Australian embryologists have found evidence that suggests that the el

Hey, taking a couple steps back there's a good deal to learn here. A couple points:

1) Remember that clarity of meaning is a big emphasis with the error types that the GMAT tests. And here that extra "that" is there to make the meaning crystal clear, that the "and" connects two things that the evidence suggests:

We've found evidence that the elephant is descended from an aquatic animal *and* that (the elephant's) trunk originally evolved as a kind of snorkel.

Without that extra "that" it's easy for the reader to see two nouns separated by "and" and see them both as objects, both as things that the elephant descended from. For example, if I were to say "DNA tests suggests that I descended from Irish immigrants and my wife's family is from Sweden" - here you can read it as I descended from two family tree branches (Irish immigrants and my wife's family, which would be a little unnerving) or that two groups (me and my wife's family) have taken DNA tests. (Then the portion "is from Sweden" stands as a fragment, but it makes you go back and re-read to unpack which two things are connected by "and" and that slows the reader down and causes confusion). A "that" after "and" makes it clear that DNA tests suggest two things - one about me and one about my wife's family.

ALSO note that in the SC problem we're talking about there's an "its" right after "and" in some choices. This is another place where clarity can be very helpful. Does "its" mean "the elephant's" or "an aquatic animal's"? With the "that" in place in choice (E), it makes it parallel with "the elephant's":

Evidence suggests 1) that the elephant... and 2)that its

That parallelism signals that both portions of "the evidence suggests" are about the elephant, thereby removing potential confusion about what "its" refers to.

2) So keep in mind that parallel structures in these kinds of constructions exists to ensure clear meanings particularly when you have lots of nouns in a sentence and a tiny connector like "and" or "or" (and potentially some pronouns, too). Parallel structure isn't just a random piece of grammar that the GMAT arbitrarily decided to test - the GMAT tests it because it adds a "reading comprehension" logical component to SC. Which brings me to:

3) One place we love teaching students to look for decision points on SC is anywhere you see a coordinating conjunction (and, or, but, etc.) followed by a structural word like "that," "which," "to," etc. Those are great signals that you're likely dealing with a parallel structure issue, and help point you back to the other parallel structural word - if I see "and that" I'm immediately going back to find a previous "that" so that I can investigate for parallelism and clarity of meaning. If I see "or which" it's the same thing...where's that other "which" so that i can assess? Sometimes (like here) those construction show up in an answer choice, but other times they appear far from the underlined portion. And *that's* where they can be all the more crucial (I'm struggling to find a link to my favorite example of this but I'll grab it and drop it in here via edit function).