A bioenergetics model is developed to examine changes in metabolic requirements over the course of human evolution. Data on (1) body size and resting metabolism, (2) brain size and metabolism, (3) activity budgets, and (4) foraging patterns for humans and other anthropoids are used to evaluate ecological correlates of variation in diet and energy expenditure. Analyses of variation in these extant species provide a framework for estimating (1) resting metabolic requirements, (2) brain metabolic needs, and (3) total energy requirements in fossil hominids. Anthropoid primates spend about 8% of resting metabolism to maintain their brains, a significantly larger proportion than in other mammals (3–4%), but still significantly less than 20–25% in humans. Total energy expenditure among anthropoids is positively correlated with day range and dietary quality. Human foragers fit this pattern, having high levels of energy expenditure, large foraging ranges, and a high quality diet. Within the fossil record, it appears that both total energy expenditure (TEE) and energy required by the brain increased substantially with the emergence of Homo erectus. For H. erectus, the percentage of resting metabolism used by the brain falls beyond the nonhuman primate range and approaches the modern human range. Additionally, TEE is 35–55% greater than in the australopithecines. The high total metabolic needs and the large proportion of energy required by the brain imply that important dietary changes occurred with H. erectus. These metabolic and dietary changes are linked to (1) the emergence of hunting and gathering, (2) the evolution of the human pattern of prolonged development, and (3) the coexistence and competition with the robust australopithecines.