The Evolution of Human Metabolism

From an evolutionary perspective, life is a game of turning energy into offspring. The strategies that species use to acquire energy, in the form of food, and allocate energy to the essential tasks of growth, maintenance, movement, and reproduction, are incredibly diverse and reflect the ecological pressures and opportunities encountered. In this talk, I discuss the deep evolutionary history of the human metabolic strategy and our divergence from other apes, focusing on metabolic changes over three timescales. First, from measures of daily energy expenditure (kilocalories per day) in living primates, we see evidence of a metabolic slow down early in the evolution of the primate order, perhaps 65 million years ago. Primates today burn roughly 50% less energy each day than other placental mammals, and this slower metabolism likely underlies the slow pace of growth, reproduction, and aging in humans and other primates. Second, metabolic rate accelerated in the human lineage as it diverged from that of chimpanzees and bonobos. The timing of this increase in daily energy expenditure likely coincides with the development of hunting and gathering with the genus Homo, beginning ~2.5 million years ago. Today, humans' greater daily expenditures fuel our big brains, big babies, and high levels of physical activity. Lastly, when we examine metabolic adaptations over a lifetime, we find that the metabolisms of humans and other species are dynamic and responsive, changing in response to physical activity and diet to keep daily energy expenditure within a narrow range. Understanding our metabolic flexibility - changing energy allocation among tasks in response to local conditions - is important for reconstructing our evolutionary past and for managing our health today.