The Origins of Facial Expressions: How Evolution Shaped Our First Language

There is a moment, around six weeks of age, when a baby first answers its mother’s smile with a smile of its own. This smile is different from the reflexive grin of a newborn whose lips happen to curl upward from gas. The eyes meet, the corners of the mouth lift, and the small muscles around the eyes move in unison. The most powerful signal binding caregiver and infant has begun to operate. Developmental psychologists call the smile that emerges at this stage the “social smile.”^[1] Without a single word or grammatical rule, this small movement of facial muscles is the first sentence of the oldest language a human being will ever use.

Long before speech existed, humans communicated with their faces. Expressions are not merely byproducts of emotion but a signaling system meticulously refined by evolution. So why is it that human beings, of all creatures, ended up with such an extraordinarily rich vocabulary of expressions?

White Sclera and Forty-Plus Muscles: What Makes the Human Face Special

Among primates, humans are the only species with conspicuously visible whites of the eye. Chimpanzees and gorillas have eyes covered with brown sclera, making it difficult to discern the direction of their gaze. By contrast, the white sclera of humans clearly reveals the position of the pupil even from a distance. Evolutionary anthropologists explain this through the “cooperative eye hypothesis.” Because signaling to companions where one was looking proved advantageous in hunting and child-rearing, human eyes evolved to reveal rather than conceal the direction of gaze.^[2]

The facial muscles themselves are also unusual. The human face contains over forty muscles involved in producing expressions.^[3] These muscles attach directly to the skin rather than to bone, so even a slight contraction can change the shape of the lips, eyebrows, and bridge of the nose. Other primates also produce expressions, but none can generate the same variety of combinations as humans. Chimpanzees and bonobos have expressions for threat, submission, and play, and dogs—who co-evolved with humans—have been shown to possess more developed muscles above the eyebrows than wolves, producing the well-known “puppy-dog eyes.”^[4] Yet building thousands of expressions through fine combination and recombination is the unique specialty of the human face.

The person who first elevated this subject to a serious scientific topic was Charles Darwin. In The Expression of the Emotions in Man and Animals, published in 1872, he argued that facial expressions were not culturally learned but products of evolution.^[5] Darwin compared the expressions of infants, the mentally ill, people of different races, and various animals, pointing out that certain patterns repeat across species—teeth bared in anger, eyebrows raised in fear. He proposed that these expressions had once served practical functions (for example, baring the teeth as preparation for biting) but had over time been pared down to signals carrying meaning alone.^[5]

Darwin’s book lay forgotten for a time before being rediscovered by mid-twentieth-century psychologists, becoming the starting point for modern emotion research.

Paul Ekman and the Six Universal Expressions

The figure who took up Darwin’s hypothesis most rigorously was the American psychologist Paul Ekman. In the late 1960s, Ekman traveled to the Fore people of Papua New Guinea. To this tribe, who had had almost no contact with outside civilization, he showed photographs of Western faces and asked, “What situation might this person be in right now?” The results were striking. The Fore consistently linked the photographed expressions to the same six emotions: anger, disgust, fear, happiness, sadness, and surprise.^[6]

Building on this research, Ekman proposed his “basic emotions theory.” The six emotions of anger, disgust, fear, happiness, sadness, and surprise, he claimed, are expressed with the same facial movements by all humans regardless of culture, and these are universal signals etched into us by evolution.^[6] He later added contempt to expand the list to seven.

In 1978, Ekman went on, with his colleague Wallace Friesen, to publish the Facial Action Coding System (FACS). FACS is a system that breaks down all muscular movements of the face into roughly forty units called “Action Units” and codes them symbolically.^[7] For example, a genuine smile (the so-called Duchenne smile) requires the simultaneous activation of the zygomaticus major, which lifts the corners of the mouth (AU12), and the orbicularis oculi, which crinkles the skin around the eyes (AU6). A polite, ceremonial smile lifts only the corners of the mouth while the eyes remain still. FACS is used today across a wide range of fields including animation, psychotherapy, lie-detection research, and AI facial recognition.^[7]

Ekman also introduced the concept of micro-expressions—fleeting expressions that flash across the face for between 0.04 and 0.5 seconds. Even when someone tries to conceal their true feelings, the genuine emotion briefly surfaces in that instant. His work also gained popular recognition by serving as the basis for the American TV drama Lie to Me.^[8]

Counterarguments to Universality: Does Culture Really Not Teach Expression?

For nearly fifty years, Ekman’s model was the standard in emotion research. But in the 2010s, powerful counterarguments began to emerge. A leading figure has been Lisa Feldman Barrett, a neuroscientist at Northeastern University.

In her 2017 book How Emotions Are Made, Barrett presented a meta-analysis of more than 200 studies, showing that even the same emotion can be expressed very differently across cultures, contexts, and individuals.^[9] When sad, for instance, not all humans turn down the corners of the mouth and draw the brows together. In some cultures sadness is masked with a blank expression, in others with a smile. Barrett argues that expressions are not universal “fingerprints” but the result of predictions the brain constructs as it interprets each situation. This view is called the “theory of constructed emotion.”^[9]

Another criticism is that the photographs Ekman used depicted exaggerated expressions in the first place. In real daily life, people do not produce such cartoonishly clear expressions. Critics also point out that in the Fore experiment, respondents were given a list of six emotion words to choose from in advance, which may have biased the results.^[10]

It would be a mistake, however, to reduce this debate to “Ekman was wrong.” Some core elements of facial signaling clearly do recur across species. Crying and laughter, the wide eyes of fear, the wrinkled nose of disgust—these are observed even in newborns. What recent consensus holds is that on top of those basic signals, culture builds a thick layer of its own.^[10] In the United States, expressing anger directly may signal strength; in Japan, masking the same anger behind a smile may signal maturity. The face is a double-layered structure, with social grammar overlaid on a biological foundation.

The Expression Deficit of the Digital Age

From the late twentieth century onward, this oldest of interfaces faced new challenges. Communication technologies began transmitting messages while leaving the face behind.

The telephone carried only voice; text messages carried only letters. In email, the same sentence could be read as a joke or as an attack, and to fill that ambiguity, people invented new symbols. On September 19, 1982, Scott Fahlman, a computer scientist at Carnegie Mellon University, proposed on a campus bulletin board that “jokes be marked with :-) and serious posts with :-(.”^[11] It was the first emoticon. Humans had begun to redraw the lost face with letters.

This trend evolved in 1990s Japan into pictographs—emoji. The starting point was the design of 176 small images for mobile phones in 1999 by Shigetaka Kurita, a designer at NTT Docomo.^[12] Today, more than 3,000 emojis are registered in the Unicode standard, and by far the most-used among them is the “Face with Tears of Joy” (😂). A small yellow circle has come to stand in for the expressions humans once made themselves.

Yet emojis have not become a complete substitute for the face. The COVID-19 pandemic that began in 2020 made this clear. When masks covered the lower half of the face, people could no longer accurately read each other’s smiles or frowns. Researchers at the University of Wisconsin and the University of Bamberg in Germany reported that recognition accuracy for emotions such as happiness, anger, sadness, and fear dropped meaningfully under masked conditions.^[13] Video calls have similar limitations. On screen, gazes do not align, micro-expressions lose resolution, and slight delays throw off the rhythm between expression and speech. This explains the strange fatigue we feel after an hour of video conferencing: the brain keeps searching for facial signals while the face on the screen fails to provide them in full.

Why Don’t Aliens in Sci-Fi Have Expressions?

When science fiction films and novels portray highly evolved alien civilizations, one of the most common devices is the “expressionless face.” Smooth gray skin, motionless lips, large unblinking eyes. The Heptapods of Arrival (2016), and the silence of humanity standing before the monolith in 2001: A Space Odyssey (1968), are examples.

A common misconception lurks here: the idea that “advanced civilizations use telepathy or precise language, so expressions are unnecessary.” There is no real basis for that conclusion. Expressions are not an inefficient remnant; they are an efficient signal that transmits broadband information at high speed. The depiction of expressionless aliens is closer to a directorial device chosen by writers to emphasize “otherness.” We instinctively fear those whose expressions we cannot read—the same reason masks frighten us.

Conversely, alien creatures meant to feel familiar and warm tend to have expressive faces. The long, wrinkled face of E.T. (1982), the rich smiles and frights of the Na’vi in Avatar (2009), and the way Groot in Guardians of the Galaxy conveys every emotion through expression alone with only a single word—all of these borrow the grammar of the human face to win the empathy of human audiences. In the end, the faces of aliens in science fiction are less an inference about extraterrestrial biology than a mirror reflecting the meaning that human society itself attaches to facial expression.

Back to the Newborn’s Smile

Let us return to where we began. The six-week-old baby gazing at its caregiver’s face grows up, goes to school, makes friends, falls in love, sits in meetings at work, and exchanges greetings with family over video calls. Along the way, the words at their disposal grow into the tens of thousands, and the channels through which they send messages multiply into voice, text, image, and video. And yet no matter what medium intervenes, people still search for the other person’s face. They want to read whether the person on the other side of the screen is genuinely laughing, somehow sad, or holding back anger.

The face has remained an interface that no other invention has been able to fully replace. Even with more than 3,000 emojis and video calls in 4K resolution, we still want to meet and look at one another in person. This is not because we are inefficient or old-fashioned. It is because the resolution of a signaling system that evolution has spent hundreds of thousands of years refining still surpasses that of any digital medium. The first language a human being learns is, in the end, the most precise language we ever use.