Current Molecular Biology Reports https://doi.org/10.1007/s40610-020-00128-x EVOLUTIONARY DEVELOPMENTAL BIOLOGY (R DIOGO AND E BOYLE, SECTION EDITORS) Muscles Lost in Our Adult Primate Ancestors Still Imprint in Us: on Muscle Evolution, Development, Variations, and Pathologies Eve K. Boyle1 & Vondel Mahon2 & Rui Diogo1 # Springer Nature Switzerland AG 2020 Abstract 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. Keywords Primate evolution . Muscles . Soft tissues . Variations . Polymorphisms . Anomalies Introduction with congenital malformations has been a significant focus of the medical and anatomical literature since the nineteenth cen- The description of muscular variations in adults within the tury [1–9]. Attempts to better understand the human muscu- normal human population and muscular anomalies in humans loskeletal system as it compares with those of other mammals, particularly our closest living relatives (the apes), have a sim- This article is part of the Topical Collection on Evolutionary ilarly long history [10–15]. Recent technological advances in Developmental Biology studying human development, such as whole-mount immuno- staining, have made it possible to better understand the onto- * Rui Diogo genetic origins of variable and anomalous muscles in humans [email protected] and compare them with what we know about musculoskeletal – 1 Department of Anatomy, Howard University College of Medicine, development in other species [16 19]. The study of evolution- Washington, DC 20059, USA ary developmental pathologies (Evo-Devo-Path) is an emer- 2 R Adams Cowley Shock Trauma Center, University of Maryland gent field that formally integrates these approaches to study Medical Center, Baltimore, MD 21201, USA the links between anatomical variations, anomalies, Curr Mol Bio Rep development, and evolution, focusing in particular on human population, and close resemblance to a character possessed by anatomy and the anatomical consequences of birth defects all members of an ancestral population. However, authors within an evo-devo framework [20–30]. such as Diogo et al. [19] would not strictly apply some of Evo-Devo-Path provides a particularly useful lens for eval- the criteria of Hall [35] to designate structures that are usually uating previously suggested hypotheses for how and why at- present in human embryos such as a tail or the muscles avistic muscles—ancestral muscles that reappear in a species dorsometacarpales of the hand, as atavisms of embryos, even after evolutionary loss in previous generations—persist in if they will disappear before birth and are present in all em- both adults within the normal human population and humans bryos. This is precisely because they want to avoid adult- with congenital anomalies [20, 31]. Haeckel, an early theorist centric views of evolution. That is, within this view, the tail in comparative embryology, proposed that ontogeny recapit- present in humans, be it in all human embryos or in adults with ulated phylogeny, i.e., that the ontogeny of embryo recapitu- congenital malformations, is always seen as atavistic because lates the adult stages of its ancestors [20, 32]. Haeckel’sreca- the crucial criterion is that it was lost in the evolution of our pitulation is no longer considered valid, in part due to the work adult ancestors. Another main complication concerns the fact of von Baer, who argued that the ontogeny of an embryo that some of the muscles might eventually be present in more mainly goes from a more general, e.g., tetrapod, embryonic than 2% of the normal population, that is, they should be configuration to a more specific, e.g., human, configuration, designated as polymorphic traits and not as variations because that is, some of the similarities between for instance humans per definition, variations are present in less than 2% of the and salamanders or mice mainly refer to the embryonic, not population [20]. So, an important distinction needs to be the adult stage [20, 33]. made. If the muscle was present in our direct adult ancestors Recent studies have confirmed, using more modern visual- and is now present as an adult polymorphism in our species, as ization techniques, that in early developmental stages, some of it is the case with the palmaris longus (present only in about the muscles of human embryos do resemble those of the em- 85%) of humans, this constitutes an adult non-atavistic poly- bryos of other animals, particularly our closest living relatives, morphism. But if the muscle was lost at some point in the the nonhuman primates [19]. In particular, researchers have evolution of our adult ancestors and is now present as an adult demonstrated that several muscles that were lost in our adult polymorphism in our species, then in phylogenetic terms, it ancestors—some of them more than 250 million years ago— would be considered an evolutionary reversion [36]. do usually develop in early human ontogenetic stages only to To provide a stronger foundation for discussing these key be reabsorbed or fused with other structures in later stages, issues, we rely on primate comparative anatomy and develop- before birth, in most humans [19, 20, 34]. That is, these ob- mental research in humans to undertake a comprehensive re- servations indicate that the developmental pathways related to view of fourteen atavistic muscles/groups of muscles (using the formation of such complex structures persist over the evo- Diogo et al.’s[20] less strict definition of atavism) that are lution of various taxa even when they are absent in the adult present in adult humans either as variations—or polymor- normal phenotype of those taxa [20, 35, 36]. phisms,ifprevalenceifhigherthan2%—or as pathological While the persistence of developmental pathways is the anomalies. For each muscle, we describe their presentation as ultimate cause for why muscles that were phylogenetically a variation/potential polymorphism and/or anomaly, their pre- lost in our adult ancestors’ evolution still “imprint” us, the sentation in nonhuman primates, data on prevalence in proximate causes of how these muscles persist are not clear. humans, and what we know, if anything, about their develop- The persistence of certain atavistic muscles in some adult ment in early human ontogenic stages. By doing this, our goal humans could be due to developmental delay/arrested devel- is to not only provide a review for evolutionary developmental opment, by which lingering muscle slips are present in later biologists but also include information relevant for compara- ontogenetic stages due to the delay of muscle reabsorptions/ tive anatomists, functional morphologists, and biological an- fusions that should normally occur in earlier stages [21, 37]. thropologists, as well as to clinicians, as atavistic muscles are Specifically, it has been suggested that decreased apoptosis in often misdiagnosed in medical studies, can compress or dis- skeletal muscles could be the mechanism leading to the pres- place adjacent structures, and can be related to exercise- ence of several supernumerary muscles