Humans evolved their relatively large brain, compared to other primates, thanks to an increased metabolic rate, reports a paper published in Nature. The study also suggests that humans may have evolved a much higher body fat percentage than other primates to provide the energy reserves required to fuel their more extensive metabolism.

Humans are different from other primates in that they live longer, breed more, have more body fat, relatively smaller guts, and relatively large brains. These traits are metabolically expensive, which suggests there are significant differences in energy expenditure and allocation between humans and apes. However, the mechanisms underlying these differences have remained unknown.

Herman Pontzer and colleagues took direct measurements of daily energy expenditure in large samples of humans (141 people) and all known species of great ape, as well as revisiting archival data. They show that humans have evolved a faster metabolism and larger energy budget, with total energy expenditure (TEE) greater than that of chimpanzees and bonobos, gorillas, and orangutans by approximately 400, 635 and 820 kcal per day, respectively. They find that much of the increase in TEE is attributable to humans’ greater basal metabolic rate - the amount of energy required to keep the body functioning at rest, measured in kcal per day - which indicates increased metabolic activity in their organs. Finally, the authors show that humans evolved a much higher body fat percentage than other primates, and suggest that this coevolved with greater TEE to mitigate the inherent risk of increased energy demands.