Hmm, yes, I do see it differently. To me, "... men were believed to have descended from Martians" is not a nonrestrictive clause. Unless I've got it wrong (always a possibility), a nonrestrictive clause is a descriptor that can be deleted without changing the basic meaning of the sentence. The clause following "which" cannot be deleted in this case; the result wouldn't even be a complete sentence.
You could recast the sentence to put the idea in a nonrestrictive clause:
A new theory, which proposed that men were descended from Martians, was put forth.