ABSTRACT
For years human bipedalism seemed a puzzle because we run inefficiently compared to similarly-sized four-legged animals. However, it was eventually established that humans are considerably more efficient walkers than their ape relatives. Because our ancestors came from an ape background, they only needed to improve on ape walking to provide an evolutionary reason for bipedalism.
However, bipedalism goes way back in hominin history, probably 7 million years. This raises doubt that bipedalism was much more efficient early on. Other causes have been proposed: Among them, that food trees became more widely spaced due to climate change, magnifying any efficiency difference; that standing up exposed the body to less direct sun; and, in a view espoused in the chapter, that life in a mosaic environment of savanna, woodland, grassland, and forest, called for carrying tool materials that in places were otherwise unavailable. Judging from chimpanzee hunts, including the occasional use of crude spears, our earliest ancestors may have had similar capabilities, suggesting a hunting-carrying explanation.
Besides its obvious consequences for cognition, bipedalism resulted in anatomical changes from head to toe. The head became balanced over the trunk, the spinal column curved, the thorax became a cylinder, the pelvis lowered and broadened, the arms shortened, the fingers straightened, the legs lengthened, the feet developed arches, and the sideways angle of the big toes nearly closed. Our gait became stiffer, and more energy was returned between steps. But what would we do with our freed-up hands?
