Human beings may be smarter than other apes due, in part, to our higher metabolisms, a new study suggests. This could answer the question of how our species is able to operate such large brains while still reproducing more often, and living longer, than our primate cousins.

Caloric use of food energy by great apes, including chimpanzees, gorillas and orangutans, was examined, and compared with human metabolisms.

"This research demonstrates that, pound-for-pound, humans use substantially more energy than do other apes. That may seem like a fairly simple conclusion, but precise data on energy use in apes had not previously been available. It has been widely assumed that humans would have had to cut corners in order to afford large brains, but that does not seem to have been necessary," Emery Thompson of the University of New Mexico said.

Great apes were fed water laced with isotopes of hydrogen and oxygen. Urine of the animals was examined in order to reveal how ratios of those tracers changed over time. This change is driven by the production of carbon dioxide, revealing the total amount of energy burned by the animals.

This data was compared with examination of how the ratio changed in 141 people. Human beings were found to burn calories 27 percent faster than chimpanzees, allowing our bodies to supply large quantities of energy to our brains.

Body fat in humans — the fattest of the great apes — was found to also play a role in the large, complex brains that are the hallmark of our species. When brains run out of food energy, the organs are able to burn fat in order to continue fueling neural cells. Because we burn calories faster than other great apes, our bodies require a larger "spare tank" of energy.

This evolutionary development among our distant ancestors — in effect, "turning up the heat" within our bodies — may have played a vital role in the rise of modern civilization.

Future investigation will examine how the results of this study may be affected by primates behaving with radically different lifestyles. This could include apes in captivity versus in the wild, and comparing sedentary to physically active human subjects.

Examination of how metabolism may have affected the development of human brains was published in the journal Nature.

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