The Mobility of the Human Face: More than Just the Musculature.
Level 5 - mechanism / opinion, no new human data
Level 5 by design analogy (comparative post-mortem anatomical/histological bench study)
PubMed 27870342 · doi:10.1002/ar.23451
What was done
Histological analysis of full-thickness tissue samples taken near the zygoma region across four anthropoid species (Homo sapiens, n=3; Pan troglodytes, n=3; Hylobates muelleri, n=1; Macaca mulatta, n=3) and two prosimian species (Tarsius bancanus, n=1; Otolemur crassicaudatus, n=2). Paraffin-embedded sections were evaluated via light microscopy to determine the presence or absence of the superficial musculo-aponeurotic system (SMAS) connective tissue layer.
What was found
The SMAS layer was detected in all examined anthropoid specimens (humans, chimpanzees, gibbons, and macaques) but was absent in both prosimian species (tarsiers and galagos). The abstract reports no numerical or quantitative morphometric data.
Why it matters
The findings suggest that connective tissue structures like the SMAS, rather than differences in mimetic musculature alone, may be an anatomical factor enabling the greater facial mobility and complex display repertoires seen in anthropoids compared to prosimians.
Limits
The study is limited by very small sample sizes (1 to 3 individuals per species, total N=13), sampling from only one facial region (near the zygoma), and purely qualitative histological assessment without direct biomechanical or functional mobility measurements.
Cited by
- contradicts Great apes have facial musculature and control similar to humans, whereas monkeys and dogs lack the musculature to produce those types of facial expressions.