Nonverbal cues are half of what makes communication understood; the way our face gestures a certain emotion and how we present ourselves in everyday life shine through our vocalic projection to emphasize our feelings. And well, the same can be said for our primate friends.
In the American Journal of Primatology Bridget Waller, an evolutionary psychologist at the University of Portsmouth who studies the facial expression phenomenon, and Lyndsay Cherry, write, “[t]hrough facial signals, [...] individuals can potentially send and receive information and may benefit from coordinating their behavior accordingly.”
To their findings, two of the gorilla signals include the “bare-teeth” signal: mouth open and both rows of teeth visible, or the “play face” signal: where a gorilla’s mouth is open but no teeth are showing. And according to Waller the play face might be a foundation for human laughter. A third facial expression was also found that appears as half play face, half bared-teeth face; resulting in a grin “wherein only the top teeth are bared.”
In an interview with BBC, Waller noted that gorillas show their teeth to inform a playmate they will not harm them, how teeth are a “subordinate” display. She said, “[p]eople think we smile when we’re happy, but that’s not true[.] [...] You smile when it’s appropriate in a situation. You smile at someone in the corridor—you don’t laugh at them.”
Overall, the way primates exchange facial expressions can be likened to the way humans do, too – and when examined properly, can be proved to be derived from our distant ancestors.