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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.
This evolution of human brains was not without its difficulties for mothers, because ______.
选项 A、human brains started to grow larger
B、our ancestors walked upright on two legs
C、it altered our ancestors’ skeletons
D、it makes the passage of big-brained infants difficult
答案D
解析 参见文中第3段，其大意是，人类进化为两足动物逐渐改变了我们祖先的骨骼。与此同时，头向上倾，骨盆的形状改变，以适应垂直的行走姿势。其结果是产道的一部分变窄，这使得人头婴儿越来越难顺产。故D为正确答案。
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