Human brains use about 20% of the body's energy when at rest. By comparison, the brains of other apes use about 8% of rest-time energy. How did human brains evolve to become such energy-sucking supercomputers? Here are some competing theories of the main driver for evolutionary changes in brains: - ecological and environmental challenges - social competition (Machiavellian Hypothesis) - social co-operation The last two can be grouped under the general rubric of "social brain" hypotheses. The link between brain size and sociability has been suggested as early as 1850, when Dujardin noticed that mushroom bodies^[structures in insect brains that handle learning and memory] are substantially enlarged in honeybees.^[Dujardin, F. (1850). Mémoire sur le système nerveux des insectes. Ann. Sci. Nat. Zool. 14, 195–206.] Robin Dunbar has observed that in primates, the ratio of neocortex to total brain size increases in proportion to the species' typical group size. --- **Sources** Dunbar, R. I. M. (1998). The social brain hypothesis. Evol. Anthropol. 6, 178–190. Bregman, Rutger, and Elizabeth Manton. Humankind: A Hopeful History. 1st english-language edition. New York: Little, Brown and Company, 2020.