Re: Intelligent alien life
True, though seriously, if Silicon could build polymers like Carbon can I think we would see them naturally on Earth. We do see complex Silicon molecules and they are called Rocks. Despite old Star Trek episodes, you can watch them for a long time without them moving very far.
The other possible chemical substitution that is discussed is liquid Ammonia as a solvent instead of Water. That would be a seriously cold world in which I doubt we could survive ourselves.