I can’t stand TV sitcoms and Hollywood movies featuring monkeys and apes. Every time I see a dressed-up simian actor produce one of their silly grins, I cringe. People may think they’re hilarious, but I know their mood is the opposite of happy. It’s hard to get these animals to bare their teeth without scaring them — only punishment and domination can call forth these expressions. Behind the scenes, a trainer is waving his electric cattle prod or leather whip to make clear what will happen if the animals fail to obey. They are terrified.
The bared-teeth grin is not to be confused with a wide-open mouth and intense staring eyes. That fierce face, which looks like an intention to bite, acts as a threat. In a grin, the mouth is closed, but the lips are retracted to expose the teeth and gums. The row of bright white teeth makes it a conspicuous signal, visible from far away, yet its meaning is the exact opposite of a threat.
Many questions surround the grin, such as how this toothy expression became a friendly one in our species and where it came from. The latter question may seem odd, but everything in nature is a modification of something older. Our hands came from the forelimbs of land vertebrates, which derived from the pectoral fins of fish. Our lungs evolved out of fish bladders.
The grin, it seems, derives from a defensive reflex. For example, when we peel a citrus fruit — an action that risks spraying drops of acidic juice into our face — we automatically pull our lips back from our teeth. I’ve observed baboons grinning to avoid perforating their lips while eating a succulent cactus.
