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  3. On Mother’ Day, it’s customary to speak about the sacrifices our mothers made to improve our lives. But mothers also deserve cre

On Mother’ Day, it’s customary to speak about the sacrifices our mothers made to improve our lives. But mothers also deserve cre

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问题     On Mother’ Day, it’s customary to speak about the sacrifices our mothers made to improve our lives. But mothers also deserve credit for the pivotal (关键的) role they’ve played in the history of human evolution. Prehistoric mothers did nothing less than seed the development of our species’ remarkable intelligence.
    The story begins at least two million years ago, when our brains started to grow larger, eventually making humans the most cognitively advanced species on earth. This evolution was not without its difficulties, particularly for mothers. That’s because, some five million to seven million years ago, soon after the human lineage (世系) branched from the ancestors of chimpanzees (黑猩猩), another peculiarity increasingly came to mark our ancestors: walking upright on two legs, or bipedalism.
    The evolution of bipedalism gradually altered our ancestors’ skeletons. By the time brain size began trending upward, the shape of the human pelvis (骨盆) had changed to accommodate the muscle attachments that facilitated walking (and running) in a more vertical posture. As a result, parts of the birth canal narrowed, making the passage of big-brained infants increasingly difficult.
    The combination of big brains and constricted birth canals was an obstetrical problem for early mothers and no doubt led to high rates of maternal and infant mortality. The infants who survived were the ones whose heads were small enough to squeeze through narrowed birth canals, but to thrive outside the womb, human development favored big brains. And so natural selection encouraged the early birth of human fetuses, before they had finished gestating (孕育). For that reason, our babies are born in immature, helpless states compared with those of the apes.
    Had it not been for the natural selection of enlarged brains, our species would have evolved in a completely different direction. There would be no theory of relativity, no knowledge of "entangled" particles or the human genome; we’d have no great art, music or novels. The excruciating (极痛苦的) pain and trauma of childbirth are the cost our species has paid for its fancy cognition. And mothers continue to pay the debt.
    But that’s hardly all prehistoric mothers gave us. They also may well have touched off the evolution of language from the sounds they made to reassure their helpless infants. Baby chimpanzees, after all, can cling to their mothers’ hairy chests and contentedly ride along, nursing on demand. But human infants, born immature, lack that dexterity. Before the advent of devices like baby slings, the burden of carrying helpless infants presented a dilemma for early mothers as they foraged for food and water.
According to the passage, which of the following human features evolved first?
选项 A、The development of human brains.
B、Upright posture of two-leg walking.
C、The change of the skeletons.
D、The use of language.
答案B
解析 参见文章第2--4段，根据人类进化的先后过程，先是双足直立，然后是骨骼形状改变，接着是大脑的增大，最后是语言的使用，因此正确答案为B。
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