The Mobility of the Human Face: More than Just the Musculature.

The human face has the greatest mobility and facial display repertoire among all primates. However, the variables that account for this are not clear. Humans and other anthropoids have remarkably similar mimetic musculature. This suggests that differences among the mimetic muscles alone may not account for the increased mobility and facial display repertoire seen in humans. Furthermore, anthropoids themselves outpace prosimians in these categories: humans > other anthropoids > prosimians. This study was undertaken to clarify the morphological underpinnings of the increased mobility and display repertoire of the human face by investigating the SMAS (the superficial musculo-aponeurotic system), a connective tissue layer enclosing the mimetic musculature located between the skin and deep fascia/periosteum. Full-thickness samples from the face near the zygoma region from the anthropoids Homo sapiens (humans, N = 3), Pan troglodytes (chimpanzees, N = 3), Hylobates muelleri (gibbons, N = 1), and Macaca mulatta (rhesus macaque, N = 3) and the prosimians Tarsius bancanus (tarsiers, N = 1), and Otolemur crassicaudatus (galagos, N = 2) were used. All samples were processed for paraffin-based histology and stained sections were viewed under light microscopy to determine if a SMAS layer could be identified. Results indicate that a SMAS layer was present in all anthropoid species but neither of the prosimian species. This connective tissue layer may be a factor in the increased facial mobility and facial display repertoire present in these species. Anat Rec, 299:1779-1788, 2016. © 2016 Wiley Periodicals, Inc.