【進化】どうしてヒトの脳はこんなに大きくなったのか

Ecology was the major factor driving the evolution of our big human brains, a Nature paper suggests. The study helps to inform a major debate in the story of human evolution.

Researchers have spent decades arguing over why the human brain has evolved to be so unusually large. A number of theories exist; among them the ‘social brain hypothesis’, which suggests that bigger brains evolved to help manage our increasingly complex social lives, and the ‘expensive tissue hypothesis’, which posits that meat-eating allowed brains to evolve at the expense of the gut. A fundamental problem with these theories, however, is that they rely on correlative data and so are unable to disentangle cause and effect.

According to a new predictive model by Mauricio Gonzalez-Forero and Andy Gardner, human brain size evolved in response to a number of different factors that were 60 per cent ecological, 30 per cent cooperation-related and 10 per cent related to competition between groups. Competition between individuals was relatively unimportant. The findings are intriguing 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 manoeuvring.