CD网："－ing form can even modify an entire clause as long as the entire clause converted into a noun phrase could function as the subject of the verb that is now in -ing form" －－ 摘自曼哈顿 Chapter 6 Modifiers 比较make sense的说法应该是event reduced number，比前面一整句描述的事情 reduced number 感觉更合适。

Dash is versatile in use and can play the role of a comma, semicolon or a colon. Dash here plays the role of a colon(:) - the colon can be used to provide further explanation for what comes before it. The dash, as a replacement of a colon, plays the same role here.

Without "that", the latter clause ( " at some time....greatly reduced their numbers") does not have much meaningful bearing to the first clause as the author intended. It is required to establish that the scientists believe even the latter clause and hence "that" is required to be repeated.

(A) is implying that our ancestors greatly reduced their own numbers (this is incorrect because it was the event that greatly reduced ancestors). When we have an independent clause followed by a participle phrase (one that starts with a gerund and serves as an adjective clause), the participle phrase modifies the subject of the sentence.

破折号是一个很神奇的符号，身后的内容既可以修饰一个词，也可以修饰一个句子。在本题中，破折号身后加上用that引导的从句或者省略这个that都是可以接受的。
A选项：reducing their numbers是伴随状语，其逻辑主语应该为其所修饰的句子的主语，即，our ancestors suffered an event的主语。但在逻辑上，下降祖先数量的应该是事件，而不是祖先本身。

that在不在都ok，不是必须不在。

破折号是一个很神奇的符号，身后的内容既可以修饰一个词，也可以修饰一个句子。在本题中，破折号身后加上用that引导的从句或者省略这个that都是可以接受的。
A. reducing their numbers是伴随状语，其逻辑主语应该为其所修饰的句子的主语
if the clause on each side of the typographical spot in question is an independent clause – if each of them can stand alone as a complete sentence – if you could sleep at night knowing you put a period and a capital letter there instead, then you can put a semicolon in there. If not, you can’t

这个题提醒注意逻辑主语。

RON说过single dash不能连接两个independent clause

破折号后面的内容。破折号是一个很神奇的符号，身后的内容既可以修饰一个词，也可以修饰一个句子。在本题中，破折号身后加上用that引导的从句（修饰词）或者省略这个that（修饰句子）都是可以接受的。

用伴随或是用从句看是有由主语引起还是宾语。
reducing their numbers是伴随状语，其逻辑主语应该为其所修饰的句子的主语，即，our ancestors suffered an event的主语。但在逻辑上，下降祖先数量的应该是事件，而不是祖先本身。

破折号是一个很神奇的符号，身后的内容既可以修饰一个词，也可以修饰一个句子。在本题中，破折号身后加上用that引导的从句（修饰词）或者省略这个that（修饰句子）都是可以接受的。
C选项：so that their numbers were greatly reduced是结果状语从句，但其是被动语态，语义上约等于：某人或某物下降了我们祖先的数量。
我们的祖先遭受到了一个事件，如此以至于，某人或某物下降了我们祖先的数量。
划线的两句话之间没有了因果关系。（肯定应该是，遭受事件，如此以至于，这件事下降了数量）

伴随状语主语为逻辑主语：reducing their numbers是伴随状语，其逻辑主语应该为其所修饰的句子的主语，即，our ancestors suffered an event的主语。但在逻辑上，下降祖先数量的应该是事件，而不是祖先本身。

A，at some time in the past our ancestors suffered an event, greatly reducing their numbers and thus our genetic variation.
“我们的祖先遭遇了一件事情，伴随的结果是减少了他们的数量。”一个是逻辑主语不对，应该是一场灾难袭击了祖先，伴随……结果；二是语意非常奇怪，event是中性词，祖先遭遇了一件事情，并不会自然地引起他们的数量减少的结果，所以这里用伴随状语非常奇怪，应该是定语，遭遇了一件减少了他们数量的事情。

纠结在AB选项
这边的排除点不是开头的that，破折号后面that存不存在都是可以的
出现ing这种逗号后面伴随的状态，一定要看清逻辑主语 A选项中：逻辑主语变成了ancestors，但是应该是events reduce

考察：
前：Scientists believe that X--- that Y. X and Y are parallel clauses. ---- OG 破折号后可以加that也可以不加
中后：reduce their number优于被动语态
关键：！！！与未画线部分联系！！！最后and平行，and thus+n，那前面也是n，排除cde，而a中reducing 的逻辑主语变成了 ancestors，错误

Some anthropologists believe that。。。——that。。。破折号前后that从句平行。
C少了at ----“at some time”

“ In D, which seems to refer to an event, implying that our ancestors' numbers were reduced FROM an event -- a nonsensical meaning.”

so as to身后的动词的主语必须是其主句的主语。在本选项中是our ancestors。但在逻辑上，下降了祖先数量的应该是“事件（an event）”，而不是祖先自己。另外，引导词that应该放在sometime in the past身前。

破折号是一个很神奇的符号，身后的内容既可以修饰一个词，也可以修饰一个句子。在本题中，破折号身后加上用that引导的从句或者省略这个that都是可以接受的。