The human skeleton is a bio-cultural entity that reacts and adapts to experiences of change, representing accumulated life histories in bone. Biological anthropologists often record these changes within individuals to interpret broad population shifts, such as changing social roles and economic systems. Musculoskeletal markers, often referred to as “entheseal changes,” have long been used by bioarcheologists to reconstruct lifeways and infer demographic indicators of archaeological populations. These distinctive biological changes refer to any transformation on skeletal surfaces where tendons and ligaments attach to bone. Many studies have reported promising results in identifying patterns of activity and lifestyle within certain cultural groups, such as social hierarchy, sexual division of labor, and occupation-specific activities, but are often sample specific and fail to capture large-scale analytical impact. Notably, very few studies on entheseal changes have included individuals whose deaths occurred post-19th century, forgoing the opportunity to observe entheseal changes in populations with evolved biosocial categories of occupational type, body-mass index, and socioeconomic status.
This project investigated several recurring themes seen in empirical research, as well as explored potential forensic applications of entheseal changes within modern populations, by examining a large sample size showcasing human variation across time and space to further understand entheseal changes and their role in the human body. The project sample included over 900 individuals representing 160 years of American history to see if factors unique to different time periods significantly affected entheseal changes. Skeletal collections and associated archival information from different geographical regions in the United States, including historic Cuyahoga County in Cleveland and San Francisco Bay Area in California, as well as modern Central and Southeast Texas and East Tennessee, were selected to diversify the research sample to better visualize human variation at entheseal sites. Observed entheseal changes of three entheseal sites in the arm, the m. subscapularis, common extensor origin, and m. biceps, were scored following the Coimbra Method published by Henderson et al. in 2016. Additionally, exemplar entheseal changes were visually captured through 2D and 3D reconstructions to build a reference library for future teaching, learning, and collaboration. Resulting entheseal scores were analyzed using a combination of unsupervised and supervised statistical methods, including approaches such as factor analysis, latent class analysis, and random forest models, to test whether there were true universal variables affecting entheseal changes of people living in the past and present to better understand underlying bodily changes regardless of time or geography.
Results indicated that entheseal changes are likely multifactorial in origin, most consistently affected by increasing age, but also influenced by the differences between biological sex, activity based on labor type, populational affinity, and body mass index. The results generally agreed with many studies already published in the literature, showcasing typically higher changes in males (except for the common extensor origin, which displayed the opposite), older individuals, and those that worked more manual labor jobs. However, as this study combined both historic and modern in complex statistical models, it provided an opportunity to explore entheseal sites within the context of secular changes involving differential workloads, social stratification, discrimination, and obesity. Noted differences between the entheseal changes in historic and modern groups reflected embodied experiences related to historic racial and social discrimination, as well as modern increases in overall obesity levels and decreases in general workload. Additionally, general patterns of decreased bone quality were observed in modern entheseal sites when compared to their historic counterparts, possibly reflecting a negative effect caused by decreasing modern activity levels.
Entheseal changes are likely not going to be absolute markers of activity or the perfect identification markers for forensic analysis, as their appearance is likely both biologically driven or culturally significant yet contextually dependent per individual and subsequent population. However, entheseal changes have been noted to have varying levels of interpretive power when paired with other osseous changes, perhaps best exemplified in “osteobiographical” research focusing on pairing an individual’s identity and social experiences with observed changes in their skeletal morphology. Additionally, due to such strong overall evidence in connecting entheseal changes to older ages regardless of time or location, a future forensic potential for entheseal changes may exist in building prediction models for age. Whether this could be successfully applied in a forensic context is still not clear, but to best accommodate the forensic profiles and inventories seen today, future exploration should utilize expanded reference samples and entheseal sites to include other entheseal sites and more diversity. Overall, the true potential of entheseal changes appears to lie in their ability to add another piece to the overall picture of an individual’s life and to reflect how their identity remains visible within their skeleton. (Publisher abstract provided.)
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