Ecology was the main driving factor which pushed the early humans to develop larger and larger brains, a new study concludes.

Image credits: Chris Stringer, Natural History Museum, United Kingdom.

If there’s one thing that differentiates humans from other animals, it’s our brains. The human brain is abnormally large and has been so for a very long time. It has tripled in size since the time of our Australopithecines ancestors and has since become almost six times larger than what you’d expect for a placental mammal of human size.

The matter is not as simple as it seems, because having a big brain also consumes a lot of resources, and no creature ever spends a lot of resources without gaining something in return. It might be tempting to look back and think that it was all worth it since we are now the dominant species. But for our ancestors a couple of million years ago, that wasn’t a consideration — they were reaping more immediate benefits.

So what were these benefits, and why did the brains start growing in the first place?

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There are several theories, the leading one being the so-called ‘social brain hypothesis’, which suggests that as human communities grew larger and larger, the brain developed to keep up with all of them. Another theory suggests that meat-eating allowed brains to evolve at the expense of the gut. However, neither of these theories has conclusive evidence backing them up, instead relying only on correlations.

This is where Mauricio González-Forero and Andy Gardner’s study enters the stage. The two developed a predictive model for studying brain growth, assessing what the factors that drove brain growth were. It’s the first study with causal, quantitative evidence regarding brain growth.

“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,” the researchers write in the study.

They found that the factors are 60% ecological, 30% cooperative and 10% related to group competition. Therefore, this tends to disprove both theories mentioned above.

“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.”

An interesting consequence of the study is that social complexity appears to be the effect of brain growth, and not the cause. In other words, it’s not that the brain grew to keep up with our social circle, but our social circle grew because we were able to keep up with it. It also suggests that the way our brain developed (which essentially, makes for human nature) is more likely to stem from ecological problem-solving than social or

The study has been published in Nature.