The sentence says that Australian embryologists have found evidence that suggests something. What does it suggest? Let?s focus on this part:
- that suggests
- that the elephant is descended from an aquatic animal,
- and its trunk originally evolving as a kind of snorkel
The evidence suggests two things:
- The elephant is a descendant of an aquatic animal
- The trunk of the elephant originally evolved as a kind of snorkel (or if you don?t know the meaning of snorkel, you can think it as "kind of something")
There are two clear problems with this sentence ? both in latter part of "and":
- "its trunk originally evolving as a kind of snorkel" doesn?t have a verb. The "trunk" is followed by a verb-ing modifier. Logically, the idea communicated in this part seems to be suggested by the evidence. Therefore, it needs to be parallel to the clause before "and". In other words, we need to change "evolving" (a modifier) to a verb in this part to make it correct.
- The other problem is that we need to repeat "that" after "and" to make it parallel to "that the elephant is _". Please remember that we cannot take "that" as common to the two clauses. Why do we need to repeat it? Because without such repetition, one can possibly understand the part after "and" to be another independent clause and not something suggested by the evidence.
There?s another problem that is not-so-major and about the quality aspect of the sentence. It doesn?t seem very polished to write two thats one after the other - "that suggests that". One way to change the same is by writing "suggesting that".
(Please note that "is descended" is absolutely correct, no matter how wrong it may sound to you. This construction is same as "This plant is grown from hybrid seeds". "Grown" and "descended" are both past participle forms of the corresponding verbs. Saying "the elephant has descended" would mean that the elephant has done the action of descending.)
(Another point to note here is that "suggest" is one of the bossy verbs requiring a subjunctive verb after them e.g. I suggest that he be called. However, there?s another use of "suggest", in which it means "indicates". This is the way "suggest" is used in this sentence.)