A study in the latest issue of Biology Letters shows that human facial expressions are very similar to those of other primates, and are produced in similar ways.
"Some of the basic human expressions have clear counterparts in other species, such as the human smile -- equivalent to the bared-teeth display in other primates -- and laughter—equivalent to the primate play face, used during play," lead author Bridget Waller of the University of Portsmouth's Center for Comparative and Evolutionary Psychology, told Discovery News.
While smiles show teeth that could potentially bite onlookers, the relaxed nature of the rest of the face reveals that the tooth showing is not tied to aggression.
"Based on our facial muscle analyses so far, chimpanzees seem to show the most similarity to humans," Waller said.
Here, a wide-eyed chimp looks relaxed, yet alert, as he surveys his territory.
When having one's picture taken, the subject usually prepares to look alert, instead of having eyes downcast or otherwise looking away. This was especially true for older photography sessions, which required more time for the sitter that, in this case, was David Davis, a former Supreme Court Justice.
There is little doubt that these chimps are curious about what the middle one is holding. Waller thinks that many expressions are produced subconsciously. The observers' responses may also happen at a subconscious level.
An important difference between human and other primate facial expressions is seen in the human look of determination. These women appear to be both curious and determined to investigate something online.
In children, a look of determination incorporates components of anger, Waller said. "Primates have a similar expression used during bluff displays, but this doesn't seem to be produced when they are engaging with a difficult task."
She and her colleagues believe humans could use the look to solicit help from others.
The wide-open eyes and mouth -- with teeth flashing -- of this marmoset suggest that he is feeling dominant. When an individual is submissive, he or she will be greater inclined to protect the face by closing the eyes and mouth more.
Actor Jim Carrey, shown in wax at Madame Tussauds museum, often exaggerates his facial expressions for comedic effect. He is definitely not conveying shyness, but rather dominance.
Sometimes facial expressions are paired with entire body displays. This siamang appears to be informing others who is king of the jungle. While our interpretations of non-primate expressions are inherently human-centric, it is clear that the photographed siamang -- at least in this moment -- is not afraid to make his presence known.
Wrestler Bobby Fish flaunts his rippling muscles in this victorious pose. Other research has found that such spontaneous expressions of triumph and pride, commonly seen at the Olympics and other sporting events, have their roots in non-human primate displays.
"We believe that the triumph expression signals victory and achievement, which in turn signals dominance and aids in establishing status in a hierarchy," David Matsumoto, who conducted the research, told Discovery News. "This enables social coordination and enhances reproductive success."