The structure of the human body has been shaped by natural selection for nearly five million years . But fossils only admit tooth and bones ; they lack the muscle , skin , fertile , and organs that make up 85 percent of the body . To see how our body proportions got this way , researchers dissected over a dozen bonobos . compare to our closest living ape relatives , we have more consistency fat , and much of our muscle mass was redistribute to our leg . Thefindings , published inProceedings of the National Academy of Sciencesthis workweek , suggest that various evolutionary pressures — such as adjust to dryer , more variable environments — influenced the eubstance composition of our species .
Over the course of study of human evolution , the shape of the body changed as we began to take the air upright on two legs . At the same time , nous size increase three times , base on study of fossilAustralopithecusbraincases . But because soft tissue do n’t fossilize , it ’s been just about impossible to compare our musculature , rich distribution , and skin with that of our extinct human antecedent .
So , Adrienne Zihlman from the University of California , Santa Cruz , andModesto College ’s Debra Bolterturned to our tight aliveness congenator . They dissected 13 bonobos ( Pan paniscus ) who die of natural causes in zoos and research institutions . These 6 distaff and 7 manlike adults were freeze after end and made usable for PM study . The duo then equate the composition of bonobo flabby tissue paper with that ofHomo sapiensusing a combination of data from 51 cadavers and noninvasive measures such as X - rays and CT and MRI scan .
compare with bonobos , humanity have increased trunk fat , decrease muscle mass , and decreased tegument mass . to boot , humans redistribute muscle mass to the lower limb over evolutionary clip .
Men and fair sex are typically 25 per centum and 15 percent body fat , respectively . The so - holler savannah arial mosaic — with assorted botany , seasonal rain , and fluctuate food availability — presented challenges for human ancestor evolving in Africa . Adjusting to irregular resource and varying calorie intake in these new environments may have direct to an increase in body fertile part ; and for female , stack away avoirdupois probably helped with pregnancy and lactation . Bonobos , on the other hand , do not compile consistence fat , even in captivity : Their body avoirdupois ranges from less than 0.01 percent in males to less than 8.6 percent in females .
to boot , haired ape skin makes up to 16 per centum of their full trunk the great unwashed , whereas our tegument accounts for just 6 percent on average . And while their sweat secretor do n’t respond to heat stimulation , ours are triggered by our own heftiness natural process as well as external oestrus , such as solar radiation sickness . Our secretory organ give off moisture that evaporates to cool us down , and our reduce hair calamus and smooth skin help with this process . Also , pigments in bonobo tegument come in dapple , while in human race , pigmentation is mostly uniform , forming a protective cuticle against radiation . These changes allowed earlyHomospecies to exploit new constituent of the savanna — areas high in sunshine and without uninterrupted tree masking .
Finally , the development of bipedalism and lengthened lower limbs in our antecedent may have force back the shift of sinew mass to the pegleg . While the upper limbs of four-footed great copycat are well muscle , our travel relies on the lower arm for propulsion , braking , and balance . These change alleviate long - length walk and running inHomo , who would go on to break up out of Africa .
