Rethinking Hominin Air Sac Loss in Light of Phylogenetically Meaningful Evidence

Area of Science:

• Paleoanthropology
• Evolutionary Biology
• Bioacoustics

Background:

• The presence of laryngeal air sacs in hominins is debated, with some inferring their presence in Australopithecus afarensis and absence in later hominins.
• Existing interpretations often rely on limited fossil evidence and assumptions about hominoid vocal anatomy.

Purpose of the Study:

• To challenge prevailing assumptions regarding the evolution of laryngeal air sacs in hominins.
• To re-evaluate the evidence for air sac presence/absence in early hominins, particularly Australopithecus afarensis.
• To advocate for a data-driven approach to understanding hominin vocal anatomy evolution.

Main Methods:

• Comparative analysis of hyoid morphology in extant African apes and fossil hominins.
• Critical review of existing literature and fossil evidence concerning hominin laryngeal air sacs.
• Examination of sexual dimorphism patterns in early hominins to assess hypotheses on vocal evolution.

Main Results:

• Hyoid-air sac morphology relationships in apes may be due to convergent evolution, not direct homology.
• The "ape-like" hyoid bulla in Australopithecus afarensis may persist after air sac loss, not indicate their presence.
• The fossil record of hominin hyoid bones is extremely sparse, limiting definitive conclusions.
• Sexual selection hypotheses for air sac loss are challenged by atypical early hominin sexual dimorphism.

Conclusions:

• Current evidence for laryngeal air sac presence or absence in early hominins is minimal and requires cautious interpretation.
• Inferences about the role of sexual selection and communication in air sac loss are not well-supported by available data.
• A rigorous, evidence-based approach is crucial for reconstructing the evolution of hominin vocal anatomy.