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COLUMN: Fat reserves separate us from apes



Humans: we reproduce and use more energy in day-to-day life, but live longer than our primate cousins.

This means humans are an exception to a frequently observed biological rule; organisms that reproduce faster than their relatives have shorter life spans because they invest more energy in reproduction and less in bodily maintenance and growth.

According to the biological rule, humans should invest heavily in bodily maintenance — the brain is the most energy-hungry organ in our bodies — and should therefore not live as long as our evolutionary relatives.

A recent study featuring an international team of scientists, including Robert W. Skumaker from the Center for the Integrative Study of Animal Behavior at IU, determined fat reserves have helped develop our brains beyond that of our closest primate cousins. So thank you, fat reserves.

To understand how humans solve the energetic paradox, they looked at the total energy expenditure of humans and a variety of apes, like chimpanzees, bonobos, orangutans and gorillas.

To measure total energy expenditure, subjects are given a dose of heavy water, which is enriched with heavy isotopes of hydrogen and oxygen. The scientists can then determine how many water molecules the body of an animal, or human, subject split apart while metabolizing calories.

They found humans expend hundreds more kilocalories in a day than all species of apes and this difference is due to the energy needs of our organs, or as the scientists refer to it, basal metabolic rate.

The second highest BMR in the primate tree belongs to our closest relatives, chimps and bonobos of the genus Pan.

This confirms the prediction humans invest more heavily in bodily maintenance than great apes, but still leaves the question of how we manage to live so long unanswered.

The authors also measured the body fat content of study participants and animal subjects.

They found the ability to store fat reserves for times of famine is not one that we share with our ape relatives. Humans in the study were found to have far more body fat than apes.

The average human female in the study had roughly twice the body fat of an average female orangutan, the second “fattest” species in the study.

Even the fittest of athletes still have more fat reserves on their body than the average chimpanzee, which is the leanest ape.

This ability to stash energy for later use is what the authors believe allows humans to produce more children and maintain larger brains than other apes, especially when combined with a food-sharing culture.

A culture of food sharing ensures that everyone can eat, no matter their hunting or food gathering abilities. This is especially important in keeping children and pregnant females well fed and ensuring the continuation of the species.

So essentially this study demonstrates getting fat and sharing might have allowed us to develop the brains that separate us from our closest animal relatives.

Keep that in mind next time you hear someone complain about not having their “beach body.” That’s not what our ancestors would have wanted.

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