Evolutionary explanations for the existence of discrete emotions such as fear and joy are one of many theoretical approaches to understanding the ontological nature of emotions. Historically, evolutionary theoretical approaches to emotions, including basic emotion theory, have postulated that certain so-called basic emotions (usually fear, joy, anger, disgust, and sadness) have evolved over human phylogeny to serve specific functions (for example, fear alerts a human mind of imminent danger). So-called basic emotions are often linked causally to subcortical structures of the brain, including the amygdala (pronounced uh-MIG-duh-luh). In other words, subcortical structures have historically been considered the causes of emotions, while neocortical (neo- meaning new, recent and cortical meaning relating to cortex) structures, especially the prefrontal cortex, are almost invariably understood as the cause of reason. Those ideas about the brain are old; they're traceable at least to Aristotle and were later incorporated into Paul MacLean's mistaken model of brain organization, the "triune brain." These ideas have led to the widespread, erroneous belief that animal brains, including human brains, evolve in a linear fashion, such that, along the course of evolution, new layers of brain tissue are stacked upon older layers of brain tissue, much like the formation of sedimentary rocks. Brain evolution is a lot more complicated than that.

Evolution and natural selection has been applied to the study of human communication, mainly by Charles Darwin in his 1872 work, The Expression of the Emotions in Man and Animals. Darwin researched the expression of emotions in an effort to support his materialist theory of unguided evolution. He proposed that much like other traits found in animals, emotions apparently also evolved and were adapted over time. His work looked at not only facial expressions in animals and specifically humans, but attempted to point out parallels between behaviors in humans and other animals.

Evolutionary psychologists consider human emotions to be best adapted to the life our ancestors led in nomadic foraging bands.

Darwin's original plan was to include his findings about expression of emotions in a chapter of his work, The Descent of Man, and Selection in Relation to Sex (Darwin, 1871) but found that he had enough material for a whole book. It was based on observations, both those around him and of people in many parts of the world. One important observation he made was that even in individuals who were born blind, body and facial expressions displayed are similar to those of anyone else. The ideas found in his book on universality of emotions were intended to go against Sir Charles Bell's 1844 claim that human facial muscles were created to give them the unique ability to express emotions. The main purpose of Darwin's work was to support the theory of evolution by demonstrating that emotions in humans and other animals are similar. Most of the similarities he found were between species closely related, but he found some similarities between distantly related species as well. He proposed the idea that emotional states are adaptive, and therefore only those able to express certain emotions passed on their characteristics.

In the 1872 work, Darwin proposed three principles. The first of the three is the "principle of serviceable habits", which he defined as we have certain habits or we perform different actions in certain states of mind, which get associated when that state of mind is induced, even when its not needed then. He used as an example contracting of eyebrows (furrowing the brow), which he noted is serviceable to prevent too much light from entering the eyes. He also said that the raising of eyebrows serves to increase the field of vision. He cited examples of people attempting to remember something and raising their brows, as though they could "see" what they were trying to remember.

The second of the principles is that of antithesis. While some habits are serviceable, Darwin proposed that some actions or habits are carried out merely because they are opposite in nature to a serviceable habit, but are not serviceable themselves. Shrugging of the shoulders is an example Darwin used of antithesis, because it has no service. Shoulder shrugging is a passive expression, and very opposite of a confident or aggressive expression.

The third of the principles is expressive habits, or nervous discharge from the nervous system. This principle proposes that some habits are performed because of a build-up to the nervous system, which causes a discharge of the excitement. Examples include foot and finger tapping, as well as vocal expressions and expressions of anger. Darwin noted that many animals rarely make noises, even when in pain, but under extreme circumstances they vocalize in response to pain and fear.

Paul Ekman is most noted in this field for conducting research involving facial expressions of emotions. His work provided data to back up Darwin's ideas about universality of facial expressions, even across cultures. He conducted research by showing photographs exhibiting expressions of basic emotion to people and asking them to identify what emotion was being expressed. In 1971, Ekman and Wallace Friesen presented to people in a preliterate culture a story involving a certain emotion, along with photographs of specific facial expressions. The photographs had been previously used in studies using subjects from Western cultures. When asked to choose, from two or three photographs, the emotion being expressed in the story, the preliterate subjects' choices matched those of the Western subjects most of the time. These results indicated that certain expressions are universally associated with particular emotions, even in instances in which the people had little or no exposure to Western culture. The only emotions the preliterate people found hard to distinguish between were fear and surprise. Ekman noted that while universal expressions do not necessarily prove Darwin's theory that they evolved, they do provide strong evidence of the possibility. He mentioned the similarities between human expressions and those of other primates, as well as an overall universality of certain expressions to back up Darwin's ideas. The expressions of emotion that Ekman noted as most universal based on research are: anger, fear, disgust, sadness, and enjoyment.