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STRUCTURAL UNDERPINNINGS. “Sherwood et al. (2005) found relatively greater volume of facial nerve nuclei in the great apes and humans than in all other Old World primates, suggesting increased differentiation of the facial muscles and greater utilization of the visual channel in social communication.” http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full View in LinkedIn

SOCIAL NOT PHYLOGENETIC. “Neurobiological evidence also indicates that there are considerable socioecological variables involved in the evolution of facial displays among primate species. species that live in large, complex social groups had more facial nerve neurons than species that live in small social groups, indicating more potential control over mimetic musculature.” http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full View in LinkedIn

NEW THINKING. “This ‘phylogenetic model’ of morphology has recently been challenged. Work in wide phylogenetic, ecological, and social environment ranges of primates (and some non-primate mammals) has shown that social environment variables play a considerable role in the adaptive morphology of mimetic musculature.” http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full View in LinkedIn

TRADITIONAL MODEL. “Under this ‘phylogenetic’ model, complexity of mimetic muscle morphology increased in a simple linear, step-wise fashion up the phylogenetic scale until humans, where the ultimate in complexity was achieved. This view has traditionally also been applied to facial display repertoire, with the most simple, undifferentiated displays being rooted in the prosimians, ever increasing in a step-wise, linear fashion up to humans, where the most complex, subtle, and graded displays are found.” http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full View in LinkedIn

THE ARTICULATE LIP. “Some mimetic musculature in humans is also used during speech. Human lips act in part as ‘articulators’ during speech, refining the sounds that come from the larynx into specific, meaningful speech units.” http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full View in LinkedIn

THE INDEFATIGABLE TONGUE. “A recent study by Sanders et al. (2013) showed that human tongue musculature has a greater percentage of slow-twitch fibers than tongue musculature from chimpanzees. Authors of that study correlated this evolutionary innovation in muscle physiology of the human tongue with the ability of the human tongue to slow down and produce more specific and longer contractions during speech, relative to how the tongue behaves in chimpanzees during vocalizations.” http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full View in LinkedIn

DEEP CLUES. “These differences in the ratio of slow-twitch to fast-twitch myosin fibers can inform our understanding of muscle function and preceding evolutionary pressures.” http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full View in LinkedIn

FATIGUE FACTOR. “It is well established that human mimetic musculature is dominated by fast-twitch myosin fibers. Our facial muscles are able to contract quickly and spontaneously (think of how quickly and automatically we smile at the sight of a familiar friend or a funny joke) but it is difficult to hold that contraction longer than a few seconds before fatigue sets in.” http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full View in LinkedIn

THE EPHEMERAL SMILE. “The mimetic muscles that control smiling, dominated by quick-to-fatigue type II myosin fibers, typically fire that smile quickly but we tire after just a minute or so of holding that smile for family photos.” http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full View in LinkedIn

TYPE II FIBERS. “In humans, these types of fibers tend to dominate in muscles of the face and in the human hand. Furthermore, the potential instantaneous force that each fiber type can generate differs, with slow-twitch myosin fibers generating a lower instantaneous force compared with fast-twitch.” http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full View in LinkedIn