The facial expression musculature in primates and its evolutionary significance.

Facial expression is a mode of close-proximity non-vocal communication used by primates and is produced by mimetic/facial musculature. Arguably, primates make the most-intricate facial displays and have some of the most-complex facial musculature of all mammals. Most of the earlier ideas of primate mimetic musculature, involving its function in facial displays and its evolution, were essentially linear "scala natural" models of increasing complexity. More-recent work has challenged these ideas, suggesting that ecological factors and social systems have played a much larger role in explaining the diversity of structures than previously believed. The present review synthesizes the evidence from gross muscular, microanatomical, behavioral and neurobiological studies in order to provide a preliminary analysis of the factors responsible for the evolution of primate facial musculature with comparisons to general mammals. In addition, the unique structure, function and evolution of human mimetic musculature are discussed, along with the potential influential roles of human speech and eye gaze.