Darwin would have loved Botox.

I don’t mean that he would have been first in line at the doctor’s office to get a needle jabbed into his famously furrowed brow. I mean that Darwin would have loved to use Botox as a scientific tool—to eavesdrop on the intimate conversation between the face and brain.

For much of his life, Darwin was ob­sessed with faces. On a visit to the London Zoo, he gave mirrors to a pair of orangutans and watched them grimace and pucker their lips as they stared at their reflections. He passed many an afternoon gazing intently at photographs of crying babies and laughing women. He showed his friends pictures of a man whose facial muscles were distorted in various ways by electric shocks and quizzed them about what emotion the man seemed to be feeling. To find out if all humans expressed emotions in the same way, he wrote up a list of 16 questions, which he sent to dozens of acquaintances around the world. His list of questions began:

1. Is astonishment expressed by the eyes and mouth being opened wide, and by the eyebrows being raised?

2. Does shame excite a blush when the colour of the skin allows it to be visible? and especially how low down the body does the blush extend?

3. When a man is indignant or defiant does he frown, hold his body and head erect, square his shoulders and clench his fists?

Darwin took the answers he got from his correspondents—from such places as Borneo, Calcutta, and New Zealand—and combined them with the rest of his notes on faces to publish a book in 1872 entitled The Expression of the Emotions in Man and Animals. Most scientists in Darwin’s time considered the face a mystery, its expressions having been set at the time of Creation. But Darwin argued that the look of happiness or grief on a person’s face was the product of evolution, just as our hands evolved from fish fins.

As evidence, Darwin pointed to the results of his poll. People the world over made faces using the same basic patterns of muscle contraction, starting from infancy. In his book Darwin printed pictures of people getting electric shocks, which were taken by the French physician Guillaume-Benjamin-Amand Duchenne. Simply by running current through different parts of a person’s face, Duchenne could produce expressions of happiness, fear, anger, and disgust. Expressions were reflexes, Darwin argued, instinctive patterns etched in our faces and brains.

To trace the history of our faces, Darwin wrote, we need only look at our fellow animals. Although human faces were unique in some ways, they also bore some striking similarities to those of other species: “He who will look at a dog preparing to attack another dog or a man, and at the same animal when caressing his master, or will watch the countenance of a monkey when insulted, and when fondled by his keeper, will be forced to admit that the movements of their features and their gestures are almost as expressive as those of man.”

Darwin described facial expressions as a “language of emotion.” They served as a way for us to communicate before we had words. They helped us not only to understand the emotions of others but to share them as well. “The free expression by outward signs of an emotion intensifies it,” he wrote. “Even the simulation of an emotion tends to arouse it in our minds.”

Darwin’s ideas turned out to be prophetic, but when you read The Expression of Emotions in the 21st century, you can’t help noticing how quaint his scientific research was. Scientists who study faces today do not rely on their pet dogs or letters from friends in Brazil. They trace the development of the face in embryos, scan brains, read the electrical activity of muscles, and record grins and pouts on high-speed video.

Our faces, these scientists have shown, acquired some of their basic form more than half a billion years ago. It was then that early fish evolved muscles on their heads to suck in food and water. All the muscles of our faces develop from a strip of cells at the base of the embryonic head, just as they do in lampreys, which belong to one of the oldest lineages of vertebrates alive today.

The transition to land brought major changes to the faces of our ancestors. They stopped breathing water through gills, and the gill-supporting muscles in the face took on new functions, like controlling the throat to swallow food. At the same time the muscles that moved the jaws became bigger as land vertebrates evolved a more powerful bite.

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.

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, 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.

During unconscious facial mimicry, Schilbach discovered, several regions of the brain become active. One of those, the left precentral gyrus, becomes active when people get the urge to move their facial muscles (such as when a song makes them sad). Other regions (the right hippocampus and the posterior cingulate cortex) become active when we have emotional experiences, helping to retrieve emotional memories. Another part of the brain that becomes active during facial mimicry (the dorsal midbrain) relays emotional signals to the rest of the body, bringing on the physical feelings that go along with emotions, like a racing heartbeat.

When humans mimic others’ faces, in other words, we don’t just go through the motions. We also go through the emotions.

Recently Bernhard Haslinger at the Technical University of Munich realized that he could test the facial feedback theory in a new way. He could temporarily paralyze facial muscles and then scan people’s brains as they tried to make faces. To block facial feedback, Haslinger used Dysport, a Botox-like drug available in Europe.

Botox and Dysport are brand names of a toxin made by the spore-forming bacterium Clostridium botulinum. Botulinum docks on the surface of neurons, blocking the release of a transmitter called acetylcholine. In small amounts botulinum can be fatal. In far, far smaller amounts, it can simply paralyze a small patch of muscles for a few weeks. Haslinger has used Dysport in people with movement disorders like dystonia to help reduce unwanted muscle movement. But Botox and Dysport are best known as treatments to mask aging. Injections into the muscles that make frowns can slow the growth of lines around the eyebrows.

For his brain experiment, Haslinger and his colleagues gave 19 women Dysport injections. Two weeks later the scientists scanned their brains as they showed the women a series of angry or sad faces and asked them either to imitate or just to observe the expressions. Haslinger then ran the same experiment on 19 women without Dysport and compared the two sets of scans.

When the women made sad faces, the same brain regions became active in both those with Dysport and those without. But making angry faces triggered different patterns. In the Dysport-free women, a region known as the amygdala—a key brain region for processing emotions—became active. In the women with Dysport, who could not use their frown muscles, the amygdala was quieter. Haslinger also found another change, in the connections between the amygdala and the brain stem, where signals can trigger many of the feelings that go along with emotions: Dysport made that connection weaker.

Of course neuroscience labs are not the only place where people get shots of Dysport or Botox. According to the American Society of Plastic Surgeons, in the United States doctors administer millions of injections of Botox each year, many of them to people’s faces. Haslinger’s research suggests that this is part of a massive, unplanned experiment.

In June 2008 in the Journal of the American Academy of Dermatology, a team of cosmetic surgeons suggested this experiment is making all of us happier. People with Botox may be less vulnerable to the angry emotions of other people because they themselves can’t make angry or unhappy faces as easily. And because people with Botox can’t spread bad feelings to others via their expressions, people without Botox may be happier too. The surgeons grant that this is just speculation for now. Nevertheless, they declare that “we are left with the tantalizing possibility that cosmetic procedures may have beneficial effects that are more than skin deep.”

Maybe. But for all the Botox youthfulness plastic surgeons may want to think about, neuroscience raises a darker possibility. Making faces helps us understand how other people are feeling. By altering our faces we’re tampering with the ancient lines of communication between face and brain that may change our minds in ways we don’t yet understand.