If you teach him to pose and ignore the lack of spray tan, a chimpanzee will wipe the floor at a bodybuilding competition. Male bodybuilders might measure in at five percent body fat when they strut their stuff, but the untrained male chimpanzee averages 0.005%!

The difference is even starker when you compare apples to apples, so to speak. The bodies of female chimpanzees are roughly 3.6 percent fat. The average woman might have 24 - 31 percent. Male chimps have 0.005%. Human males have 12 - 20 percent.

For how similar humans and chimpanzees are -- we share about 99% of our DNA -- these disparities are remarkable, especially when you consider that human males generally require at least 4% body fat to remain in good health, and women require about twice that. So what accounts for the marked difference in leanness between our two species? Put more simply, why are humans so much fatter than chimps? Anthropologists Adrienne Zihlman and Debra Bolter, respectively based out of UC-Santa Cruz and Modesto College, presented an idea in yesterday's issue of the Proceedings of the National Academy of Sciences.

As a genus, humans, from Homo sapiens (that's us) to our extinct ancestors Homo neanderthalensis and Homo erectus, are wanderers. Over the vast majority of our history, which spans hundreds of thousands of years, we have roved from place to place, inhabiting a wide range of habitats. We moved with the seasons, we moved to find food, we moved -- perhaps -- just to move. Our adaptability was our key adaptation, an evolutionary leg-up on the competition. The ability to store fat was vital to this lifestyle. Body fat cushions internal organs, but it also serves as a repository of energy that can be readily broken down and used to power muscles. Humans might fatten up at one environment, then move on to another. When food was scarce, we could count on our fat to sustain us, at least temporarily.

Chimpanzees, on the other hand, are localized to specific environments where food is often plentiful, primarily the forests of West and Central Africa. Fatty stores of energy aren't required, but strength to climb food-bearing trees is. Natural selection favored brawn, causing chimps to shed fat as unnecessary weight.

Interestingly, this may have hindered chimpanzees' brain development. Human brains are about three times larger than chimp brains, and this may be because we exchanged muscle for fat. Muscles and brains are metabolically expensive, requiring gobs of energy to function. With less muscle and more fat, humans had more energy to dedicate to brains.

While we hunger for information about our origins, good data can be hard to find. Zihlman and Bolter substantiated their idea by measuring the body compositions of thirteen naturally deceased bonobos, one of two species of chimpanzees. That the body fat of the bonobos was so low was "unexpected," they say. Pregnant or lactating females had higher levels of fat, as much as 8.6%, in order to have enough energy to nurse their babies. But regardless of age, body mass, or captivity, all males measured in at below 1/100th of a percent of body fat. Under no conditions did they store an appreciable amount of fat.

While fat storage aided our ancestors on the savannah, it's not doing denizens of the modern world any good. Copious food and a pampered lifestyle cause many humans to build a body fat composition of 35% or higher. While that might've been beneficial ten thousand years ago, it's of limited use now.

Source: Adrienne L. Zihlmana and Debra R. Bolter. "Body composition in Pan paniscus compared with Homo sapiens has implications for changes during human evolution." June 2015. PNAS. www.pnas.org/cgi/doi/10.1073/pnas.1505071112

(Image: AP)