This question has a naturally associated question which is how common is life? Not just on earth but anywhere in the universe. To begin making assumptions of the likelihood of life outside of our planet we have to first understand how life arose here first. Since we have not found any evidence of life outside of our planet as of yet, even though our search is only just beginning, we must conclude that at least intelligent life is not that common of a thing. Intelligent meaning capable of building devices using electromagnetic waves that we could detect. If intelligent life were extremely common you think seti would have found something by now. If we have a difficult time finding the distinction of the first forms of life and inanimate molecules here on earth, how would be able to distinguish a life form of an alien race. Maybe they don't use electromagnetic waves but are into some quantum entanglement for communication. It is possible there are other life forms that don't use water, proteins, etc and are of a completely different order than just molecular configurations. It just so happens on this particular planet intelligent life takes the form brains, bones, flesh, etc.. it could be a different story somewhere else. I would like to hear what an astrobioligist would think about an alien made up of some bosonic particles or something alike.