Evolution works on a strict energy budget. Each adaptation burns through a certain number of calories, and each individual can only acquire so many calories in the course of a day. You can’t have flapping wings and a huge body and venom and fast legs and a big brain. If you want to expand some departments, you need to make cuts in others. That’s why, for example, animals that reproduce faster tend to die earlier. They divert energy towards making new bodies, and away from maintaining their own.
But humans, on the face of it, are exceptional. Compared to other apes, we reproduce more often (or, at least, those of us in traditional societies do) and our babies are bigger when they’re born and we live longer. And, as if to show off, our brains are much larger, and these huge organs sap some 20 percent of our total energy.
“We tend to have our cake and eat it too,” says Herman Pontzer from Hunter College. “These traits that make us human are all energetically costly. And until now, we didn’t really understand how we were fueling them.”
Some scientists have suggested that we paid for our large brains by reducing the size of another costly organ—the gut. Others have said that we freed up some energy by evolving to walk more efficiently. These explanations all assume that humans work to the same energy budget as other apes. We supposedly have the same metabolic rates as them and the same number of calories to burn, so we had to fuel our energetically costly traits by making trade-offs. “It’s all framed as a zero-sum game,” says Pontzer. And that, he thinks, isn’t right.
By finally measuring the daily energy expenditures of humans and other apes, Pontzer has found that we actually burn hundreds more calories every day. Even when we’re all at rest, our metabolic rates are higher. So, we didn’t balance our energy sheets by making cuts. We just raised the total budget. We are the gas-guzzling primate.
Pontzer and his team measured the energy expenditures of 56 chimpanzees, bonobos, gorillas, and orangutans that lived in zoos and sanctuaries, as well as 141 people from the United States, Ghana, South Africa, Seychelles, and Jamaica.
They gave each individual a glass of water containing distinctive isotopes of hydrogen and oxygen. Both isotopes were eventually expelled in the volunteers’ urine, sweat, and water vapor, but the oxygen was also lost through exhaled carbon dioxide. By measuring the rate at which both isotopes were lost, and subtracting the hydrogen from the oxygen, the team could accurately track how much carbon dioxide the apes were exhaling. And that, in turn, precisely reflected how many calories they were burning.
The team found that after adjusting for size, the humans were burning 400 more calories every day than the chimps and bonobos, 635 more than the gorillas, and 820 more than the orangutans.
That’s not because the apes were captive and the humans were free-range. The team carefully chose volunteers who didn’t work in manual labor, and who were just as active as the zoo-bound apes. Their bodies weren’t any more active but their organs sure were. Even at rest, they were burning hundreds more calories a day than their ape cousins.
Besides, Pontzer and others have shown that, surprisingly, captive mammals burn the same amount of energy as their wild kin. That’s true for pandas, monkeys, and even humans. A Hadza hunter-gatherer, though extremely active, has roughly the same energy expenditure as desk-bound moi. “It’s really surprising,” says Pontzer. “We think of daily energy expenditure as a function of how active we are, but it’s more about a species’ evolutionary history than its lifestyle.” We, for example, seem to have supported our bigger brains, longer lives, and faster reproductive cycles by upping our energy budgets.
“This was a long-lasting gap in our knowledge,” says Karin Isler from the University of Zurich, “and the results are very important to understand the peculiarity of human evolution within the broader picture of primates and mammals.”
“This resets our understanding of the human energetic landscape,” adds Rachel Carmody from Harvard University. That is, we can see many aspects of human life not as ways of making cuts to compensate for costly brains, but as means of boosting our energy budgets altogether.
We are a more calorific diet of tubers and meat. We cooked our food and walked more efficiently, allowing us to extract more net calories from the same amount of foraging. We divided that foraging between hunters and gatherers, and we started sharing our food to a degree not seen in other apes. And we started keeping more fuel in the tank. Compared to other apes, Pontzer found that humans have much more body fat—23 to 41 percent compared to just 14 to 15 percent in gorillas and 8 to 9 percent in chimps.
“Our data set gives us a unifying framework, says Pontzer. “Rather than everything having to trade off against everything else, we now see that everything’s working towards providing a larger energy budget with the smallest costs and biggest safety margins.”
He adds, “When papers come out in human evolution, the title is often ‘We’re right’, and the subtitle is ‘Everyone else is wrong’. Our subtitle is: ‘Everyone made a lot of important observations and now we can make better sense of them.’