The structure of the human body has been shaped by natural selection for nearly five million years. But fossils only include teeth and bones; they lack the muscle, skin, fat, and organs that make up 85 percent of the body. To understand how our body proportions got this way, researchers dissected over a dozen bonobos. Compared to our closest living ape relatives, we have more body fat, and much of our muscle mass was redistributed to our legs. The findings, published in Proceedings of the National Academy of Sciences this week, suggest that various evolutionary pressures—such as adjusting to drier, more variable environments—influenced the body composition of our species.

Over the course of human evolution, the shape of the body changed as we began to walk upright on two legs. At the same time, brain sizes increased threefold, based on studies of fossil Australopithecus braincases. But because soft tissues don’t fossilize, it’s been just about impossible to compare our musculature, fat distribution, and skin with that of our extinct human ancestors.

So, Adrienne Zihlman from the University of California, Santa Cruz, and Modesto College’s Debra Bolter turned to our closest living relatives. They dissected 13 bonobos (Pan paniscus) who died of natural causes in zoos and research institutions. These 6 female and 7 male adults were frozen after death and made available for postmortem study. The duo then compared the composition of bonobo soft tissue with that of Homo sapiens using a combination of data from 51 cadavers and noninvasive measures such as X-rays and CT and MRI scans.

Compared with bonobos, humans have increased body fat, decreased muscle mass, and decreased skin mass. Additionally, humans redistributed muscle mass to the lower limbs over evolutionary time.

Men and women are typically 25 percent and 15 percent body fat, respectively. The so-called savanna mosaic—with mixed vegetation, seasonal rainfall, and fluctuating food availability—presented challenges for human ancestors evolving in Africa. Adjusting to unpredictable resources and variable calorie intake in these new environments may have led to an increase in body fat percentage; and for females, storing fat likely helped with pregnancy and lactation. Bonobos, on the other hand, do not accumulate body fat, even in captivity: Their body fat ranges from less than 0.01 percent in males to less than 8.6 percent in females.

Additionally, hairy ape skin makes up to 16 percent of their total body mass, whereas our skin accounts for just 6 percent on average. And while their sweat glands don’t respond to heat stimulation, ours are triggered by our own muscle action as well as external heat, such as solar radiation. Our glands give off moisture that evaporates to cool us down, and our reduced hair shafts and smooth skin help with this process. Also, pigments in bonobo skin come in patches, while in humans, pigmentation is mostly uniform, forming a protective shield against radiation. These changes allowed early Homo species to exploit new parts of the savanna—areas high in sunlight and without continuous tree cover.

Finally, the development of bipedalism and lengthened lower limbs in our ancestors may have driven the shift of muscle mass to the legs. While the upper limbs of quadrupedal great apes are well muscled, our locomotion relies on the lower limbs for propulsion, braking, and balance. These changes facilitated long-distance walking and running in Homo, who would go on to disperse out of Africa.