⠀Anthropologists once thought that the ancestors⠀⠀⠀ of modern humans began to walk upright because⠀⠀⠀ it freed their hands to use stone tools, which they⠀⠀⠀ had begun to make as the species evolved a brain of(5)⠀⠀increased size and mental capacity. But discoveries⠀⠀⠀ of the three-million-year-old fossilized remains of⠀⠀⠀ our hominid ancestor Australopithecus have yielded⠀⠀⠀ substantial anatomical evidence that upright walking⠀⠀⠀ appeared prior to the dramatic enlargement of the(10) brain and the development of stone tools.⠀⠀⠀⠀⠀⠀⠀⠀Walking on two legs in an upright posture (bipedal⠀⠀⠀ locomotion) is a less efficient proposition than walking⠀⠀⠀ on all fours (quadrupedal locomotion) because several⠀⠀⠀ muscle groups that the quadruped uses for propulsion(15) must instead to provide the biped with⠀⠀⠀ stability and control. The shape and configuration⠀⠀⠀ of various bones must likewise be modified to allow⠀⠀⠀ the muscles to perform these functions in upright⠀⠀⠀ walking. Reconstruction of the pelvis (hipbones) and(20) femur (thighbone) of "Lucy", a three-million-year-old⠀⠀⠀ skeleton that is the most complete fossilized skeleton⠀⠀⠀ from the australopithecine era, has shown that they⠀⠀⠀ are much more like the corresponding bones of the⠀⠀⠀ modern human than like those of the most closely(25) related living primate, the quadrupedal chimpanzee.⠀⠀⠀ Lucy’s wide, shallow pelvis is actually better suited to⠀⠀⠀ bipedal walking than is the rounder, bowl-like pelvis of⠀⠀⠀ the modern human, which evolved to form the larger⠀⠀⠀ birth canal needed to accommodate the head of a(30) large-brained human infant. By contrast, the head of⠀⠀⠀ Lucy’s baby could have been no larger than that of a⠀⠀⠀ baby chimpanzee.⠀⠀⠀⠀⠀⠀⠀⠀If the small-brained australopithecines were not⠀⠀⠀ toolmakers, what evolutionary advantage did they(35) gain by walking upright? One theory is that bipedality⠀⠀⠀ evolved in conjunction with the nuclear family:⠀⠀⠀ monogamous parents cooperating to care for their⠀⠀⠀ offspring. Walking upright permitted the father to⠀⠀⠀ use his hands to gather food and carry it to his mate(40) from a distance, allowing the mother to devote more⠀⠀⠀ time and energy to nurturing and protecting their⠀⠀⠀ children. According to this view, the transition to⠀⠀⠀ bipedal walking may have occurred as long as ten⠀⠀⠀ million years ago, at the time of the earliest hominids,(45) making it a crucial initiating event in human evolution.

1. The primary purpose of the passage is toA. present an interpretation of the chronological relationship between bipedal locomotion and certain other key aspects of human evolutionB. compare the evolutionary advantages and disadvantages of bipedal locomotion to those of quadrupedal locomotionC. argue that the transition to a nuclear family structure was a more crucial step in human evolution than was the development of stone toolsD. analyze anatomical evidence of bipedal locomotion to show that the large brain of modern humans developed at a later stage of evolution than was previously believedE. use examples of muscle and bone structure to illustrate the evolutionary differences between modern humans, australopithecines, and chimpanzees