Evolution in the News - June 2018
by Do-While Jones

Think Big

How did we get so big-headed?

The headline of a three-paragraph article on page 17 of the 2 June 2018, issue of New Scientist caught our eye. It claimed to reveal, “The real reason our brains evolved.” Spoiler alert: It didn’t. However, it reminded us that we haven’t addressed brain evolution recently, so we looked into it. The article was based on a study published last month in the professional journal, Nature, so we went straight to the source.


The human brain is unusually large. It has tripled in size from Australopithecines to modern humans and has become almost six times larger than expected for a placental mammal of human size. Brains incur high metabolic costs and accordingly a long-standing question is why the large human brain has evolved. The leading hypotheses propose benefits of improved cognition for overcoming ecological, social or cultural challenges. However, these hypotheses are typically assessed using correlative analyses, and establishing causes for brain-size evolution remains difficult. Here we introduce a metabolic approach that enables causal assessment of social hypotheses for brain-size evolution. Our approach yields quantitative predictions for brain and body size from formalized social hypotheses given empirical estimates of the metabolic costs of the brain. Our model predicts the evolution of adult Homo sapiens-sized brains and bodies when individuals face a combination of 60% ecological, 30% cooperative and 10% between-group competitive challenges, and suggests that between-individual competition has been unimportant for driving human brain-size evolution. Moreover, our model indicates that brain expansion in Homo was driven by ecological rather than social challenges, and was perhaps strongly promoted by culture. Our metabolic approach thus enables causal assessments that refine, refute and unify hypotheses of brain-size evolution.


The leading hypotheses for the evolution of brain size make different suggestions as to which cognitive challenges have been the most important in driving brain expansion. ‘Ecological-intelligence’ hypotheses emphasize challenges posed by the non-social environment, for example, finding, caching or processing food (Fig. 1). By contrast, ‘social-intelligence’ hypotheses emphasize challenges posed by the social environment, for example, cooperating for resource extraction, manipulating others, avoiding manipulation or forming coalitions and alliances to outcompete others (Fig. 1). 1

First, “Why did anyone expect our brains to be six times smaller?” Clearly, their expectations were wrong because our brains are the size they are. End of story.

Second, although this isn’t the first time that someone thought thinking has been responsible for brain evolution, this is the first time that someone has been crazy enough to claim to know what kind of thinking caused brain evolution. They modeled how hard it is to think about different things, and came up with their 60-30-10 conclusion. We question their thinking.

Third, individual competition is the foundation of survival of the fittest (Darwinian Evolution). They found individual competition “unimportant for driving human brain-size evolution.” Hmmmmm.

One of the editors of Nature was sane enough to recognize,

Finally, because the model aims to explain brain size in humans only, the results have no clear significance for debates about the evolution of intelligence in other species. 2

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1 Mauricio González-Forero & Andy Gardner, Nature, 24 May 2018, “Inference of ecological and social drivers of human brain-size evolution”, pp. 554–557, https://www.nature.com/articles/s41586-018-0127-x
2 Richard McElreath, Nature, 23 MAY 2018, “Sizing up human brain evolution”, https://www.nature.com/articles/d41586-018-05197-8