Generally speaking, larger brains* are more capable of storing and processing information and therefore achieve higher intelligence. Although this is a little simplistic (and does not factor in the degree of neuronal connectivity - where “smarter” brains are also more intra-connected), human brains have been getting progressively bigger and more intelligent. According to the cognitive psychologist Prof. Steven Pinker, this improvement is occurring at a rate of ~3 IQ points per decade! As flattering as this is for humankind, the question remains: WHY are our brains getting bigger?
Dr. Michael Muthukrishna from the London School of Economics and his colleagues study social intelligence. They theorised that the development of culture and its associated demands on information storage and processing are a key driver behind an increase in brain size. They named this the Cultural Brain Hypothesis (CBH), and set out to test for it.
Culture is a product of sociality. Sociality evolves when the benefits of living in close association with conspecifics exceed the costs. These benefits include foraging success and predator avoidance. Due to these benefits, sociality is a widespread evolutionary phenomenon. By its very nature, sociality requires group living and often leads to the social transmission of information between group members, including to younger generations. Socially transmitted behaviours within a group are often broadly termed as “culture”, and though culture is most often discussed in reference to humans, it is certainly not limited to us. According to the CBH, the presence of culture presents a considerable information load on the members of the cultured society, and may require larger brains to store and process this information – leading to the observed upward trend in brain volumes.
Dr. Michael Muthukrishna and colleagues set out to test the CBH using both computational models and empirical data. In both cases, they found that progressions from asociality to culture correlated with increased brain volumes in many animal lineages, supporting the CBH. This trend was found to be most pronounced in humans, whose brains have trebled in size over the last few million years.
Increases in brain volume are a little puzzling due to the huge energy requirements of brain tissue. For the average human the basal metabolic rate of the brain is 10.8 calories per hour, 20% of their total basal calorific requirement. Therefore, larger brains have evolved for a reason, and their benefits must warrant their energy costs, such as the facilitation of social living and culture. Muthukrishna’s study builds upon existing research into the influences of ecological, environmental and social factors on brain size, helping us piece together the evolutionary puzzle. A better understanding of the mechanisms underlying changes in brain volume can give us a clearer picture of where we have come from, where we are now, and where we are heading.
*In this article discussions of brain size refer to the brain:body ratio, i.e. beagle dogs and stegosaurus dinosaurs have/had brains of ~70g. However, a beagle weighs ~10kg, and a stegosaurus weighed ~1.5 tonnes, which goes some way to explain why dogs are hailed as highly intelligent animals… and dinosaurs are not. And yes, the human lineage has become larger and heavier over time, however according to published literature the brain:body ratio has not remained constant, but instead, has increased.