Some anthropologists believe that the genetic homogeneity evident in the world's people is the result of a "population bottleneck" — at some time in the past our ancestors suffered an event, greatly reducing their numbers and thus our genetic variation.
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The sentence talks about what some anthropologists believe.
What do they believe? That the genetic homogeneity is the result of a population bottleneck.
What kind of homogeneity? The homogeneity that is evident in the world’s people.
Then, the sentence has em dash, which is a very versatile punctuation and can provide any kind of additional information relevant to the sentence. In this question, em dash is used to enclose an independent clause.
As we read the sentence after em dash, we encounter an independent clause. This part says that at some time in the past, our ancestors suffered an event. No information about the ‘event’ is provided. The sentence then introduces a verb-ing (reducing) acting as a clause modifier and providing the result of the previous clause. The sentence means that as a result of our ancestors’ suffering of an event, the numbers of our ancestors and thus our genetic variation greatly reduced. Given the lack of information about “an event”, it doesn’t seem logical to talk about the result of the event. Also, the verb-ing modifier (reducing) doesn’t make sense with the subject of the clause (our ancestors).
The original sentence thus has the abovementioned problems.
(A) Incorrect. For the reasons explained in the sentence analysis.
(B) Correct. This option corrects both the problems in the original sentence by replacing the verb-ing modifier with the relative clause (that greatly reduced). In this option, the relative clause modifies “event” and thus provides additional information about it. Our ancestors suffered an event. What kind of event? That greatly reduced their numbers and thus our genetic variation.
(C) Incorrect. For the following reasons:
- We need a preposition (at) with “some time” for it modify the clause “our ancestors suffered”. Or we can use “sometime”, which is an adverb, standalone. For the difference between some time and sometime, please refer this article. In general, “sometime” means “at some point”, and “some time” means “a period of time”.
- The non-underlined part “and thus our genetic vision” doesn’t make sense within the sentence structure of this option.
(D) Incorrect. For the following reasons:
- Error no. 1 of option C
- “from which” construction stands for “their numbers were greatly reduced from the event”. Clearly, the numbers were reduced not FROM the event but BY the event.
- Error no. 3 of option C
(E) Incorrect. For the following reasons:
- Error no. 1 of option C
- The purpose of “that” is not clear. It illogically seems to modify “past” or “some time in the past”.
- “so as to” indicates purpose. This option illogically means that the ancestors suffered with an intention to reduce their numbers.