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.