Brain size linked to ecology: Scientists discover it wasn’t a complex social network that grew the prehistoric human brain; it was how far they traveled
11/24/2018 // Russel Davis // Views

Social group size has nothing to do with the evolutionary process that increased the human brain's size, a new study reveals. The scientific community has previously believed that the expansion of peer-to-peer and social interaction, more commonly known as the social brain hypothesis, may have contributed to the increase in brain size by enabling early humans to move about in bigger and more complex social groups and manage these relations. However, a team of international researchers at the University of Durham in the U.K. and the University of Zurich in Switzerland has challenged this long-held notion.

The research team analyzed more than 140 primate species of primates -- such as apes, monkeys, chimpanzees, and lemurs -- as part of the study. The experts then pooled the data and compared them against two major theories: social brain hypothesis and a competing hypothesis that focused on ecological factors. The competing hypothesis has emphasized that the increase in human brain size could be a result of various ecological factors such as eating habits and the size of the home range where the species live.

The results show that very little data support the social brain hypothesis. On the other hand, the research team had observed a stark correlation between ecological factors and human brain size, particularly when it comes to home range.

"We found evidence in both datasets for associations between brain size and ecological variables (home range size, diet and activity period), but little evidence for an effect of social group size, a correlation which has previously formed the empirical basis of the social brain hypothesis. However, reflecting divergent results in the literature, our results exhibited instability across datasets, even when they were matched for species composition and predictor variables. We identify several potential empirical and theoretical difficulties underlying this instability and suggest that these issues raise doubts about inferring cognitive selection pressures from behavioural correlates of brain size," the researchers noted in the Proceedings of the Royal Society B website.


Greater home range contributes to increase in brain size

The experts stress that early humans and hominids had greater home range compared with primates and other monkeys. This increase in home range provided early humans with greater access to a wide array of food sources and ecological niches, the experts explain. Likewise, the scientists note that a far-reaching home range has benefited ancient humans, particularly the Homo erectus who had been able to hunt larger animals.

The scientists also note that using the social brain hypothesis falls short on effectively determining whether brain size indicates cognitive ability. The research team has stressed that relying on brain size alone may not provide a clear picture of the species' cognitive ability as nocturnal primates have large areas of grey matter that focuses on both smell and hearing, while diurnal primates such as humans have more neurons that promote vision. (Related: New study examines complexity of dolphin culture; researchers determine that brain size correlates with “human-like” behaviors and societies.)

"Our research has shown the traditional approach of looking for correlations between brain size and behavior may not tell us much. To better understand the evolution of the brain, we need to stop thinking of it as one single organ, and instead view it as a complex mosaic, in which different components specialize in different functions. It is unlikely the way these different components evolve can be explained by a single factor, such as social complexity," lead author Lauren Powell has told Daily Mail online.

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