Hey Duong,

Think of it this way:

(D) scientists have gathered evidence that suggests a much earlier emergence of complex life-forms
than the emergence which was previously thought

First off, it's an incorrect modifier "which" (requires a comma). But beyond that, the logic here is doesn't make any sense. We want to be comparing times, not some specificemergence. Do you see how the word "that" creates a specific emergence we're comparing to. But that's not right.

(E) scientists have gathered evidence which suggests a much earlier emergence of complex life-forms than the emergence previously thought.

In both of these, the correct comparison would be the equivalent "I ate an earlier breakfast than I did most days" not "I ate an earlier breakfast than that I ate most days."

In the second example, "that" refers to a specific breakfast, which makes no sense.

Hope that helps a little! Don't forget that both (D) and (E) have an incorrect, commaless "which," so you don't really need to worry about this little stuff!

-t