How Did Our “Social Brain” Evolve?

Beginning about 2.5 million years ago a particular organ underwent a threefold increase in volume, leading to the creation one of nature’s most complex, social, and efficient structures: the human brain. The explanations as to what drove this progressive development are still debatable, yet evolutionary neuropsychologists believe that it may be due to a variety of selection pressures, some of which include physical (e.g., climate, diet, and food availability) and social (e.g., group size, coalition formation, and parent care). But instead of differentiating drivers of neurological evolution, scientists have recently been trying to develop explanatory frameworks that integrate the various social and ecological correlates of our inherent neurological complexity.

One of the first questions we should ask ourselves when exploring the origins of the social brain is, “What came first? The physical selection pressure, or the social selection pressure?” In other words, did our ancient ancestors become intelligent by eating something first, which in turn developed the uniqueness of our cognitive capabilities as social animals, or was it that we started to feel the need to interact, socialize, and communicate first, before deciding what to eat for dinner? A recent paper published in 2017 in the journal Nature Ecology & Evolution by DeCasien, Williams, and Higham titled, “Primate brain size is predicted by diet but not sociality,” became one of the first papers to discover that brain size, evolutionarily speaking, was driven more by dietary factors than by social factors.

Alexandra DeCasien and her colleagues found that what a species eats predicts its brain size more than whether it is social or not. In fact, species whose diet is predominantly made up of fruit have, on average, larger brains than those that specialize on eating leaves. By utilizing the largest data set ever brought to bear on this question, with more than three times the number of primate species than have previously been used, the researchers were able to call into question the long-held “social brain hypothesis” — which asserted that social complexity was the predominant evolutionary force driving human intelligence.

A 2007 article published in Science by Dunbar and Shultz, titled “Evolution in the Social Brain,” suggested, to the contrary, that the particular demands of more intense forms of pair-bonding became a critical factor for triggering our evolutionary development of intelligence. Their analysis showed that, among primates, relative brain size (usually indexed as relative size of the neocortex) correlates with many indices of social complexity including social group size, the frequency of coalitions, male mating strategies, the prevalence of social play, and the frequency of social learning. The fact that only higher primates exhibit a correlation between social group size and relative brain (or neocortex) size when compared to other mammals strongly suggests that at some early point in our evolutionary history, we used cognitive skills to create relationships that are not just for reproduction. However, DeCasien and her colleagues challenge the emphasis on social, rather than ecological, explanations.

Some studies have shown that polygynandrous primate species have the largest brains. This is consistent with the notion that systems that promote high social relationships between the greatest numbers of individuals may be the most cognitively demanding. Conversely, other studies have shown that monogamous species have the largest brains, due to the fact that monogamy may require greater cognitive deception skills, in order to obtain extra-pair copulations that require conflict resolution and coordination abilities for bond maintenance. So even though the social brain hypothesis may not be the full answer as to why we are so intelligent in general — why did we develop such a system? What has it offered us as a species? And by what mechanisms, evolutionarily speaking, does it operate?

Dr. Ralph Adolphs, a specialist in the neural underpinnings of social behavior at Caltech, published a paper in 2009 in the Annual Review of Psychology, titled “The Social Brain: Neural Basis of Social Knowledge,” that proposed certain neural networks may have evolved over time to create our modern-day social brain. The amygdala, a part of the brain that is usually associated with the fear response and motivation, seems to have been developed in humans in order to attach emotional value to faces within a social context. Adolphs describes how this enables us to recognize expressions such as fear and trustworthiness, while the posterior superior temporal sulcus has been developed to aid us with seeing the world from another’s perspective. The posterior superior temporal sulcus and temporo-parietal junction help enable us to realize that other people may have different knowledge from us, and are therefore worth learning from, speaking with, and even may sometimes have false beliefs about the world.

Another brain region, the medial prefrontal cortex (MPFC), could also be crucial in allowing us to think about the mental states of others and ourselves. The MPFC may have developed over time to register our full engagement in real-time social interactions, or even when simply observing social interactions. Many times these situations involve predicting people’s behavior in regards to their current beliefs and intentions. So much of what defines the quality of our social experience comes from our ability to accurately perceive what others are thinking (i.e. “person perception”) and what we are thinking ourselves (i.e. “self-perception”).

In his 2009 paper, Adolphs describes how our ability to make inferences about what is going on inside other people (their intentions, feelings, and thoughts) during social situations may account for some of the uniqueness of our species in regards to the development of culture, civilization, and language. Due to our social nature as a species, language was able to stimulate trade, commerce, social connectivity, and bonding behaviors. Studies on the development of human speech and language utilizing functional MRI and EEG (electroencephalograph) analysis have shown that infants learn rapidly and robustly from linguistic exposure in social settings, and that the timing and the context of social interactions are both critical.

A paper published by Cold Spring Harbor Laboratory Press in 2014 by Patricia K. Kuhl, titled “Early Language Learning and the Social Brain,” described how recent brain imaging results of infants listening to speech show that hearing speech activates and develops motor planning areas (Broca’s area and the cerebellum) in the infant brain. When infants begin life, they are neurologically “prepared” to be social, learn any language, and have an especially extraordinary capacity to learn from social experiences during prime periods of cognitive development. This may be because action-perception systems in the human brain enable learning from other humans in social situations in a particularly special way.

A 2009 paper by Uta Frith and Chris Frith titled, “The social brain: allowing humans to boldly go where no other species has been,” describes that it is only recently that we have begun to explore the biological basis of our social abilities, language, and its neurobiological evolution. The neurological basis of complex human social communication has been elucidated through various comparative studies of other species, studies of disorders of social cognition, and developmental psychology. Researchers have discovered that the use of deliberate social signals, intention, and connection can serve to increase one’s reputation, trust, and facilitate group learning — a key arbiter for developing culture and society.

Even though social psychologists have been investigating social behavior for more than a century, their work has developed in isolation from the rest of neurobiology. The concept of the “social brain” demands a marriage of social psychology and evolutionary neurobiology in order to pave the way to a greater understanding of why we find ourselves in such a need for communication.

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