the largest fossil dataset that is currently available. We systematically compiled published information on fossil crania from the Middle and Upper Palaeolithic of Eurasia, dating to roughly 80-20 thousand years ago. Cranial injuries -considered typical for Neanderthals- are a particularly reliable trauma archive, because they heal with only a minor bone remodelling and therefore leave visible lesions even after full recovery.

For each specimen, we recorded whether trauma was present, the taxon, sex, age at death, preserved skeletal element(s), the preservation percentage of each skeletal element and location. We then used generalized linear mixed models (GLMMs) to assess differences in trauma prevalence with taxon, sex, age and preservation as explanatory variables, while accounting for variation among skeletal elements and locations.

Our systematic literature survey revealed 21 specimens with one or several cranial lesions (9 Neanderthals and 12 Upper Palaeolithic modern humans) in our sample of 114 specimens of Neanderthals and 90 specimens of Upper Palaeolithic modern humans. At the level of skeletal elements, this corresponds to 14 out of 295 cranial elements of Neanderthals, and 25 out 541 cranial elements of Upper Palaeolithic modern humans, exhibiting at least one traumatic lesion.

We calculated separate models to predict trauma prevalence at the specimen and the skeletal-element level. Our analysis comprised two sets of four GLMM models each that were based on hierarchically nested subsets of the raw data. The first set followed an element-based approach, with skeletal elements being the unit of analysis; the second set was based on individuals. Trauma was modelled as a binary response variable in all models, either per skeletal element or per specimen.

None of the models showed a quantitative difference in cranial trauma prevalence between Neanderthals and Upper Palaeolithic modern humans. Instead, we found a significantly higher prevalence of trauma in males compared to females. Furthermore, trauma prevalence significantly increased with preservation status, indicating a greater probability to detect trauma on more complete skeletal elements or individuals. Finally, in the element-based models, trauma prevalence varied between age classes with distinct patterns for the two taxa: Neanderthals had a significantly higher prevalence  of trauma when young, whereas Upper Palaeolithic modern humans showed a similar prevalence of trauma across age cohorts. Although a similar pattern appeared to be present in the specimen-level models, the interaction failed to reach statistical significance.

On the basis of our results, we reject the hypothesis that Neanderthals exhibit more cranial trauma than Upper Palaeolithic modern humans in western Eurasia -rather, we show that the two taxa exhibited a similar overall prevalence of cranial injuries. Trauma prevalence derived from skeletal remains must not be equated to the actual numbers of injuries that were experienced during an individual’s lifetime and comparisons of crude trauma frequencies should be considered with caution, because the methods used for their estimation are not always comparable among studies.

Trauma prevalence was further affected by the preservation state of skeletal remains; more complete crania or cranial elements were more likely to have preserved traumatic lesions. We therefore caution against quantitative trauma analyses that do not address preservation bias.

Both taxa showed mostly healed traumata and we did not find a markedly higher prevalence of trauma among ‘old’ skeletal elements in either group. This finding contradicts the expectation that healed traumatic injuries accumulate with increasing age as a result of longer exposure to dangerous situations, given that cranial defects remain visible over long-term periods owing to the limited regenerative bridging capacity of cranial bone healing -even if they survived their injuries- had an increased risk of dying relative to individuals who were never injured. Thus, our observed age pattern across taxa is consistent with the well-documented increased mortality risk of trauma survivors.

Our study addresses the controversial topic of trauma prevalence in the Palaeolithic by reassessing cranial trauma data using a state-of-the-art methodological approach. It is, to our knowledge, the largest population-level investigation of Neanderthal cranial trauma to date and accounts for differential skeletal preservation and contextual explanatory variables using Upper Palaeolithic modern humans as a comparative sample. The available evidence indicates similar overall trauma prevalence in Neanderthals and Upper Palaeolithic modern humans in western Eurasia, rejecting earlier hypotheses of highly traumatized Neanderthals. Beyond this overall similarity, our observed age-dependent differences between the taxa also suggest possible differences in the likely age of trauma acquisition or in the mortality risk of trauma survivors.

(Source: “Similar cranial trauma prevalence among Neanderthals and Upper Paleolithic modern humans”, by Judith Beier et al.)

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