Anatomical Evidence for a Uniquely Human Depressor Anguli Oris and a Novel Helplessness Signaling Hypothesis.

The downward pulling of the mouth corner in humans is attributed to the depressor anguli oris (DAO), yet the evolutionary history of this muscle remains unresolved. Classical comparative accounts have often assumed that nonhuman primates (NHPs) possess a homologous lower component within the caninus-triangularis complex (CTC), but this proposition has not been systematically tested. Here, we examined the architecture of the CTC across 10 primate species using a multimodal anatomical approach combining gross dissection, morphometric analysis, histology, and fiber-level quantification. We analyzed 30 NHP specimens and 61 human specimens to determine whether NHPs possess a discrete mandibular muscle homologous to the human DAO. In all human specimens, the CTC consisted of two distinct bellies, the levator anguli oris and DAO, whereas in NHPs, including great apes, the CTC appeared as a continuous sheet lacking a discrete mandibular DAO homolog. Morphometric and fiber-level analyses supported this distinction, revealing a characteristic human pattern around the labial commissure consistent with bipartite organization. Although the present study does not test behavioral function directly, it motivates the hypothesis that this derived muscular architecture may have been recruited into an expressive facial signal associated with helplessness during infancy in the context of cooperative breeding.