The human brain is unusually large. It has tripled in size from Australopithecines to modern humans1 and has become almost six times larger than expected for a placental mammal of human size2. Brains incur high metabolic costs3 and accordingly a long-standing question is why the large human brain has evolved4. The leading hypotheses propose benefits of improved cognition for overcoming ecological5,6,7, social8,9,10 or cultural11,12,13,14 challenges. However, these hypotheses are typically assessed using correlative analyses, and establishing causes for brain-size evolution remains difficult15,16. 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.
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We thank L. Lehmann for advice on the model set-up and for support at the University of Lausanne, where this project was initiated; P. Avila, S. H. Montgomery, S. B. Laughlin, M. Falcone, T. Clutton-Brock, and M. B. Morrissey for discussions; and A. Whiten for feedback. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 701464. A.G. was funded by a NERC Independent Research Fellowship (NE/K009524/1). The contents of this paper reflect only the authors’ view and not the views of their funders.
Nature thanks A. R. DeCasien and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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