Why did the human brain size evolve? A way forward

Why the human brain size evolved has been a major evolutionary puzzle since Darwin but addressing it has been challenging. A key reason is the lack of research tools to infer the causes of a unique event for which experiments are not possible. We describe how the analogous problem of why there is day and night has been successfully addressed in physics and learning from that experience, we outline a strategy to address why the human brain size evolved: hypotheses are expressed in mechanistic models that yield quantitative predictions for evolutionary and developmental trajectories of brain and body sizes, the predicted trajectories are compared to data, and models are chosen by their ability to explain the data. By pursuing this strategy, we present results from one model that predicts evolutionary and developmental trajectories for six hominin species. We compare these predictions to data, finding that the model recovers multiple but not all aspects of hominin evolution and development. Counterintuitively, the human brain size evolves in this model as a spandrel, that is, as a byproduct of selection for something else, specifically, preovulatory ovarian follicles. Our analysis describes an alternative way forward to infer why the human brain size evolved.