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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

FAST-TWITCH MYOSIN. “Type II fibers consist of a number of isoforms (different sub-types) but overall they are able to contract more quickly than type I fibers and use less energy. As a trade-off, they are quick to become fatigued and cannot hold the contraction as long as type I fibers.” http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full View in LinkedIn

SLOW-TWITCH MYOSIN. “There are several types of myosin proteins but the most abundant and best understood for mammalian skeletal muscle physiology are type I (slow-twitch) and type II (fast-twitch) myosin. Type I fibers take more time and more energy to contract. As a trade-off, they are slow to fatigue and hold the contraction longer. In humans, these types of fibers tend to dominate in muscles of the limbs (except for the hand) and spine.“ http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full View in LinkedIn

Galena close up. View in LinkedIn

SO ENDS this section on the evolution of the human face, driven by the type of musculature and the social pressures for communication. The superficial beauty of the human mask captivates us all, but the effects are just skin deep. Below is a structure welded by forces over long periods of time, designed for a social function. We are easily misled, as was the intention, if ever there was one. View in LinkedIn

SOCIAL FACIAL MASK. “Overall, these qualitative case studies add to the growing body of evidence that primate mimetic musculature form and evolution are adaptive to social, communicative pressures. We know that mimetic musculature in extant species is adaptive to social variables (such as group size and dominance ‘style’), but future studies may be able to extrapolate our current knowledge to taxa represented only in the fossil record.” http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full View in LinkedIn