In the 19th century, Charles Darwin put forward theories about the origin of species, but also about the evolution of emotion. In his 1872 book The Expression of the Emotions in Man and Animals, he argued that there is a set of primal emotions, such as fear, love, anger, disgust and so on, that are present throughout different species.

These occur across mammalian species, even in societies with limited interaction with the outside world, implying a distinct hereditary link. As Galen described, these emotions are indeed processed by the brain and our ability to do this is itself a result of evolution.

The role of emotions in evolution is as a unique example in psychology, and a tool in future evolution and speciation. How has this ability evolved and what part will it play in evolution to come?

Primal emotions are an innate part of the human psyche – they have had the most profound effect on our evolution. One such emotion is fear, which has been researched in great detail by American neuroscientist Joseph E. LeDoux, founder of the Emotional Brain Institute in New York.

LeDoux separates our fear-based response into two circuits – one an instinctive survival response, and the other involving emotion and critical thinking. The tiny, almond-shaped part of the brain known as the amygdala is programmed to react regardless of whether there is input from the ‘thinking’ part of the brain, the prefrontal cortex.

The amygdala response is more of a defensive survival mechanism – it detects threats, and initiates appropriate responses. This is not a wilful mechanism; according to LeDoux, it could go back to the beginning of life.

The emotion of fear, controlled by the prefrontal cortex, is subjective and can be conditioned. It quantifies emotional responses to this survival mechanism, although this is a much slower reaction.

The development of these responses as a whole can be linked to the growth in the size of the amygdala. When human brains are compared to primates in antecedent stages of evolution, the amygdala volume is greater – this results in a more developed fear complex.

In some ways, the evolution of emotion is a cyclical process. Physiological development allows for increased emotional processing capability, which in turn allows for the enactment of natural selection, which leads back again to advancements in physiology. Our emotions play a crucial role in evolution.

Considering fear again, it plays a crucial role in the ‘fight or flight’ mechanism for survival. When we sense a threat, our amygdala initiates the defensive survival mechanism that LeDoux described. As a result, the sympathetic nervous system, which controls our stress-based response, is activated. Stress hormones, such as adrenaline, are also released in the endocrine system. Our body is now prepared to fight, or to run away.

This mechanism plays a crucial role in the survival of organisms, and as a result their capability to reproduce. Animals that respond more actively are more likely to survive. It is because of this response that the inheritance of those advantageous alleles (variations of genes) that caused their survival can occur in natural selection, contributing to an increased rate of evolution.

Considering sexual reproduction, a critical factor is the mating behaviours exhibited by varying species. These behaviours are influenced by the various emotions experienced by humans and other species when seeing members of the opposing sex, such as love and lust. The variety of behaviours contribute to sympatric speciation – meaning organisms with differing behaviours do not mate, leading to a separation of their gene pools. This eventually leads to the gene pools becoming two separate species.

Importantly, emotions such as love contribute to parental and alloparental care (care for young that are not the carer’s own offspring), which helps protect offspring for future reproduction. This is supported by the synthesis of oxytocin, a hormone known colloquially as the ‘love hormone’, which is triggered when such emotions are felt, strengthening these feelings.

Our individual feelings may seem menial at times, but it’s incredible to consider how much of an impact they can have on our environment. After all, evolutionary changes in humans can have widespread impacts on other species. These primal emotions have truly stood the test of time and will only grow in significance in generations to come.