Similarities exist between humans and other primates in terms of physical appearance, but we now know that we may be even more similar than we previously thought. New research has shown that chimpanzees and humans possess similar abilities to produce facial expressions.
An expression that a chimpanzee makes most often has a human counterpart, the bare-teeth display which is one of the most studied facial expressions in ethology.
The chimpanzee species use this expression to “increase social attraction and affiliation” and is “homologous to the human smile” (Parr and Waller 2006, Preuschoft and van Hooff 1997, van Hooff 1972). Both of these expression are characterized by “retraction of the lip corners exposing… the upper and lower teeth” as well as the similar meaning they both display (Parr and Waller 2006).
This realization can open the door to future studies to see if particular facial expressions are shared by common ancestry between humans and other primates. Researchers have recently developed a standardized system used to measure facial movement in chimpanzees that is directly comparable to humans.
FACS and ChimpFACS
This system is called ChimpFACS based on the well known human Facial Action Coding System (FACS) used to study the “biological basis for human expression and emotions” (Parr and Waller 2006). Both of these systems are unique in that they enable scientists to examine subtle facial movements and compare human facial expressions, based on similarities in musculature.
A study by Davila-Ross et al. (2015) “investigated specific types of smiles that accompany laugh sounds and found that these smile types have the same evolutionary origin as human smiles when they are laughing”. The results suggest the possible idea that the positive expressions of ancestral apes may explain these smile types in humans (Davila-Ross et al., 2015).
Human Display Rules
A trait that is unique to humans is what psychologists call display rules, social or cultural norms about how to appropriately express emotions. These display rules are expressed through microexpressions in which tiny muscle movements around your eyes and mouth react to these rules (Pfister et al., 2014).
This phenomenon is the ability to display, or in some cases, hide emotions through facial expressions. Microexpressions include, for example, dropping your jaw when surprised, wrinkling your nose when disgusted, pressing your lips together when angry or pouting your lip when sad. This unique characteristic in humans is something that should be tested in other species, at least in chimpanzees based on other similarities in some facial expressions.