Learn Something New About Evolution by Watching These Baby Monkeys Smile

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Photo: Clive Harris/Getty Images

There are some things that are just better seen than described, and newborn monkeys smiling in their sleep is one of them. Luckily, a team of researchers from Kyoto in Japan recently caught that very thing on video; instead of trying to explain to you here what said smiling looked like, I’m going to do you a favor and tell you to watch:

Besides being so gosh-darn adorable, the footage, which was recorded as part of a new study in the journal Primates, may also help to shed some light on the origins of our own facial expressions. The researchers based their paper on seven newborn macaque monkeys, monitored as they napped between procedures for an unrelated study. The observations sessions lasted for an average of 44 minutes, during which time each of the monkeys showed at least one “spontaneous smile” — a facial movement also found in human babies, often during sleep and typically with no discernible cause. (Real smiles, or “social smiles” kick in at about 8 weeks.)

It’s not the first time another species has demonstrated the same behavior — “About a decade ago we found that chimp infants also display spontaneous smiles,” study author Masaki Tomonaga, a primatologist at the University of Kyoto, said in a statement — but it’s the first time we’ve seen it happen on such a far-flung branch of the primate family tree. “Since we see the same behavior in more distant relatives,” Tomonaga said, “we can infer that the origin of smiles goes back at least 30 million years, when old world monkeys and our direct ancestors diverged.”

Some scientists, as the New York Times reported, believe that spontaneous smiles are a strategic evolutionary measure, a way of ramping up a baby’s cuteness so its parents will be more attentive. The study authors, though, argue that something else must be at play, noting in the paper that even human fetuses spontaneously smile — a fact that suggests the movement “occur[s] for the benefit of the fetuses, neonates, and infants themselves, and not for other people who happen to see the smiles.” Their explanation: Spontaneous smiling “may facilitate the development of the zygomaticus major muscle,” they wrote, “which is implicated in smiling-like facial expressions.” Like the real, non-spontaneous one you may have after watching a tiny monkey take a nap.

Watch Baby Monkeys Shed Light on the Origin of the Smile