B - Incorrect: This answer changes the meaning of the original sentence, stating that it was contended that the planet's oceans house millions of species and not that all of the million of species can be traced back to a single unicellular ancestor, as the original sentence and the answer choice answer (A) suggests.

C, D, E - Incorrect: The problem with these answers, in addition to changing the meaning of the sentence (C), is that they are all also uneconomical, unidiomatic, or awkward.

A - Correct: This choice uses the correct idiom contend that. This answer is stated as simply as possible, without changing the meaning of the original sentence or violating conventions.

A is the correct answer.