Laughter and its role in human evolution. Image credit: Unsplash.
You have probably never considered how common laughter is in your everyday conservations. However, it is an integral component of our social interactions. Whilst we tend to associate laughter with jokes, most of the laughter we produce has nothing to do with humour. Instead, we use laughter for various communicative functions, such as to show agreement and understanding or to ease social tension.
Laughter is also a form of conversational punctuation, often occurring at the end of sentences and phrases. But why is this behaviour so pervasive in our everyday conservations? In other words, why did laughter evolve in humans? To answer this question, the origins, functions, and timing of the evolution of laughter need to be explored, which will show that laughter is no joking matter.
An important point to discuss first is that humans have two distinct types of laughter. First is Duchenne laughter: an involuntary, reactive, and emotional response. External events and positive emotions drive this laughter. On the other hand, non-Duchenne laughter is more voluntary, controlled, and deliberate. Although Non-Duchenne laughter is used more in conversational settings, it is sometimes referred to as “the dark side of laughter” as it is sometimes used to mock and degrade others.
Mechanistic distinctions also exist between these two laughter types and their underlying neural circuitry. As such, Duchenne and non-Duchenne laughter likely evolved at different times for divergent functions. The rest of this article will focus on Duchenne laughter as it evolved first and then facilitated the evolution of the second type of laughter.
Origins of laughter
Laughter is not unique to humans. Rather, several species exhibit laughter. For example, rats produce ultrasonic vocalisation patterns during juvenile play and tickling. Like human laughter, these vocalisations occur during cheerful and playful social situations and are absent during stressful conditions. Whilst these vocalisations appear functionally akin to human laughter, human laughter likely has different evolutionary origins. This is because of the evolutionary distance between primates and rodents. Additionally, rodents lack the facial expressions that are associated with laughter in human and non-human primates.
Whilst we tend to associate laughter with jokes, most of the laughter we produce has nothing to do with humour.
The other animals we share laughter with are the non-human primates, such as gorillas, orangutans, chimpanzees, and bonobos. Primate laughter exists in the form of play vocalisations. The primary function of these vocalisations is to signal good and harmless intentions during play. It is thought that spontaneous human laughter evolved from these play vocalisations in apes. Evidence for this comes from analysing the acoustics of play- and tickling-induced laughter in chimps and adult humans. This analysis found chimp laughter shares key characteristics with human laughter.
Phylogenetic analysis of great ape and human laughter further supports the hypothesis that human laughter evolved from ape play vocalisations. Phylogenetic analyses investigate the evolutionary history and relationships between organisms. Based on genetic data and laughter acoustics, phylogenetic trees support the idea that laughter is homologous between apes and humans (i.e., it has a single evolutionary origin).
Neuroscience also points to a link between primate and human laughter. The neural circuitry that underlies involuntary laughter in humans is also found in primates. This neural circuitry is known as the emotional network and consists of the pregenual anterior cingulate cortex, temporal pole, and nucleus accumbens. The evolutionary conservation of the emotional network further suggests that human laughter evolved from primate laughter.
However, human laughter differs from primate laughter in the fine detail of its structure and its physiological properties. For example, there has been a shift from an exhalation-inhalation sequence in non-human primates to an exhalation-only arrangement in humans. Furthermore, whilst play vocalisations in apes are only used during play, laughter in humans is produced more flexibly and is a part of everyday social interactions. So, human laughter evolved from ape play vocalisations, but what caused laughter to become such a pervasive part of everyday conversation?
Why did human laughter evolve?
One idea comes from Robin Dunbar, who hypothesises that laughter was used as a form of chorusing by early hominins (the taxonomic tribe containing H. sapiens and extinct members of the human lineage). Dunbar suggests that laughter evolved as a new form of social bonding to increase group size beyond what grooming permits. Laughter is thus proposed to have evolved as a transitional phase between primate bonding mechanisms and more recent culture-based bonding behaviours in modern humans, like singing and storytelling. But why was a new form of social bonding needed to enable a larger group?
Social grooming, the grooming of others to remove dirt and debris to keep skin/hair clean, is the primary mechanism underpinning social bonding in anthropoid primates (apes and monkeys). However, grooming is a time-consuming form of social bonding, and the time available for social grooming is limited due to competing demands from other activities necessary for survival, like foraging. This time limit restricts the size of groups that can be bonded in this way because, as group size increases, more time needs to be devoted to social grooming.
The maximum time spent on social grooming by any primate species is 18.5% of the day, and the maximum group size bonded this way is 44.3. However, the mean observed group size for contemporary humans is 154, and studies show that, on average, humans devote only 20% of their day to social interaction. So, humans spend the same amount of time as primates on social bonding but can achieve larger group sizes. Therefore, we must have evolved more efficient bonding mechanisms to enable larger group sizes. Dunbar suggests this was initially achieved through the evolution of laughter.
People’s sense of bonding with others increased significantly after laughing with them.
Both social grooming and laughter upregulate endorphins. Endorphins are opioid neuropeptides involved in pain modulation and facilitating social bonding. Therefore, this convergence suggests that grooming and laughter increase social bonding via the same mechanism. Evidence for this comes from experiments measuring changes in pain thresholds in humans before and after laughter. The results of these experiments found that laughter consistently elevates pain thresholds, a proxy for endorphin upregulation. Further evidence comes from assessing endorphin receptor activity before and after watching a comedy video using positron emission tomography (PET) scans. The results found that endorphin receptor activity positively correlated with the frequency of laughter, thus providing evidence that laughter upregulates endorphins.
Furthermore, there is evidence that laughter increases human social bonding in psychological experiments. In these experiments, subjects were asked to rate their perception of their relationship with other group members before and after watching a control or comedy video. This study found that people’s sense of bonding with others increased significantly more after laughing with them than in the absence of laughter.
Laughter enables larger social groups than grooming permits because laughter is more efficient. Social grooming is time-consuming, as individuals can only interact with one group member at a time. In contrast, the average laughter group size is 2.7. As both the speaker and audience laugh together, laughter is nearly three times more efficient for social bonding than grooming.
Therefore, there is evidence that laughter increases social bonding via the same endorphin mechanism as grooming and is a more efficient way to bond, supporting Dunbar’s hypothesis. The final question is: when did human laughter evolve from ancestral ape play vocalisations?
When did human laughter evolve?
Likely times for the evolution of laughter are points in human evolution when a significant increase in group size would cause an increase in required grooming time beyond the limit of 18.5% of the day. There are two major stepwise increases in estimated group size in human evolution. The first is the appearance of the Homo genus about 2.4 million years ago. At this point, group size increased to roughly 75, which is above the 44.3 maximum and would require more than 18.5% of the day for social bonding if grooming was the only mechanism. However, calculations estimate that for the extra 30 or so group members, only 22 minutes of laughter a day would be needed to bond them. This would keep the percentage of time spent on social bonding below 18.5%.
Laughter is not just some funny quirk of humans but rather an ancient form of communication that played an essential role in the evolution of human sociality.
The second stepwise increase is the appearance of the archaic humans 600,000 years ago, with an estimated group size of 122. Calculations estimate it would have required an extra 55 minutes of laughter to support the additional 77.5 members. Time budgets were tightly constrained in later Homo; thus, spending an extra 55 minutes daily in sustained laughter seems unlikely. Therefore, archaic humans likely evolved different mechanisms more efficient than laughter to enable their larger group sizes.
In contrast, an extra 22 minutes a day spent laughing seems possible. Therefore, a plausible time for the evolution of laughter was 2.4 million years ago to solve boding demands in early Homo. This timing coincides with when hominins evolved a more nomadic lifestyle in open, high predation-risk habitats. Primates typically solve the risk of increased predation by living in larger groups, and so this transition could explain why a larger group size evolved at this time.
So, laughter is not just some funny quirk of humans but rather an ancient form of communication that played an essential role in the evolution of human sociality. This social bonding mechanism is still pervasive in our interactions today, showing that laughter really is no joke.