Humans Paid for Bigger Brains With Gas-Guzzling Bodies – The Atlantic

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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.

Humans Paid for Bigger Brains With Gas-Guzzling Bodies – The Atlantic