Rethinking Hominin Air Sac Loss in Light of Phylogenetically Meaningful Evidence

The evolution of laryngeal air sacs in hominins has been a subject of considerable debate, with particular attention given to the inferred presence of air sacs in Australopithecus afarensis and inferred absence in Middle and Upper Pleistocene hominins. We challenge several assumptions prevalent in relevant discourse and assert that (1) while exhibiting morphological similarity, it cannot be ruled out that relationships between hyoid morphology and air sac morphology in extant African great apes may reflect convergence; (2) while the only known A. afarensis hyoid exhibits "ape-like" bulla, this feature may have persisted following the loss of air sacs, and not be indicative of their presence per se; (3) because there are currently only five known hominin hyoid bones represented in the fossil record (with a single specimen predating the Middle Pleistocene) the evidential basis for interpreting air sac presence or absence is minimal; and (4) inferences toward a role of sexual selection and communicative behavior in explicating the loss of air sacs in the hominin lineage are undermined by the atypical sexual dimorphism patterns in early hominins. We advocate for a cautious approach to interpreting hominin behavior and evolution which prioritizes data over speculation, and underscore the need for rigorous evidence when constructing evolutionary narratives about early hominin vocal anatomy and its evolution.