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Anthropologists once thought that the ancestors of modern humans began to walk upright because it freed their hands to use stone

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问题 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 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 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 must instead be adapted 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 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 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 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 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 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, making it a crucial initiating event in human evolution.  
The primary purpose of the passage is to
选项 A、present an interpretation of the chronological relationship between bipedal locomotion and certain other key aspects of human evolution
B、compare the evolutionary advantages and disadvantages of bipedal locomotion to those of quadrupedal locomotion
C、argue that the transition to a nuclear family structure was a more crucial step in human evolution than was the development of stone tools
D、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 believed
E、use examples of muscle and bone structure to illustrate the evolutionary differences between modern humans, australopithecines, and chimpanzees
答案A
解析 To discern the primary purpose of a passage requires an understanding of the key messages within the passage. The passage states that a once-favored view was that modern human beings’ ancestors began to walk upright around the same time that they began to use stone tools.
However, this argument has been weakened by archaeological discoveries indicating that these ancestors began to walk upright before the enlargement of the brain that led to the development of such tools.
Furthermore, the passage indicates that these ancestors had a shallow pelvis that would actually suit walking upright better than the bowl-shaped human pelvis, a development that allows for children to be born with larger brains and therefore heads.
Given that walking upright is less efficient than walking on all fours, the passage suggests that some sort of evolutionary advantage came
from walking upright; the passage suggests that this advantage may have been that it allowed monogamous parents to cooperate in the care of offspring.
A      Correct. As the review above indicates,
the passage is primarily concerned with the chronological relationship between bipedal locomotion and certain other important developments in human evolution, such as fitness for cooperative parental roles.
B      While the passage does mention the relative efficiency of bipedal and quadrupedal locomotion, it does so primarily to motivate the larger purpose of the passage. That is, it justifies the need to explain the development of bipedal locomotion. In other words, if bipedal locomotion were more efficient rather than less efficient, this efficiency boost would be sufficient to explain its evolution.
C      The passage does not discuss whether the development of the nuclear family or the development of stone tools was the more crucial step in human evolution. Rather, it simply rules out the development of stone tools as an explanation for the development of bipedal locomotion and proposes the transition to a nuclear family as a possible explanation.
D      The passage does present such evidence, but it does so merely as one step in pursuit of the primary purpose of presenting the chronological relationships among the evolution of bipedal locomotion and other key human developments.
E      The passage does use such examples, but it does so merely as one step in pursuit of the overall, primary purpose of presenting the chronological relationships among the evolution of bipedal locomotion and other key human developments.
The correct answer is A.
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