The “expensive brain” framework proposes that the costs of an increase in brain size can be met by any combination of increasing the total energy turnover or reducing energy allocation to other expensive functions, such as maintenance (digestion), locomotion, or production (growth and reproduction). Here, we explore its implications for human evolution. Using both comparative data on extant mammals and life-table simulations from wild extant apes, we show that primates with a hominoid lifestyle face a gray ceiling that limits their brain size, with larger values leading to demographic nonviability. We argue that cooperative care provides the most plausible exaptation for the increase in brain size in the Homo lineage.
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