Here goes:
Alternate explanations for rarity of intelligence:
3a) Interstellar travel is prohibitively difficult. The fact that the galaxy isn't obviously awash in intelligence is a sign that FTL travel is impossible or extremely unfeasible.
Barring technology indistinguishable from magic, building any kind of STL colonizer would involve a great investment of resources for a questionable return; intelligent beings might just look at the numbers and decide not to bother. At most, the typical modern civilization might send probes out to the nearest stellar neighbors. If the cost of sending a ton of cargo to Alpha Centauri is say, 0.0001% of your civilization's annual GDP, you're not likely to see anyone sending million-ton colony ships to Alpha Centauri. In which case intelligent life might be relatively common in the galaxy without any of it coming here; even the more ambitious cultures that actually did bother to make the trip to the nearest stars would tend to peter out over time rather than going through exponential expansion.
3b) Interstellar colonization is prohibitively difficult. If sending an STL colony expedition to another star is hard, sending one with a large enough logistics base to terraform a planet will be exponentially harder.
There are something on the order of 1000 stars within 50 to 60 light years of us. Assuming more or less uniform stellar densities, if the probability of a habitable planet appearing around any given star is much less than 0.1%, it's likely that such planets will remain permanently out of reach for a sublight colony ship. In that case, spreading one's civilization throughout the galaxy depends on being able to terraform planets across interstellar distances before setting up a large population on those worlds. Even if travel across short (~10 ly) interstellar distances is not prohibitively difficult, there might still be little or no incentive to colonize the available worlds beyond one's own star system. After all, if you're going to live in a climate-controlled bunker on an uninhabitable rock where you can't step outside without being freeze-dried or boiled alive, you might as well do it somewhere closer to home.
NOTE: This amounts to "super-difficult life," but it does not require that there are few intelligent species in the galaxy. If the emergence of life is (for lack of a better term) super-duper-difficult, or if most planets are inhospitable enough to make it impossible, then we could have many thousands of intelligent species in the galaxy without any of them being likely to reach each other.
3c) Interstellar colonization might be "psychologically" difficult. For instance, what if the next logical step in the evolution of modern civilization is an AI singularity, possibly coupled with some kind of uploading of consciousness into machines? Either way, our descendants of 200 years from now might well be, to our eyes, a civilization of robots. To a society of strong AIs, interstellar colonization is liable to look a little different. Traveling to even the nearest stars, you will be cut off from the rest of your civilization by a transmission gap on the order of 10^20 cycles just because of the lightspeed limit.*
That might sound like an even worse idea to them than spending a long lifetime in cryogenic storage and having a twenty year round trip communication cycle with Earth does to us. In which case they're likely to stay at home and come up with elaborate social activities or simulations to spend their time, because interstellar colonization is just too unpleasant to bear considering.
*Assuming roughly 1 THz computing, for relatively near stellar neighbors. This estimate is probably too low, but I need some numbers and I am nowhere near an expert on artificial intelligence or the probable limits of computer technology.
For a machine-phase civilization, the only one of these that seems plausible is 3c, but I can't think of any reason why no one in a given civilization would want to leave, and assuming growth of any kind, resource pressure alone will eventually drive expansion. If the need for civilization is so psychologically strong, copies can be shipped and revived only after specialized systems have built enough infrastructure to support them.
It seems far more likely to me, given the emergence of multiple civilizations in a galaxy, that some technical advance inevi...
We have a sample of one modern human civilization, but there are some hints on how likely it was to happen.
Major types of hints are:
Data for:
Data against:
To me it looks like life, animals with nervous systems, Upper Paleolithic-style Homo, language, and behavioral modernity were all extremely unlikely events (notice how far ago they are - vaguely ~3.5bln, ~600mln, ~3mln, ~200k or ~600k, ~50k years ago) - except perhaps language and behavioral modernity might have been linked with each other, if language was relatively late (Homo sapiens only) and behavioral modernity more gradual (and its apparent suddenness is an artifact). Once we have behavioral modernity, modern civilization seems almost inevitable. Your interpretation might vary of course, but at least now you have a lot of data to argue for your position, in convenient format.