Smiling baby monkeys and the roots of laughter
When human and chimp infants are dozing, they sometimes show facial movements that resemble smiles. These facial expressions -- called spontaneous smiles -- are considered the evolutionary origin of real smiles and laughter.
"About a decade ago we found that chimp infants also display spontaneous smiles," says study author Masaki Tomonaga. "Since we see the same behavior in more distant relatives, we can infer that the origin of smiles goes back at least 30 million years, when old world monkeys and our direct ancestors diverged."
|Japanese macaque infants show spontaneous smiles |
[Credit: Kyoto University Primate Research Institute]
Some researchers have argued that infants' spontaneous smiles exist to make parents feel that their children are adorable and to enhance parent-child communication. On the other hand, this study suggests that spontaneous smiles don't express feelings of pleasure in chimpanzees and Japanese monkeys; rather, the smiles are more similar to submissive signals (grimaces) rather than smiles (play faces). The team interpreted that spontaneous smiles facilitate the development of cheek muscles, enabling humans, chimpanzees, and Japanese monkeys to produce smiles, laughs, and grimaces.
So is smiling special to monkeys and primates? Tomonaga says he won't rule out the possibility.
"There are case reports about mice laughing when they get tickled and dogs displaying facial expressions of pleasure. It may be the case that many mammal infants display spontaneous smiles, in which case smiling would have an older evolutionary origin. Who knows? " he says with a smile.
The findings are published in the journal Primates.
Source: Kyoto University [August 04, 2016]