Identifying Locomotor Adaptations in the Hominoid Clavicle and the Implications for Interpreting Hominin Behavior

One of the hallmarks of the emergence of hominins is a transition to bipedalism, but debates continue on the importance of arboreal behaviors including climbing trees for food, shelter, and safety during and after this evolutionary transition. The australopithecines, a diverse group of early hominins living in Africa from 4.2 to 1.98 million years ago, exhibit a mosaic of novel lower limb adaptations tied to bipedality and largely primitive upper limb features, providing a unique opportunity to explore the evolving role of the upper limb and shoulder as hominins became obligate bipeds. To assess the integrated function of the shoulder, I explore the morphology of the clavicle in living hominoids via four approaches: a 3D geometric morphometrics analysis of shape throughout ontogeny, quantification of diaphysis cross-sectional geometry through ontogeny, an investigation of trabecular structure in adults, and identifying how morphological variation affects bone strain during locomotion using finite element modeling. The fossil hominins Australopithecus afarensis (KSD-VP-1/1; DIK-1-1), Australopithecus africanus (StW 431g; StW 573f, StW 582), Australopithecus sediba (UW 88-38), and Homo naledi (UW 101-258; UW 101-1229) were then analyzed and compared to the extant apes. Together, the results from all analyses stress the importance of continued arboreality and support extending hypotheses of locomotor diversity amongst the early, habitually bipedal hominins to include variation in arboreal behaviors. No fossil individuals included show the same combined trend in cortical geometry, trabecular structure, and external shape, but the similarities and differences amongst them most likely reflect the impact of the surrounding environmental conditions on locomotor behavior. The results presented here also provide a potential explanation for why modern humans and chimpanzee clavicles are so alike despite very different uses of the upper limb and positioning of the shoulder girdle. Analyses considering external shape (Chapters 1 and 4) highlight the evolutionary time and magnitude of imposed bone strain it takes to cause external changes in clavicle shape. Additionally, since the release of the upper limb from strictly locomotor demands likely decreased the magnitude of strain habitually faced by the clavicle, any modification in clavicular morphology reflective of this transition are identifiable in the internal structure of the bone (Chapters 2 and 3) long before any changes in external shape. In sum, this research has provided novel insight into the locomotor regime of Australopithecus, supporting arboreal behaviors as a critical component of daily life and highlighting the variation in upper limb loading even between relatively contemporaneous hominins. Most importantly, the results presented here provide evolutionary context for the similarity between modern human and chimpanzee clavicular morphology, allowing for a better interpretation of the fossil material moving forward.