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.
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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。