Some organisms are brainless but thriving. Brains are expensive to produce and maintain, and in the human lineage, they have grown so large as to incur a substantial metabolic burden as the brain develops.

According to a new paper published in Nature, ecology was the major factor driving the evolution of our big human brains. After decades of arguing over why the human brain has evolved to be so unusually large, the newly published study helps to inform a major debate in the story of human evolution.

A number of theories attempted to explain the status of the human brain. Among those theories is the social brain hypothesis which suggests that bigger brains evolved to help in managing our increasingly complex social lives, and the expensive tissue hypothesis, which suggests that meat-eating allowed brains to evolve at the expense of the gut. But the fundamental problem with these theories is that they rely on correlative data and so are unable to determine the relationship between cause and effect.

A human brain stops growing by the age of ten, long before the body reaches physical maturity, and this costly and fast process of brain growth has been proposed to cause a delay in body growth.

According to a new predictive model by Mauricio González-Forero and Andy Gardner from the University of St Andrews in the UK, human brain size evolved in response to a number of different factors that were 60% ecological, 30% cooperation-related, and 10% related to competition between groups.

This competition between individuals was relatively unimportant. The findings are puzzling because they suggest that social complexity is more likely to be a consequence rather than a cause of our large brain size, and that human nature is more likely to stem from ecological problem-solving and cumulative culture than it is from social interactions.