As per a study titled Smiling baby monkeys and the roots of laughter, the origin of smiles goes back at least 30 million years, when old world monkeys and our direct ancestors diverged.
The study was conducted by team of researchers including Fumito Kawakami, Masaki Tomonaga and Juri Suzuki of Kyoto University in Japan and was published on 3 August 2016.
Findings of the Study
• The study found that 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.
• Spontaneous smiles are facial movements that are characterised by a raised lip corner occurring during irregular sleep or drowsiness without known causes.
• They showed that this not only happens to higher-order primates like humans and chimpanzees, but also in newborn Japanese macaques, which are more distant relatives in the evolutionary tree.
Background
Lead researcher Fumito Kawakami caught macaque infants smiling when they were receiving routine health checkups. In total, researchers 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 REM sleep and they show more lop-sided smiles compared to symmetrical, full smiles.
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