On the other hand, there is also an assumption that if life can survive in very different conditions to our own, that would automatically preclude the possibility that other life may have evolved in conditions which are quite similar.
I don't think the "common humanoid ancestor" holds much scientific ground. Humans are apes. We share 97% of our DNA with chimpanzees. That is a greater percentage than chimps share with other apes.
However, with all the possibilities out there, I would not completely discount the remote possibility that somewhere there may be another species which has evolved along similar lines to our own. Whether they are close enough that we will ever make contact - that may be another story. And I also doubt very much that we would be able to interbreed. But the development of similar forms, I wouldn't consign to impossibility just yet.