Clavicular evidence for continued arboreality in Australopithecus afarensis

The emergence of the hominin lineage is marked by the transition to bipedalism, but debates persist regarding the role of arboreal behaviors during and after this shift. Uncertainties surrounding the locomotor habits of Australopithecus partly stem from the mosaic nature of their skeleton, with pelvic and lower limb traits principally indicating bipedalism, while upper limb morphology retains primitive features largely associated with arboreality in extant apes. Analyses of internal bone structure coupled with new fossil evidence may provide new insights in this regard. Here, we first describe the clavicles of the juvenile Australopithecus afarensis individual DIK-1-1, then the morphology is further investigated alongside adult Au. afarensis, as represented by KSD-VP-1/1, using geometric morphometrics and cortical cross-sectional geometry to identify and interpret potential functional signals. Our findings challenge the notion of a distinct modern human clavicular morphology as separate from chimpanzees, revealing significant overlap in external shape between Homo and Pan. Conversely, internal cortical geometry exhibits developmental plasticity in both extant apes and Au. afarensis, supporting its utility in exploring locomotor adaptations. Furthermore, similarities in internal cortical geometry between Au. afarensis and modern apes support hypotheses of continued arboreal behavior-including suspension and climbing-throughout life in the former. The discordance between external and internal morphology highlights the potential to identify unrecognized functional signals in hominin clavicles previously categorized as 'primitive' and cautions against oversimplified taxonomic assignments based solely on external shape. Ultimately, these results emphasize the importance of taking a comprehensive view of morphology to better understand locomotor evolution in early hominins and underscore the relevance of continued arboreality in Au. afarensis.