Your rule is correct; but those aren't "two independent clauses". The clause because cancer cells are extremely diverse in their metastatic ability is dependent/subordinate, as it cannot stand on its own.
The rules for subordinate clauses are more complex and less definite. In the case of subordinate conjunctions, like because, I believe most style guides allow but do not prescribe the comma, depending on context. If the subordinate clause is felt to be very closely connected to the main clause, and/or if it is very short, style guides would probably advise that you leave out the comma; if not, then write a comma. This advice is of course somewhat vague; it leaves considerable freedom to authors.
In your example, I find it hard to judge the 'closeness' of the connection between the clauses. I would say it could go either way. The subordinate clause is not short, however, which would weigh in favour of a comma. So I would probably write a comma there, but I'd say it's not compulsory.
There is also a kind of sentence where the subordinate clause has focus in the sentence, where it contains the most salient point. But that is not the case in your example.
He did not hate Alyona Ivanovna. He killed her because he needed the money.
In this case, a comma would be wrong and unacceptable. Because the focus lies on the subordinate clause, the main clause cannot stand on its own pragmatically (narratively), i.e. this would distort the story line, even though it's possible grammatically (syntactically):
He did not hate Alyona Ivanovna. He killed her, because he needed the money.
This doesn't really make sense as a story or sequence of statements.