Actually, what we keep finding, over and over again, is that our situation in the universe is not unique but ordinary. It is far more likely, more "ordinary" that we will find life wherever there is free water than for life on Earth to be unique.
If you see "Rare Earth" as an explanation for Fermi's Paradox, I think a more likely reason is the vast distances between stars.
Think it through; think it through. In the face of "billions and billions" they throw 0.001's and 0.0001s.
- Earth in the right orbital distance. All we seem to find are "hot gas giants" orbiting very near their primary.
- Earth with a single large moon that was formed by an impactor about the size of Mars, which came from precisely the right direction and angle. Any variation in these variables: no moon. The moon stabilizes our precession and hence gives us predictable and repeaing seasons.
- An astounding amount of water for a rocky planet.
- The "snowball earth", which killed off 98% of all life 600 million years ago, and then killed 98% of the cold-adapted survivors when the temperature overshot to as high as 70C.
Etc. It appears that the authors assemble sufficient multipliers (.0001x0.001x0.00001x....) to wipe out the billions and billions.
I would like to believe there are huge numbers of intelligent species. But I find the arguments in Rare Earth compelling. As Fermi asked, WHERE ARE THEY?.
As for the vast distances, some races could be nearly immortal, or possess technology to make themselves so. As I have commented, a technological civilization a few centuries or millennia in advance of ours would have seemingly godlike powers. Surely they could attain 0.05 "C". There has been ample time for self-replicating robot probes, running at 5% of "c", to visit every single star in the Galaxy. If several ET civilizations figure this out (as we have) there ought to be a veritable traffic jam of probes swarming about the Solar System. Again, Fermi asks: "Where are they?"
--Boris