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  3. Sexual Reproduction Birds do it. Bees do it. But dandelions don’t. The prodigious spread of these winsome weeds underscores a

Sexual Reproduction Birds do it. Bees do it. But dandelions don’t. The prodigious spread of these winsome weeds underscores a

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问题                            Sexual Reproduction
   Birds do it. Bees do it. But dandelions don’t. The prodigious spread of these winsome weeds underscores a little-appreciated biological fact. Contrary to human experience, sex is not essential to reproduction. Asexual organisms can often churn out multiple generations of clones, gaining a distinct edge in the evolutionary numbers game. And therein lies the puzzle: If sex is such an inefficient way to reproduce, why is it so widespread?
   Sex almost certainly originated nearly 3. 5 billion years ago as a mechanism for repairing the DNA of bacteria. Because ancient earth was such a violent place, the genes of these unicellular organisms would have been frequently damaged by intense heat and ultraviolet radiation. " Conjugation" — the intricate process in which one bacterium infuses genetic material into another — provided an ingenious, if cumbersome, solution to this problem, although bacteria continued to rely on asexual reproduction to increase their numbers.
   Animal sex, however, is a more recent invention. Biologist Lynn Margulis of the University of Massachusetts at Amherst believes the evolutionary roots of egg and sperm cells can be traced back to a group of organisms known as rotests that first appeared some 1. 5 billion years ago.(Modern examples include protozoa, giant kelp and malaria parasite. ) During periods of starvation, Margulis conjectures, one rotest was driven to devour another. Sometimes this cannibalistic meal was incompletely digested, and the nuclei of prey and predator fused. By joining forces, the fused cells were better able to survive adversity, and because they survived, their penchant for union was passed on to their distant descendants.
   From this vantage point, human sexuality seems little more than a wondrous accident, born of a kind of original sin among protozoa. Most population biologists, however, believe sex was maintained over evolutionary time because it somehow enhanced survival. The mixing and matching of parental genes, they argue, provide organisms with a novel mechanism for generating genetically different offspring, thereby increasing the odds that their progeny could exploit new niches in a changing environment and, by virtue of their diversity, have a better chance of surviving the assaults of bacteria and other tiny germs that rapidly evolve tricks for eluding their hosts’ defenses.
We can infer from the passage that sex first appeared in______.
选项 A、bacteria
B、multicellular organisms
C、rotests
D、protozoa
答案A
解析 细节题型，答案是A。本题考查有性繁衍最早出现在哪种生物形态上。根据第二段第一句“Sex almost certainly originated nearly 3．5 billion years ago as a mechanism for repairing the DNA of bacteria．”中的originate一词可基本锁定A选项“细菌”。B选项中的organisms一词同样出现在第二段，但修饰词unicellular被置换为multicellular，为干扰项。C选项rotests出现在第三段，对应的时间点为1．5 billion years ago。D选项protozoa亦出现在第三段，对应的时间点为modern。依次排序可确定A为正确答案。本题核心：此题涉及时间排序，可通读全文寻找时间标识词确定答案。
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