One of our species' trademark expressions is the smile. And smiling is a form of communication. It's not just about how you feel, it's about how you want to make others feel. We use smiles to put each other at ease, to bring people together. So it may seem a little weird that we show off our teeth when we want to be friendly, because teeth are for biting. And many other mammals do bare their teeth as a way to make a threat. Just think about the snarl of a wolf or the open-mouth display of a baboon. It's a shorthand way of saying, like, "Back off if you don't want these to go inside of you!" It's pretty unusual that our smile evolved as a sign of friendliness. But there may be some clues to why it happened in the behavior of our primate cousins. See, many primates show off their teeth for non-aggressive reasons. For example, rhesus macaques make what's called a silent bared-teeth expression as a sign of submission. When a dominant or aggressive opponent is threatening them, they'll show off their teeth as a signal that they don't want any trouble. In other primates, including some macaques and baboons, the same toothy expression shows up while the monkeys are greeting, grooming, and embracing each other. And one of our closest cousins, chimpanzees, show a clear connection between the silent bared-teeth expression and social bonding: the more they flash their teeth at each other, the better they get along. If that sounds familiar, it's because it is also true in humans! Smiles make us feel more comfortable with each other.