Mounting comparative evidence focusing on the evolution of speech suggests that it may have evolved from the rhythmic facial expressions of ancestral primates, such as the lip-smacking gesture in rhesus macaques, geladas, and chimpanzees. Clearly, precise and highly regulated movement of the lips is an integral part of human speech. Further, the lips themselves act as articulators in human speech as part of the supralaryngeal portion of the vocal tract. Disruption of this correlated acoustic and visual behavior alters production of visemes and reduces speech intelligibility, a phenomenon described by the McGurk Effect. Movements of the lips work together in a highly correlated mechanism with the sounds produced during speech to generate “visemes”, visual phonemes that aid in the perception of speech.
#Shaved chimpanzee muscles plus
Speech is distinguished from other types of primate vocalizations in its combination of bimodally rhythmic, acoustic output plus visual output, – which depends upon contraction of facial muscles to move the lips. As primates, humans also use facial displays/expressions as part of our visual signal repertoire but human social communication is achieved primarily through a coarticulation of visual (facial displays/expressions) and auditory pathways with the unique evolutionary innovation of speech –. In addition many primate species use auditory pathways as a means of communication. Part of the visual signal repertoire includes facial expressions of emotion and many primate species have well-documented, complex repertoires of facial displays/facial expressions –. Most primates use visual signals as part of their social communication repertoire. This variation among primate species seems to be largely dependent upon factors such as group size, time of day activity, and environment. Primates have a phylogenetically conserved arrangement of facial muscles but there is variation within the order in the complexity of expressions and displays produced. In mammals, this musculature takes on an additional function as a means to produce visual communication signals, deforming the facial mask into expressions of emotion.
#Shaved chimpanzee muscles skin
Among skeletal musculature, facial muscles are unique because they attach into the skin of the face and scalp, controlling size and shape of the openings for the eyes, external nares, and mouth. Movement of the lips is controlled by the facial musculature (or facial expression musculature), present in all vertebrates. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Ĭompeting interests: The authors have declared that no competing interests exist. All relevant data are within the paper and its Supporting Information files.įunding: This study was supported by contract grant number MH082282 to lap ( funding from the Samuel & Emma Winters Foundation to AMB (no website). This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.ĭata Availability: The authors confirm that all data underlying the findings are fully available without restriction. Received: JAccepted: SeptemPublished: October 22, 2014Ĭopyright: © 2014 Burrows et al. PLoS ONE 9(10):Įditor: Bernhard Fink, University of Goettingen, Germany Are there similar unique, evolutionary physiologic biases found in human facial musculature related to the evolution of speech?Ĭitation: Burrows AM, Parr LA, Durham EL, Matthews LC, Smith TD (2014) Human Faces Are Slower than Chimpanzee Faces. Recently, it was found that human tongue musculature contains a higher proportion of slow-twitch myosin fibers than in rhesus macaques, which is related to the slower, more controlled movements of the human tongue in the production of speech.
This movement necessitates a more controlled, sustained muscle contraction than that produced during spontaneous facial expressions which occur rapidly and last only a short period of time. Visemes depend upon facial muscles to regulate shape of the lips, which themselves act as speech articulators. The evolution of speech required the development of a coordinated action between visual (movement of the lips) and auditory signals in a rhythmic fashion to produce “visemes” (visual movements of the lips that correspond to specific sounds). While humans (like other primates) communicate with facial expressions, the evolution of speech added a new function to the facial muscles (facial expression muscles).
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