After spying on sleeping baby monkeys, scientists in Japan have determined that humans and chimpanzees are not the only primates to practice smiling in their sleep. Japanese macaques, a more distant relative, do it too. This pushes back the likely evolutionary origin of smiling, perhaps to 30 million years ago.
While drowsing, baby humans and chimpanzees raise the corners of their lips. These facial movements are called “spontaneous smiles.” These smiles aren’t intentional, but they lay the groundwork for voluntary smiles and laughter later on.
To confirm that spontaneous smiles aren’t reserved for people and chimps, scientists watched seven newborn macaques nap, and observed spontaneous smiles in all of them. The 58 “smiles” they saw all showed up during REM sleep, and resembled those made by human infants.
The team speculates that spontaneous smiles help with development of the zygomaticus major muscles. These cheek muscles let us lift the corners of our mouth so we can laugh or laugh.
“Spontaneous smiles may, therefore, play an important role in emotional development as the origin of positive facial expressions,” the team concluded August 2 in the journal Primates.