Smiling is one of the most universal human behaviors. A smile can express joy, politeness, embarrassment, sympathy, and a myriad of other emotions and intentions. The origins of the human smile have long fascinated anthropologists, psychologists, and evolutionary biologists. When and why did our ancestors first begin to smile? Understanding the emergence of this distinctive expression can provide insight into human emotional and social evolution.
Theories on the Origin of Smiling
Charles Darwin was one of the first scientists to systematically study emotional expressions. In his 1872 book, The Expression of the Emotions in Man and Animals, Darwin explored three theories on the origin and evolution of the human smile:
- The “Darwinian” theory argued that smiles evolved to express genuine happiness and pleasure. Positive expressions reinforced social bonds.
- The behavioral ecology view saw smiles as originating from displays of submission. Early smiles conveyed appeasement or lack of threat.
- A third perspective viewed smiles as having derived from mocking or taunting displays. Smiles served as signals during competition.
While Darwin favored the joy expression theory, modern researchers recognize merit in all three. The meaning of smiles is multi-functional and context dependent. How did scientists uncover when smiling first emerged?
Evidence for Smiling in Human Ancestors
Fossil evidence provides limited insights into the history of smiling. Soft tissue facial movements do not leave impressions on skulls and skeletons. However, anthropologists have used other types of evidence to identify when smiles likely developed.
Tool Use and Expansion of Group Sizes
In the 1990s, anthropologist Robert Provine of the University of Maryland analyzed the emergence of smiling. He tied the rise of smiling and laughter in humans to two key developments:
- The creation and use of tools, which first occurred about 2 million years ago.
- An increase in overall group size, which expanded opportunities for bonding.
Tool use and larger social networks may have favored smiling as a way to build alliances and diffuse tensions during competitive encounters over resources. Provine concluded that ritualized smiling likely emerged in Homo erectus approximately 2 million years ago.
The Link Between Laughter and Smiling
In a 2017 publication, researchers Antonio Benítez-Burraco and Cedric Boeckx proposed an earlier timeline. They argued that smiling arose with the capacity for complex language and singing in our ancestors. The authors pointed to a connection between smiling and speech breathing control. Smiling also often accompanies laughter. This suggests a link between smiling and the evolution of vocal and emotional expressions.
When did language likely emerge? Evidence of symbolic culture, such as pigments use and shell jewelry, dates back 200,000-300,000 years. This suggests key language developments occurred at least that early in the Middle Pleistocene era. Based on this, Benítez-Burraco and Boeckx concluded that ritualized human smiling likely emerged approximately 300,000 years ago.
Early Hominin Fossils
Perhaps the most direct insight comes from analysis of early human and human ancestor skulls and musculature. Anthropologists can identify the muscle attachments and shapes required to produce smiles.
In 1999, researchers C.A. Marquart and R.J. Smith examined skulls of Homo and Australopithecus species. They determined that both Homo sapiens and Homo neanderthalensis (modern and Neanderthal humans) had the facial muscles necessary for beaming smiles. More limited upward curving of the lips was possible for earlier Homo erectus. Very early hominins such as Australopithecus afarensis (Lucy’s species) lacked most facial muscles needed for anything beyond simple grimaces.
Based on skeletal structures, it appears anatomically modern human-like smiles emerged approximately 150,000-200,000 years ago in Africa with archaic Homo sapiens and early Homo sapiens. However, simple smiling expressions likely occurred ever earlier in human evolution.
Emotion Expression in Primates
Studying our closest primate relatives provides more clues about the origins of smiling. Charles Darwin conducted extensive observations of emotional expressions in animals. He carefully documented similarities and differences between human and nonhuman primate signals.
Play Face in Apes
Chimpanzees, gorillas, orangutans, and bonobos all demonstrate a distinctive “play face” when engaging in friendly, mock fighting sessions. The play face includes an open mouth with only the lower teeth exposed. The expression resembles a human smile, albeit more exaggerated. Play faces appear to invite play, convey friendliness, and help avoid misunderstandings during rambunctious interactions.
Nonhuman apes likely inherited the play face from our last common ancestor 10-20 million years ago. The ancestors of modern humans would have also displayed the play face during lighthearted interactions. Over time, the toothy expression could have been ritualized into the simpler, lip-curling smiles of humans.
Laughter in Chimps and Bonobos
Laughter evolved before the emergence of Homo sapiens, as indicated by the fact that great apes laugh during play. Chimps produce panting during rough-and-tumble sessions that is rhythmic and audible. Bonobos take laughter even further with play screams and joyful peals akin to human laughter.
Apes appear to use this vocalization to invite play and strengthen social bonds. Since laughter often elicits smiles in humans, laughter in our early ancestors may have similarly prompted smiling exchanges.
Caveats on Animal Comparisons
While animal studies provide useful clues, we must be cautious when making direct comparisons. Primate expressions are involuntary and fixed Action Patterns triggered by specific stimuli. Human smiles can occur in many more contexts and contain myriad nuanced meanings. Our smiles also likely play a larger role in complex social interactions and relationships.
The Many Forms and Functions of Smiles
Not all smiles are the same. Human smiles vary in appearance and intent based on the situation and the individual. How did this variety evolve from early ritualized expressions?
Anatomical Differences
In the 19th century, French anatomist G.B. Duchenne identified two types of smiles based on which muscles are engaged:
- Duchenne Smiles – Contract the muscles around the eyes (orbicularis oculi) as well as the mouth. These smiles form when we feel enjoyment and crinkle the corners of our eyes.
- Non-Duchenne Smiles – Only engage the mouth muscles (zygomatic major). These smiles don’t reach our eyes and often occur when being polite or acting for others.
Duchenne believed only smiles involving the eyes expressed genuine emotion. However, today we recognize that non-Duchenne smiles also serve important social functions.
Developmental Stages
Babies go through stages in smiling and laughing behavior that may parallel evolutionary shifts:
- 2 months: The social smile emerges. Babies respond to parents with reflexive smiling.
- 4 months: Full Duchenne smiles appear. Babies smile spontaneously to express joy.
- 6 months: Laughing starts in amusement or play. Smiles accompany these vocalizations.
This progression suggests laughter and joyous smiling preceded more formalized, polite smiling in our ancestors. Duchenne smiles have an evolutionary origin in spontaneous amusement and bonding.
Universality Across Cultures
Paul Ekman conducted groundbreaking research on emotional expressions. He found that people from literate and preliterate cultures easily recognize six basic facial expressions—anger, disgust, fear, happiness, sadness, and surprise. Happiness was identified based on Duchenne smiling.
This indicates that open, eye-engaging smiles universally represent genuine enjoyment, good will, and acceptance across all human populations tested. However, cultures do vary in their norms around displaying different types of smiles.
Context Dependent Meaning
Why we smile depends on multiple factors:
- Relationship between communicator and recipient
- Gender
- Social rank
- Cultural norms
- Personal motivations
For example, women tend to smile more than men in most Western cultures. Higher status individuals smile less at subordinates. The same smile can convey shared amusement or condescension depending on the context.
Learning rules around emotional displays represents a key part of socialization. While smiles originated much earlier in evolution, social norms shape their meanings.
Conclusion
Evidence suggests ritualized human-like smiling emerged relatively late in human evolution around 200,000 years ago or less. However, simple smiles based on upturned mouths occurred in earlier Homo species and laughing expressions further back. The universal Duchenne smile indicates an origin in spontaneous joy and connection.
Smiles became more complex and context dependent as social structures and cultural norms evolved. But at their core, smiles remain an important human behavior for expressing happiness, building bonds, and negotiating social roles and rankings. Though often overlooked, they represent a small but meaningful part of the human experience.