In my younger days I had a soft spot for well crafted “space opera,” with David Brin’s “Uplift” series being an excellent exemplar. And yet the reality is that part of me always felt that these were more akin to space fantasy than science fiction. The reason is that a world such as the one you see in Star Trek, where aliens often meet each other at technological parity, just did not seem intuitively plausible to me. Rather, much more likely was the dark universe Gregory Benford outlines in Great Sky River. In this imaginging intelligent life forms meet across a chasm of technological sophistication which makes the idea of a broad class of organisms with the term “intelligent life form” laughable; humans were to the “higher intelligences” in this universe as ants are to us. Benford’s novel was depressing from a human perspective, and its coldly Malthusian universe reflects the pessimism of many biologists. I first encountered this in Jared Diamond’s The Third Chimpanzee, where the author suggests that optimism in regards to “First Contact” promoted by astronomers such as Carl Sagan in his work Cosmos was incredibly naive. Diamond’s basic contention was that if the universe was full of intelligent life forms, then we had better be glad that they weren’t here yet, because it probably wouldn’t end well for human beings, using our own planet’s encounters between different civilizations as models.
But I no longer even hold to the position that the cosmos is teeming with intelligences of varied levels of sophistication. Rather, I would guess that we humans are all there is in this galaxy.* I don’t speak of this often because I haven’t thought about this issue in great depth. And with these incredibly big picture inferences deduced from sparse data points one has to admit (at least I do!) that one’s confidence is just not high. What can a puny human truly grasp?
So why would I suggest that we are the only intelligence? Basically, the Fermi paradox. Rather that outlining my inchoate thoughts I’ll point you to Nathan Taylor’s posts at Praxtime, Life on Wet Planets, and Intelligent life is just getting started. With the appropriate caveat that we don’t really know much about this in any deep sense, it strikes me that major bottleneck for the emergence of intelligent life is the transition from simple unicellular life forms to multicellular organisms. Therefore the prediction from this model is that the universe is filled with life, but of the single celled kind. As Taylor lays out time almost ran out for the emergence of intelligent life on this planet (the sun is getting brighter, and it seems like that runaway greenhouse is inevitable ~1 billion years into the future).
Yet please note that we are likely just the first intelligent life form. If we go extinct soon before developing a form of automaton which can populate the galaxy there is plenty of time for other organisms similar to ourselves to emerge. The local universe is relatively young when measured in terms of the future existence of G (or K) class stars. That means the “responsibility” of being the first intelligent galactic species is somewhat attenuated on a cosmic scale.
Addendum: It is possible that the universe is teaming with intelligent non-technological life forms, and the upward ratchet of cultural complexity of Homo sapiens is a major bottleneck. I doubt that, therefore I have omitted a qualifier of technological intelligences, because I do think that if intelligences were numerous then many would have become technologically sophisticated.
* The whole space of possibilities is so much larger than our galaxy that I am somewhat wary of making broad assertions about the universe.