When our ancestors evolved into mammals, their faces changed again. New muscle attachments spread from the jaws to the skin itself. They boosted the senses of mammals—muscles on the sides of the face could swivel ears, while muscles around the lips controlled whiskers. But mammals could do more with these muscles than sense the world. They could also communicate. A mammal could bare its teeth or turn back its ears to send signals to other mammals.
Facial expressions, Darwin argued, were “a language of emotion.” They served as a way for us to communicate before we had words.
If the mammal face is an instrument for communication, the primate face is a Stradivarius. When primates evolved about 60 million years ago, big blocks of facial muscles broke into small bits of specialized tissue. Some did nothing but raise eyebrows. Some exclusively puckered lips. Nerves developed new branch patterns across the face to control the new muscles, and the face-controlling regions of the brain grew.
Anne Burrows, a physical anthropologist at Duquesne University, has been dissecting primate faces and finds that they have a lot more in common with ours than anatomists once thought. She has found muscles in the chimpanzee face [pdf], for example, that were once believed to be uniquely human.
Not only are the same muscles in the same place, but the primate brain uses them to make many of the same expressions. Bridget Waller, a psychologist at the University of Portsmouth in England, and her colleagues demonstrated this with a new twist on Duchenne’s old research. Duchenne could only put electrodes on the skin of his subjects; Waller and her colleagues today can insert fine needles into the muscles themselves. Researchers can also insert those needles into anesthetized chimpanzees. They have found that in most cases, a facial movement produces the same expression whether on the face of a chimpanzee or of a human.
Why did one small group of mammals evolve such sophisticated faces? The answer probably has to do with the intensely social life of primates. Natural selection may have favored primates that could make a wide range of expressions and that could read those expressions in others. The right expression could let a primate stare down a rival or cement a bond. It might even have kept bands of primates from descending into civil war. Seth Dobson, an anthropologist at Dartmouth College, found support for this idea in a study he did on 12 species of monkeys and apes. He found that primates that lived in bigger groups tended to have more mobile faces.
You can see this expressive range not just in the faces of primates, but in their brains, too. Dobson found that primates in bigger groups have larger face-controlling regions in their brains. Primate brains also have impressive networks for reading facial expressions. If a monkey sees a picture of another monkey making a face—an open-mouthed threat, for example, or a submissive lip smack—a network of neurons becomes active in its brain. Some parts of the network process features of the face in order to recognize to whom it belongs. And emotion- processing parts become active as well. Different centers switch on in response to different emotions, allowing the monkey to decode the feelings behind the face.
When we see faces, we don’t just recognize them; we also make the same face, if only for a moment. If you see someone wearing a big grin, muscles on your face will start contracting in about a third of a second. The same goes for angry faces and sad ones. We respond this way whether people are looking at us or at someone else.
Mimicking faces is a deep instinct in humans—babies start doing it days after birth. And our ancestors were probably making these faces for millions of years. Earlier this year, Marina Davila Ross of the University of Portsmouth and her colleagues reported the first observation of other apes quickly mimicking faces. When orangutans play with each other, they sometimes open their mouths in the ape equivalent of a smile. Observing 25 orangutans at play, Ross found that when an orangutan sees another orangutan make an open-mouth expression, it tends to do the same in less than half a second.
We do not mimic faces simply as a side-effect of looking at other people. Experiments show that mimicry actually helps us understand what other people are feeling. Harvard University psychologist Lindsay Oberman and her colleagues demonstrated this effect with little more than a pen.
Oberman had volunteers bite down on a pen and then look at a series of faces. They had to pick the emotion they thought the faces were expressing. The volunteers could recognize sad faces and angry ones with the same accuracy as test subjects who did not have pens in their mouths. But they did a worse job of recognizing happy faces.
Biting a pen, it just so happens, requires you to use the same muscles you use to smile. Because the smiling muscles were active throughout the experiment, Oberman’s subjects apparently couldn’t feel themselves start to mimic happy faces. Without that feedback, they had a more difficult time recognizing when people were happy.
Oberman and a growing number of other psychologists believe that we empathize by mimicking faces. By putting ourselves in other people’s places, we understand what they’re feeling. To investigate how facial mimicry helps us empathize, Leonhard Schilbach of the University of Cologne and his colleagues recently made a brain-scanning breakthrough. They had volunteers watch movies of computer-animated people turning toward them and smiling while scientists scanned their brain activity and tracked their facial muscles. Later the researchers examined the muscle recordings to pinpoint the precise instant when the volunteers mimicked the faces. Then they looked at how the brains of the volunteers were acting at that instant.
