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The social brain hypothesis argues that the human brain did not increase in size to solve increasingly complex problems, but as a means of surviving and reproducing in complex social groups (Dunbar, 2009). The social brain hypothesis is one of the most largely held views when it comes to explaining primate encephalization. However, an analysis of new phylogeny and more primate samples shows that differences in human and non-human primate brain evolution come down to diet, not sociality.

Diet is one of the most important factors in regards to brain and body size. The more high-quality food an animal has, the bigger its brain and body will be. Using a larger sample (3 times as large, 140 primates), more recent phylogenies (which show inferred evolutionary relationships amongst species, not which species is ‘more evolved’ than another), and updated statistical techniques, Decasien, Williams, and Higham (2017) show that diet best predicts brain size in primates, not social factors after controlling for body size and phylogeny (humans were not used because we are an outlier).

The social scheme they used consisted of solitary, pair-living, harem polygyny (one or two males, “a number of females” and offspring), and polygynandry (males and females have multiple breeding partners during the mating season). The diet scheme they used consisted of folivore (leaf-eater), frugivore-folivore (fruit and leaf eater), frugivore (fruit-eater) and omnivore (meat- and plant-eaters).

None of the sociality measures used in the study showed a relative increase in primate brain size variation, whereas diet did. Omnivores have bigger brains than frugivores. Frugivores had bigger brains than folivores. This is because animal protein/fruit contains higher quality energy when compared to leaves. Bigger brains can only evolve if there is sufficient and high-quality energy being consumed. The predicted difference in neurons between frugivores and folivores as predicted by Herculano-Houzel’s neuronal scaling rules was 1.08 billion.

The authors conclude that frugivorous primates have larger brains due to the cognitive demands of “(1) necessity of spatial information storage and retrieval; (2) cognitive demands of ‘extractive foraging’ of fruits and seeds; and (3) higher energy turnover and enhanced diet quality for energy needed during fetal brain growth.” (Decasien, Williams, and Higham, 2017). Clearly, frugivory provided some selection pressures, and, of course, the energy needed to power a larger brain.

The key here is the ability to overcome metabolic constraints. Without that, as seen with the primates that consumed a lower-quality diet, brain size—and therefore neuronal count—was relatively smaller/lower in those primates. Overall brain size best predicts cognitive ability across non-human primates—not encephalization quotient (Deaner et al, 2007). Primate brains increase approximately isometrically as a function of neuron number and its overall size with no change in neuronal density or neuronal/glial cell ratio with increasing brain size (in contrast to rodent brains) (Herculano-Houzel, 2007). If brain size best predicts cognitive ability across human primates and primate brain size increases isometrically as a function of neuron number with no change in neuronal density with increasing brain size, then primates with larger brains would need to have a higher quality diet to afford more neurons.

The results from DeCasien, Williams, and Higham (2017) call into question the social brain hypothesis. The recent expansion of the cerebellum co-evolved with tool-use (Vandervert, 2016), suggesting that our ability to use technology (to crush and mash foods, for instance) was at least as important as sociality throughout our evolution.

The authors conclude that both human and non-human primate brain evolution was driven by increased foraging capability which then may have provided the “scaffolding” for the development of social skills. Increased caloric consumption can afford larger brains with more neurons and more efficient metabolisms. It’s no surprise that frugivorous primates had larger brains than folivorous primates. Just as Fonseca-Azevedo and Herculano-Houzel (2012) observed, primates that consumed a higher quality diet had larger brains.

In sum, this points in the opposite direction of the social brain hypothesis. This is evidence for differing cognitive demands placed on getting foods. Those who could easily get food (folivores) had smaller brains than those who had to work for it (frugivores, omnivores). However, to power a bigger brain the primate needs the energy from the food that takes the complex behavior—and thus larger brain—to obtain. This lends credence to Lieberman’s (2013) hypothesis that bipedalism arose after we came out of the trees and needed to forage for fruit to survive.

Brain size in non-human primates is predicted by diet, not social factors, after controlling for body size and phylogeny. Diet is the most important factor in the evolution of species. With a lower quality diet, larger brains with more neurons (in primates, 1 billion neurons takes 6 kcal per day to power) would not evolve. Brain size is predicated on a high-quality diet, and without it, primates—including us—would not be here today. Diet needs to be talked about a lot more when it comes to primate evolution. If we would have continued to eat leaves and not adopt cooking, we would still have smaller brains and many of the things that immediately came after cooking would not have occurred.

Since we are primates we have the right morphology to manipulate our environment and forage for higher quality foods. But those primates with access to foods with higher quality have larger brains and are thus more intelligent (however, there are instances where primate brain size increases and decreases and it comes back to, of course, diet). Sociality comes AFTER having larger brains driven by nutritional factors—and would not be possible without that. Social factors drove our evolution—no doubt about it. But the importance of diet throughout hominin evolution cannot be understated. Without our high-quality diet, we’d still be like our hominin ancestors such as Lucy and her predecessors. Higher quality diet—not sociality, drives primate brain size.

References

DeCasien, A. R., Williams, S. A. & Higham, J. P. Primate brain size is predicted by diet but not sociality. Nat. Ecol. Evol. 1, 0112 (2017).

Deaner, R. O., Isler, K., Burkart, J., & Schaik, C. V. (2007). Overall Brain Size, and Not Encephalization Quotient, Best Predicts Cognitive Ability across Non-Human Primates. Brain, Behavior and Evolution,70(2), 115-124. doi:10.1159/000102973

Dunbar, R. (2009). The social brain hypothesis and its implications for social evolution. Annals of Human Biology,36(5), 562-572.

Fonseca-Azevedo, K., & Herculano-Houzel, S. (2012). Metabolic constraint imposes tradeoff between body size and number of brain neurons in human evolution. Proceedings of the National Academy of Sciences,109(45), 18571-18576. doi:10.1073/pnas.1206390109

Herculano-Houzel, S. (2007). Encephalization, neuronal excess, and neuronal index in rodents. Anat. Rec. 290, 1280–1287.

Lieberman, D. (2013). The story of the human body: evolution, health and disease. London: Penguin Books.

Vandervert, L. (2016). The prominent role of the cerebellum in the learning, origin and advancement of culture. Cerebellum & Ataxias,3(1). doi:10.1186/s40673-016-0049-z