At Aeon:
Most ethologists agree that smiling is evolutionarily old, and that variants of it can be seen across many kinds of primates. If you watch monkeys in a group you might see them flash each other what looks like a grimace. They are communicating non-aggression; ethologists call it a ‘silent bared teeth display’. Some theorists argue that it evolved from more or less the opposite gesture, a preparation for attack. But by focusing on the teeth, I think they miss a great deal. The display really involves the whole body. If it’s flashed subtly, it might be mostly limited to the face. An extreme version, however, looks an awful lot like a whole-body protective stance. So, here is my account of how the smile came about, informed by my lab’s work on defensive reflexes.
Imagine two monkeys, A and B. Monkey B steps into the personal space of Monkey A. The result? Those bubble-wrap neurons begin to crackle, triggering a classic defensive reaction. Monkey A squints, protecting his eyes. His upper lip pulls up. This does expose the teeth, but only as a side-effect: in a defensive reaction, the point of the curled lip is not to prepare for a biting attack so much as it is to bunch the facial skin upward, further padding the eyes in folds of skin. The ears flap back against the skull, protecting them from injury. The head pulls down and the shoulders pull up to protect the vulnerable throat and jugular. The head turns away from the impending object. The torso curves forward to protect the abdomen. Depending on the direction of the threat, the arms may pull across the torso to protect it, or may fly up to protect the face. The monkey snaps into a general defensive stance that shields the most vulnerable parts of his body.
Monkey B can learn a lot by watching the reaction of Monkey A. If Monkey A makes a full-blown protective response, cringe and all, it’s a pretty good sign that Monkey A is frightened. He’s uneasy. His personal space is revved up and expanded. He must view Monkey B as a threat, a social superior. On the other hand, if Monkey A reveals only a subtle response, perhaps squinting and slightly pulling back his head, it’s a good sign that Monkey A is not so frightened. He does not consider Monkey B to be a social superior or a threat.
That kind of information is very useful to members of a social group. Monkey B can learn just where he stands with respect to Monkey A. And so the stage is set for a social signal to evolve: natural selection will favour monkeys that can read the cringe reactions of their peers and adjust their behaviour accordingly. This, by the way, is perhaps the most important point of the story: the primary evolutionary pressure is on the receiver of the signal, not the sender. The story is about how we came to react to smiles.
Then again, nature is often an arms race. …
Insightful, isn’t it? Just what you needed to know about why you catch yourself smiling when you feel at peace with the world …
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