Why are humans so smart? We must have evolved in an environment that made more intelligent individuals likely to survive. But this just raises new questions: what factors in the environment could have created an evolutionary pressure for intelligence? And how have other species that faced similar pressures ended up evolving?

One prominent hypothesis about our brains is that the human lineage became especially social, which required a suite of advanced cognitive skills to manage the relationships that were intertwined with survival. There’s evidence that social behavior is associated with bigger brains across the primate family, and the same correlation has been found in birds.