Compared to other primates, humans live longer and have more offspring, more body fat, and smaller guts. And we’ve evolved relatively large brains. This suite of traits is energetically expensive, and a faster metabolism made it possible, according to new findings published in Nature this week. 

All animals must allocate their energy to the competing needs of growth, reproduction, and maintenance. As a result, we often have to make trade-offs. A small rodent that reproduces faster than what’s expected for their body mass, for example, will typically have a shorter lifespan. Humans are a bit of a paradox: We reproduce more often and have large newborns, yet we also have the longest lifespans and largest brains of any other great ape. 

To study the mechanisms underlying our differences in energy expenditure, a large international team led by Herman Pontzer of Hunter College took direct measurements of the daily energy expenditure of 141 people and a sample of all known species of great ape: 27 chimpanzees, 8 bonobos, 10 gorillas, and 11 orangutans. The human recruits come from the U.S., Ghana, South Africa, Seychelles, Jamaica, and the other apes live in zoos and sanctuaries in the U.S. and the Democratic Republic of Congo. The team collected urine and saliva samples over the course of 7 to 10 days while all the subjects went about their daily routines. 

They found that humans evolved an accelerated metabolic rate and a larger energy budget. Our total energy expenditure (TEE) is greater than that of orangutans, gorillas, and chimps and bonobos by 820, 635, and 400 kilocalories (kcal) per day, respectively. So while chimps and the closely related bonobos expend less energy a day than we do, they use more energy than any of the other apes – even more than the largest ape of all, the gorillas. 

This increase in our energy expenditure is due to our greater basal metabolic rate, or the amount of energy that’s required when the body is at rest. That means our organs have higher metabolic activity too.

Additionally, the researchers found that humans have a much higher percentage of body fat compared to the other apes who remain pretty lean, even in captivity. This likely co-evolved with our greater total energy expenditure: A fatty reserve would help fuel our increased energy demands. "An increased metabolic rate, along with changes in energy allocation, was crucial in the evolution of human brain size and life history," the authors wrote.

^{Image in the text: Adult female orangutan. Mary H. Brown}