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题库 / GMATLA-SC-46
All Y chromosomes in existence today are descended from a single ancestor's who is thought to have lived about 140,000 years ago.
a single ancestor's who is thought to have lived
a single ancestor's who was thought to live
a single ancestor whom it is thought had lived
the Y chromosome of a single ancestor who is thought to have lived
the Y chromosome of a single ancestor whom it was thought had lived
题目分析：
略
选项分析：
选项A：a single ancestor’s 身后出现了定语从句，而a single ancestor身后的所有格显然指的是“染色体”， who is thought to have lived about 140,000 years ago应该放在其先行词ancestor旁边，不能放在所有格身后。
选项B：定语从句错误同选项(A)。另外，划线部分最后的不定式短语to live about 140,000 years ago应该改成to have lived about 140,000 years ago。请注意，在不定式短语中添加have，不表示完成时，而是表示的是“先时性”。在本题中即，“生活在140,000年之前”是发生于“认为（thought）”这个动作之前的，因此，应该把to live改成to have lived。
选项C：所有的Y染色体应该进化自（All Y chromosomes in existence today are descended from）一个染色体，而不能是进化自single ancestor，即， single ancestor应改为the Y chromosome of a single ancestor。另外，whom it is thought had lived about 140,000 years ago是一个嵌入式关系分句，若把先行词带入从句且补全省略，则有：
it is thought (that) a single ancestor had lived about 140,000 years ago
划线的部分是先行词。虽然这个从句在语法上没有错误，在其在逻辑上应改为：
a single ancestor is thought to have lived about 140,000 years ago
这点考查了不定式和从句的区别，用之于本题， 由于think是看法类动词，所以其宾语的补足语必须用不定式来表示。
选项D：Correct. 本选项在语法和逻辑上均是正确的。
选项E：嵌套式关系分句错误同(C)。
A/B: who离ancestor's最近，应该离被修饰对象ancestor最近。
C，最优的平行是People descend from people, chromosomes descend from chromosomes，而不是染色体来源于人。
E，whom错了；it was thought表示过去认为，应该为it is thought，从过去到现在都认为。
to have done, having done表示动词的先时性，表示该动作先于谓语动词发生，不属于完成时态。如：
Having finished my homework, I went to the park.
A single ancestor is thought to have lived about 140,000 years ago.
of通常修饰无生命物体，也可以修饰有生命物体，如：
名词太长,the works of Marx,engels and Yao
名词定语太长,the name of the student sitting near the window.
‘s通常修饰人，也可以用在dog、bird等有生命推，还可用于时间、距离、国家、城镇、地区、重量、价格、自然现象等.例如：half hour's walk, a mile's distance, Chian's development,a pound's weight.
本题修饰人用了of，因为人后面的定语太长
定语从句中用who，还是whom？
定语从句中,whom只能在从句中充当宾语成分,who既可当主语又可当宾语。但是若是介词后的宾语,只能用whom而不能用who.
R：There is nobody who/whom he can turn to for help.
W：There is nobody to whom he can turn for help.
RON："to have ___ed" is the only past-tense version of "to ___".
to HAVE done 表达的是to do 的过去形式的含义
同时前后逻辑语义上的agreement
is thought to have lived=be known to have made 这个用法prep08中曾经也出现过，表示之前已经发生
想起来了之前known to have done结构，这里应该也是thought to have done结构吧
emmmm网上是这样解释的
Note the sequence of tenses used here -
"who is thought to have lived 140,000 years ago" - the thinking is in the present.
to + past participle suggests that the action occurred before the preceding verb. Hence, this correctly suggests that "living" occurred before "thinking".
也就是说现在完成时可以表示动词先后顺序，不仅仅关注在持续发生这一点上
think是看法类动词，所以其宾语的补足语必须用不定式来表示
然而我就是不懂为什么要用have lived呢？从过去一直到现在都在生存？还是居住？
为什么xxxxx年以前，要用完成时而不用过去时呢？
讲得好 感谢！