Somatic Marker Hypothesis - Evolutionary Evidence

Evolutionary Evidence

To support his claims about the evolution of the human race, Charles Darwin wrote The Expression of Emotions in Man and Animals in which he noted the similarities between emotional facial expressions between humans and other animals. In this book, Darwin argues that human emotion-evoked expressions, like furrowing of the eyebrows and tears, are similar to animal facial expressions and is evidence of our evolutionary origins. Today, the existence of emotion in animals is controversial, but current research suggests that intelligent, social animals (e.g., primates, dolphins, parrots) do experience primary emotions like fear, joy, anger, and disgust. In particular, our closest living relatives, the great apes, are prime candidates for having the most developed capabilities for emotions, empathy, and theories of mind.

Within an evolutionary framework, emotions in humans and animals can be defined as coordinated cognitive and physiological responses that enable an individual to respond effectively to environmental opportunities and communicate with others. For example, anger is not simply a specific facial expression or neural activation, rather it is a set of coordinated responses that help an individual express dissatisfaction and perhaps restore relations. Emotions are adaptive because they are efficient responses that help organisms reproduce, protect offspring, maintain cooperative alliances, and avoid physical dangers. Emotions and their influence on motivation and certain physiological responses, prepare individuals for actions that are in their best interest. In humans, for example, anger is associated with enhanced blood flow to the hands for fighting an enemy, whereas fear causes less blood flow to the periphery to help escape an attack without too much blood loss.

Yet humans have a much larger array of finely tuned emotions compared to animals, even the great apes. It has been hypothesized that concurrent selection for greater social cohesion and organizational flexibility in early hominids may have been the impetus for increasing the human emotional repertoire. The elaboration of human emotions may have aided social bonds if genetic relatedness was low within a social group. Consequently, the increase in the spectrum of human emotions could also be used to tag dimensions within their environment with emotional value that could be used in the future to guide difficult decisions that had not yet been encountered by animals— such as the abstract and complex issues surrounding morals and social rules.

According to the Social brain hypothesis the demand for the ability to solve complex social problems initiated the evolutionary trend for increases in brain size observed among humans and other primates. This increase is mostly due to an increase in prefrontal cortex volume. The evolution of the human prefrontal cortex allows humans to think and process information in more abstract ways. By tagging particular stimuli with an emotion associated with a bodily change, it helps to speed up the process of decision-making when it is encountered by eliminating unsuitable choices and ensuring only advantageous options are considered and fully processed. This is most important in the social domain since social environments are complex, unpredictable and more responsive to one’s own behavior than the physical environment. Since social interactions are complex, evaluating the appropriate behavioral response in these situations requires greater brain processing capacity involving multiple brain structures. Given that there is a large amount of information that needs to be processed in a short amount of time, it is advantageous to have a “fast-track decision-making” mechanism (i.e., somatic markers), that can respond rapidly in the most advantageous fashion. In essence, the SMH provides neurobiological evidence of what has often been referred to as “hunches” or “gut-feelings”.

The SMH is supported by human lesion studies suggesting that predictions of future outcomes or consequences that are near in time require processing in the more posterior portion of the ventromedial (VM) cortices and representations of outcomes or consequences not in the immediate future are processed in the anterior portion of the VM cortices. Patients with damage to only the anterior VM cortices, are more likely to engage in behaviors that negatively impact personal relationships in the distant future. However, they will never engage in actions that would lead to immediate harm to themselves or others. This brain organization pattern appears to be rooted in evolution. The functional evolution of the prefrontal cortex involves increases in the ability to represent events that may occur in the future. The ability for humans to think about future consequences of their actions coincides with the development of the more anterior portions of the VM cortices. This is also supported by human neuroimaging studies demonstrating that tasks involving moral or ethical decisions evoke increased activity in the most anterior sections of the VM cortex. Finally, comparative studies in neuroanatomy demonstrate that advancements in the size and connectivity seen in human brains primarily relates to the evolutionarily younger anterior portions of the frontal lobes as opposed to the more ‘ancient’ posterior areas. Consequently, Damasio suggests that the ability of humans to cope with such degrees of abstract thinking quickly and efficiently coincides with both the development of the anterior regions of the VM cortex and the use of somatic markers to guide human behavior.