There has been research conducted at Kyoto University about 10 years ago that found that chimpanzee infants display spontaneous smiles. When humans and chimps sleep they oftentimes have facial expressions that resemble smiles. Kyoto University believes this was the evolutionary origin of real smiles. They have observed macaque infants and compared their spontaneous smiles to those of the humans. They both happened during irregular REM sleep and they showed lop-sided smiles compared to symmetrical, complete smiles. The research team has interpreted that these spontaneous smiles lead to the development of the cheek muscles that enable humans, chimpanzees, and monkeys to produce laughs and smiles. Research is still in the works and now they are questioning if these smiles are special to monkeys and primates.
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