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Researchers have found that the spontaneous smiles seen in hominoids are also common in Japanese macaques, revealing that the origin of smiles goes back at least 30 million years, when old world monkeys and our ancestors diverged.
Spontaneous smiles -- considered the evolutionary origin of real smiles and laughter -- are facial movements that are characterised by raised lip corner that occur during irregular sleep or drowsiness without known external or internal causes.
For the study, the team observed 58 spontaneous smiles from seven macaque infants, all of which showed spontaneous smiles at least once.
"Spontaneous macaque smiles are more like short, lop-sided spasms compared to those of human infants. There were two significant similarities; they both happened between irregular rapid eye movement (REM) sleep, and they show more lop-sided smiles compared to symmetrical, full smiles," said Lead author Fumito Kawakami from Kyoto University, in Japan.
"A major difference, though, is that the smiles were much shorter," Kawakami added.
The findings showed 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).
Further, the spontaneous smiles facilitate the development of cheek muscles, enabling humans, chimpanzees, and Japanese monkeys to produce smiles, laughs, and grimaces, the researchers said in the work published in the journal Primates.
However, some researchers argued that infants' spontaneous smiles exist to make parents feel that their children are adorable and to enhance parent-child communication.
