Patterns of interspecific variation in labial microarchitecture among anthropoid primates and the evolution of the hominin lips

The lips play a central role in both facial expression and speech, yet their microarchitecture remains largely unexamined across primates. This study presents the first systematic, histological, and morphometric investigation of labial structure in 15 primate species, including humans, non-human great apes, Cercopithecidae, Cebidae, and Lemuridae. We analyzed sagittal lip sections stained with Masson's trichrome, applying deep learning-based segmentation and manual annotations to quantify the relative proportions of muscle, adipose, and connective tissue. We also assessed the morphology of the orbicularis oris muscle (OOm) and the anatomical configuration of direct labial tractors (DLTs). Our results reveal that humans possess a uniquely curved, hook-shaped OOm, a dermis-inserting DLT system segregated from the OOm, and a distinctive tissue profile marked by high adipose and low connective tissue content. These features differ markedly from other primates, which typically exhibit a planar OOm, overlapping DLT architecture, and connective tissue-rich labial composition. Interestingly, Lemuridae share aspects of the human soft tissue profile but lack the muscular specialization. We propose that these derived human traits-especially the curved marginal OOm and anatomically decoupled DLTs-enhance the lips' capacity for fine motor control. These features may have supported the evolution of speech-related articulatory precision. While not providing direct functional evidence, our findings offer a new anatomical framework for understanding the evolution of facial expressivity and speech. This study underscores the value of soft tissue microanatomy in reconstructing the evolutionary pathways of hominin communication.