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Shoulder functional anatomy and development - implications for interpreting early hominin locomotion Open Access

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Recently, the discoverers of the primitive, yet complete Australopithecus afarensis infant scapula, DIK-1-1, called for further investigation into the ontogeny and functional morphology of the shoulder. Previously, there has been much disagreement about the functional relevance of primitive, apelike traits in hominin fossils and the goal of this thesis is to integrate experimental, ontogenetic, and comparative approaches to interpret and reconstruct early hominin locomotor activities. First, the effects of genetically-enhanced musculature and different locomotor regimes (vertical climbing vs. wheel-running) on shoulder morphology were examined in mice. These considerations provided the context for a large ontogenetic primate comparative study, which evaluated whether or not features indicative of suspensory locomotion were correlated with behavioral changes throughout development. Finally, DIK-1-1, the early Homo erectus specimen, KNM-WT 15000, and other contemporaneous hominin shoulder fossils were included to test and develop hypotheses about the functional significance of primitive features in light of the aforementioned experimental and ontogenetic results. The mouse model considerations showed that both hypermuscularity and exercise had a significant effect on shoulder morphology. The dorsal blade, particularly relative supraspinous and infraspinous fossa shape, differed among the experimental conditions. Building on these results, the primate comparative study also found that traits related to scapula spine orientation and infraspinous fossa shape distinguished suspensory from non-suspensory taxa. Furthermore, the developmental trajectory of these traits was unique in Pan and Gorilla, matching previously identified ontogenetic shifts in locomotor behavior. Finally, differences between the dorsal scapula blades of DIK-1-1 and KNM-WT 15000 were marked, with the Dikika infant most closely resembling great apes and KNM-WT 15000 being decidedly more modern human-like. Several researchers have outlined that shoulder morphology can be informative of locomotor habits. The results of this thesis suggest that, in particular, characteristics of the dorsal scapula blade successfully sorted primate locomotor groups and also changed in line with shifts in locomotor behavior throughout ontogeny. Further, direct study of hypermuscularity and locomotor differences in mice supports the idea that external forces can significantly influence shoulder morphology. In this context, the primitive aspects of the australopith shoulder are hypothesized to be an accurate representation of their locomotor attributes.

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