Psychologists from the University of Portsmouth have published a paper suggesting gorillas use human-like facial expressions to communicate moods with one another. Not only that, but two of the expressions, both of which resemble grinning, could show the origins of the human smile.
One, a 'play face,' featuring an open mouth and showing no teeth, denotes a playful mood, usually accompanied with physical contact. Another, which is open-mouthed and displaying top teeth, could be a submissive smile - as it mixes the play face and a bared-teeth expression, which indicates appeasement.The Presurfer