title: Human faces are slower than chimpanzee faces.
abstract: BACKGROUND
  While humans (like other primates) communicate with facial expressions, the evolution of speech added a new function to the facial muscles (facial expression muscles). 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). Visemes depend upon facial muscles to regulate shape of the lips, which themselves act as speech articulators. 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. 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. Are there similar unique, evolutionary physiologic biases found in human facial musculature related to the evolution of speech?
  
  
  RESULTS
  Using myosin immunohistochemistry, we tested the hypothesis that human facial musculature has a higher percentage of slow-twitch myosin fibers relative to chimpanzees (Pan troglodytes) and rhesus macaques (Macaca mulatta). We sampled the orbicularis oris and zygomaticus major muscles from three cadavers of each species and compared proportions of fiber-types. Results confirmed our hypothesis: humans had the highest proportion of slow-twitch myosin fibers while chimpanzees had the highest proportion of fast-twitch fibers.
  
  
  CONCLUSIONS
  These findings demonstrate that the human face is slower than that of rhesus macaques and our closest living relative, the chimpanzee. They also support the assertion that human facial musculature and speech co-evolved. Further, these results suggest a unique set of evolutionary selective pressures on human facial musculature to slow down while the function of this muscle group diverged from that of other primates.
authors.affiliation: Department of Physical Therapy, Duquesne University, Pittsburgh, Pennsylvania, United States of America; Department of Anthropology, University of Pittsburgh, Pittsburgh, Pennsylvania, United States of America.
authors.forename: Anne M
authors.initials: AM
authors.lastname: Burrows
authors.affiliation: Department of Psychiatry and Behavioral Science, Center for Translational Neuroscience, Yerkes National Primate Research Center, Emory University, Atlanta, Georgia, United States of America.
authors.forename: Lisa A
authors.initials: LA
authors.lastname: Parr
authors.affiliation: Department of Physical Therapy, Duquesne University, Pittsburgh, Pennsylvania, United States of America.
authors.forename: Emily L
authors.initials: EL
authors.lastname: Durham
authors.affiliation: Department of Health Management Systems, Duquesne University, Pittsburgh, Pennsylvania, United States of America.
authors.forename: Lea C
authors.initials: LC
authors.lastname: Matthews
authors.affiliation: Department of Anthropology, University of Pittsburgh, Pittsburgh, Pennsylvania, United States of America; School of Physical Therapy, Slippery Rock University, Slippery Rock, Pennsylvania, United States of America.
authors.forename: Timothy D
authors.initials: TD
authors.lastname: Smith
mesh_terms: D000819:Animal Communication; D000818:Animals; D005145:Face / Q000502:physiology*; D005149:Facial Expression; D005152:Facial Muscles / Q000166:cytology / Q000502:physiology*; D006801:Humans; D009119:Muscle Contraction; D018656:Muscle Fibers, Fast-Twitch / Q000502:physiology; D018657:Muscle Fibers, Slow-Twitch / Q000502:physiology; D009633:Nonverbal Communication; D002679:Pan troglodytes; D013045:Species Specificity; D014059:Tongue / Q000166:cytology / Q000502:physiology