When a monkey bares its teeth, flattens its ears and tightens its throat muscles, it is cornered, afraid and bracing for a fight. When a human bares his teeth, flattens his ears and tightens his throat muscles, he is smiling. How did this odd evolutionary divergence happen?
Strange as it may seem, the friendly human smile probably evolved from that much more aggressive display of fangs, said Janice Porteous, a professor of philosophy at Vancouver Island University in Canada who studies the evolution of humor and laughter. The main evidence comes from "missing link" facial expressions made by primates that signify neither "you're my enemy," nor "you're my friend."
The fear expression — bared teeth, flattened ears, taut neck — "often happens in situations where an animal is trapped, or threatened but physically can't escape," Porteous said. However, in higher primates such as rhesus monkeys, "subordinate members of the group flash that bared-teeth expression to the dominant member when they are occupying a spot that the dominant wants to occupy. The expression seems to deflect the dominant's aggression, so it's a sign of submission, non-hostility or appeasement, resulting in the dominant leaving them alone."
A facial expression that originally arose as a scare tactic turned into an admission of fear, thereby indicating non-hostility. The bared teeth said, "I recognize your superior status, so please go easy on me."
Next, came fang-flashing between friends. "Scientists find that sometimes in higher primates [such as chimpanzees] the expression also gets flashed between equals," Porteous told Life's Little Mysteries. "A couple of equals will have been parted for a long time and then meet and flash it to each other and then embrace. So it moves from showing non-hostility to showing affection or affiliation. It becomes friendly."
And thus, the smile was born. Scientists don't know how long ago it emerged among the great apes. [Why Haven't All Primates Evolved into Humans?]
Since then, the human smile has come to signify a huge range of meanings. Like those rhesus monkeys, people still grin out of fear or nervousness. Sometimes when children are in trouble and being reprimanded, they can't stop smiling — more likely a sign of submission than one of insubordination, Porteous said. We also crack a smile in response to happiness and amusement. And our subtle psychological manipulations of one another have bred more insidious varieties of smiles, too. Case in point: the smirk.
"I don't know that other animals can smirk," Porteous said, "because they don't have the complicated psychology behind that expression."
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Natalie Wolchover was a staff writer for Live Science from 2010 to 2012 and is currently a senior physics writer and editor for Quanta Magazine. She holds a bachelor's degree in physics from Tufts University and has studied physics at the University of California, Berkeley. Along with the staff of Quanta, Wolchover won the 2022 Pulitzer Prize for explanatory writing for her work on the building of the James Webb Space Telescope. Her work has also appeared in the The Best American Science and Nature Writing and The Best Writing on Mathematics, Nature, The New Yorker and Popular Science. She was the 2016 winner of the Evert Clark/Seth Payne Award, an annual prize for young science journalists, as well as the winner of the 2017 Science Communication Award for the American Institute of Physics.