There is no scientific consensus on why the human brain has such a large relative size, compared to other living beings. Numerous evolutionary theories have tried to explain this singularity, but none has been able to discern whether its growth is a cause or an effect of other factors.

One of the best known hypotheses is that our brain grew to allow our ancestors to function better in an increasingly complex society. Another hypothesis is that this increase in size is related to the fact that our ancestors began to eat meat. The greater protein contribution would have allowed the reduction of the digestive system, in favor of an increase in brain mass.

A new study published this week in Nature rejects those hypotheses. "Our results indicate that ecology has been a determining factor in the evolution of the size of the human brain, and not social aspects such as cooperation or competition", explains Sinc Mauricio González-Forero, researcher at the Faculty of Biology at the University of Saint Andrews (United Kingdom).

Among these ecological factors are problems such as finding food, storing it, and processing it to consume it. "The hunter-gatherers who live in the African savanna solve these problems through animal tracking skills, construction of tools such as bottles and leather containers, and with the production and control of fire to cook food," explains the researcher.

The study concludes that when the environment is inhospitable and individuals can continue to learn how to solve problems long after childhood - for example, because they can learn difficult techniques from other individuals - that combination between ecology and knowledge accumulation produces brains of size human.

On the trail of a larger brain

With the help of a computational model, the authors have analyzed the energy costs and benefits provided by a larger brain. The larger the size, the more energy is consumed and the less energy available for other functions, such as the reproductive organs. However, a larger brain also tends to allow the individual to solve more complex problems.

"The model calculates how large the brain should be as a result of natural selection when individuals have evolved by finding problems of different types. We have considered ecological problems and three types of social problems (cooperation, competition between individuals, and competition between groups), "explains González-Forero.

In this way, 60% of the determining factors are of an ecological nature, 30% would be related to cooperation and only 10% would be based on competition between groups. The competition between individuals would not have been relevant for the evolution of the brain.

These percentages are consistent with the fact that human psychology is characterized by its tendency to cooperate. Cooperation among individuals plus competition between groups, which involves cooperation among individuals in the group, provides a high proportion of cooperation problems - 40% - that could have shaped human psychology.

"Our model refutes the hypothesis that the human brain expanded throughout evolution due to social demands. On the contrary, we found that such demands contribute to decrease the size of the brain ", explains González-Forero.

"That does not mean that we should diminish our social interactions to promote a bigger brain, because the consequences of something like that would take hundreds of thousands of years to have an effect and could involve negative consequences that the model does not anticipate," concludes the researcher.