We share 99 percent of our DNA with chimps - so why do they stay thin, while we pile on the pounds?
Scientists think they've found the gene behind it.
"We've lost some of the ability to shunt fat cells toward beige or brown fat, and we're stuck down the white fat pathway," said Devi Swain-Lenz, postdoctoral associate in biology at Duke University.
"We're the fat primates."
Fat comes in three general types - beige, brown and white. Brown fat easily burns calories to create heat, while white fat is what gets stored around our waistlines.
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Scientists at Duke analysed human and chimpanzee DNA, and found a recurring snippet which converts fat cells from one type to another - in humans it was tightly bound to proteins and "closed for business", while in chimps it was loose and therefore "accessible".
It's possible to activate the body's brown fat by doing things like going out in the cold but unfortunately "we need to work for it", Dr Swain-Lenz said.
And don't go blaming humans' love of food and a hatred of exercise. Even the best human athletes have more fat than the laziest chimp - up to three times as much.
Dr Swain-Lenz said humans' evolution towards making more white fat than brown might have been the result of our ever-growing brains. Human brains are three times bigger than chimps', and need a lot more energy - brown fat, which burns calories for heat, isn't as useful as white.
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But now we generally have too much white fat. Dr Swain-Lenz said one day scientists hope to find a way to easily turn white fat into brown or beige.
"I don't think that it's as simple as flipping a switch. If it were, we would have figured this out a long time ago."
The research was published in journal Genome Biology and Evolution.