Muscles Lost in Our Adult Primate Ancestors Still Imprint in Us: on Muscle Evolution, Development, Variations, and Pathologies

The study of evolutionary developmental pathologies (Evo-Devo-Path) is an emergent field that relies on comparative anatomy to inform our understanding of the development and evolution of normal and abnormal structures in different groups of organisms, with a special focus on humans. Previous research has demonstrated that some muscles that have been lost in our ancestors well before the evolution of anatomically modern humans occasionally appear as variations in adults within the normal human population or as anomalies in individuals with congenital malformations. Here, we provide the first review of fourteen atavistic muscles/groups of muscles that are only present as variations or anomalies in modern humans but are commonly present in other primate species. Muscles within the head and neck and pectoral girdle and upper limb region include platysma cervicale, mandibulo-auricularis, rhomboideus occipitalis, levator claviculae, dorsoepitrochlearis, panniculus carnosus, epitrochleoanconeus, and contrahentes digitorum manus. Within the lower limb, they include scansorius, ischiofemoralis, contrahentes digitorum pedis, opponens hallucis, abductor metatarsi quinti, and opponens digiti minimi. For each muscle, we describe their synonyms, comparative anatomy among primates, embryonic development, presentation and prevalence as a variation, and presentation and prevalence as an anomaly. Research on the embryonic origins of six of these muscles has demonstrated that they appear early on in normal human development but usually disappear before birth. Among the eight muscles in the upper half of the body, mandibulo-auricularis is, to our knowledge, present in humans only as a variation, while the other seven muscles can be present as either a variation or an anomaly. All six muscles of the lower limb are present only as variations, and to our knowledge have not been described in anomalous individuals. Interestingly, although these muscles conform to most definitions of what constitutes an atavism—i.e., they were lost in our adult ancestors and now appear in some adult humans—some of them are seemingly present in more than 2% of the normal population. Therefore, they might actually constitute polymorphisms rather than variations. The research summarized here therefore emphasizes the need of future studies of the evolution, development, and prevalence of soft tissue variations and anomalies in humans, not only for the understanding of our evolutionary history but also of our phenotype and pathologies.