When human and chimp¹ infants are dozing², they sometimes show facial movements that resemble smiles. These facial expressions -- called spontaneous smiles -- are considered the evolutionary³ origin of real smiles and laughter. Researchers at Kyoto University's Primate⁴ Research Institute show that this not only happens to higher-order primates⁵ like humans and chimpanzees, but also in newborn Japanese macaques, which are more distant relatives in the evolutionary tree. 

 

"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 diverged⁶."

 

Lead author Fumito Kawakami caught macaque infants smiling when they were receiving routine health checkups. "These checkups can take quite long, so the infants tend to nap in between," says Kawakami. "We took this opportunity to empirically examine the behavior." 

 

In total they 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 spasms⁷ compared to those of human infants. There were two significant similarities; they both happened between irregular REM sleep, and they show more lop-sided smiles compared to symmetrical, full smiles," says Kawakami. "A major difference, though, is that the smiles were much shorter."

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