"They're what really sets us apart and makes us what we are," he said. "But they're all really metabolically expensive traits."

For years, scientists believed we got the energy to fuel those traits by sacrificing other qualities. This is the "expensive tissue hypothesis," which suggests that we "paid" for our big brains, so to speak, by having smaller guts or weaker muscles. Underlying this belief is the assumption that we can only spend as much energy as we can make, and that our metabolisms are only capable of making so much energy per day.

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Pontzer had an alternate theory: What if, instead of sacrificing energy for one organ to pay for another, we just increased our overall budget?

So he compared human energy expenditures to those of our closest relatives: chimps, bonobos, gorillas and orangutans. And he believes his hypothesis proved correct: Humans have a basal metabolic rate (the number of kilocalories we burn simply by being) between 200 and 600 calories larger than other apes. The gulf between our total energy expenditure and theirs was even greater — as many as 820 calories. Our ability to use up energy is abnormally high even when you adjust for things like body size and activity levels, Pontzer said.

These results, published Wednesday in the journal Nature, suggest that humans have overcome a biological constraint that holds most other creatures back.

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"Something really interesting and special in human evolution has happened that allows us to escape this trade-off regime," Pontzer said. "And that opens a lot of doors for understanding how evolution can work and how it can push things around."

This may seem intuitive, until you consider the fact that our metabolisms are pretty much pre-determined by our genes. Much like the engine of a car, metabolism converts fuel into energy at a certain speed. You can put as much gas as you want in the tank of a car from 1920, but it's still not going to move very fast. For that, you'd need to upgrade the engine.

Likewise, a chimpanzee can only bump up its energy budget so much by simply eating more food. To really get things going, it would need to actually increase the rate at which it processes that food.

Most mammals can't do that. But it seems like humans did. We became "the gas guzzlers" of the animal world, Pontzer said with a laugh.

He and his colleagues looked at metabolic rates of humans and zoo apes using a relatively new technique that gauges daily energy expenditure by measuring the amount of carbon dioxide the body is producing (carbon dioxide is a waste product of the chemical reaction that cells use to make energy). They also compared previously published measurements of basal metabolic rates in humans, orangutans and chimps. By both measures, humans had higher energy budgets than any animal other than an adult male gorilla (justifiably — they're about twice our weight).

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Like lots of scientific research, the study doesn't resolve questions so much as switch them out for new ones. So instead of asking, "what are the trade-offs?" Pontzer said, "We're now wondering, 'Oh my gosh, how do we afford this higher metabolism?'"

There are already a few explanations for this: For one thing, our diets of rich carbs, cooked meat and processed food enabled us to get more bang for our buck, nutritionally speaking. For another, the phenomenon of sharing food meant that humans could take time to go after high-risk, high-reward energy sources, like a tasty mammoth.

Pontzer and his colleagues also noticed that humans had a thicker layer of body fat than our primate relatives, which could function as a backstop against starvation when we ran into lean times.

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"Its the safety net," he explained. "It's the reserve fuel tank for our gas guzzling bodies."

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This discovery doesn't invalidate decades of research on the sacrifices humans did make to be able to afford our big brains and long-lived, many-offspring-producing bodies, Pontzer said. Trade-offs like smaller guts and more efficient gaits probably helped mitigate the risk of large energy shortfalls, much as body fat did.

Leslie Aiello, the paleoanthropologist who helped develop the "expensive tissue hypothesis" two decades ago, said that the study "stands to be a classic in the field."

"They have effectively integrated all of the earlier ideas ... into a unified theory for energy and the evolution of human characteristics," she wrote in an email to The Post. Aiello is the president of the Wenner-Gren Foundation, which funds anthropology research, and a reviewer for the Nature study, though she was not involved in Pontzer's research.

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"It is really nice to see how far things have come in the last 20 years," she added. "And how good science can move our understanding forward."