I haven't read Rare Earth yet. Someday, but my stack of books to read is taller than me.

My guess is that life is ubiquitous. It will pop up rapidly whereever it is possible, and hold out under harsh conditions. I think it is probable that there is life beneath the surface of Mars, though I doubt the Europa fad will pay off. However, I think that intelligent life (assuming it is not overly arrogant to call ourselves "intelligent"), is likely to be somewhat rare. If I am not mistaken, this is pretty much what the authors of the book Rare Earth think, though they may think that life in general is more rare than I do.

Look at the debate between Ernst Mayr & Carl Sagan. Ernst Mayr is the dean of evolutionary biologists, and Carl Sagan was certainly the arch champion of SETI. But I think Sagan's overwhelming desire for intelligent life to be ubiquitous might have gotten the better of him. As Mayr points out, if (human-like) intelligence alone is a strong factor for natural selection to work on, then there should be more than one "intelligent" species, right here on Earth.

I think it's more appropriate to say that intelligence evolves to meet the needs. We see ourselves as "intelligent", when it might be more appropriate to see ourselves as "technological". The more we learn about intelligence in non-human animals, the more it seems that they have the intelligence they need. How does a California Sea Otter know that it can crack open crabs with rocks? It has the intelligence it needs to survive, that's how.

Trish: Wasn't it Fermi who raised the question of, if life is everywhere then where is it/why aren't they [aliens] here?

Probably was; I can't remember, but they do call it the Fermi Paradox. I worked on the NASA SETI project, back in the days when it had one. I always figured the Fermi Paradox was a pretty flimsy excuse. Not to sound too ufo-cultish, but, how do you know that aliens aren't here? The fact is that we can't easily tell that the guy in the next office isn't spying for the Russkies (as the CIA, FBI & etc. can attest), because they are "well disguised". What makes one think that a real live genuine space-alien would be any more obvious? And remember, an alien spacecraft could have come and gone, without us having a clue that it was here, right up to the close of WWII in the late 40's, when radar technology reached the point that such a spacecraft could be detected. We don't know that aliens hav never been here, and we don't know that they are not here now. We may assume both, and it may even be a good assumption, but that does not excuse the lazy practice of failing to distinguish between knowledge & assumption.

I like the idea of searching for extraterrestrial intelligence, because it returns knowledge. Each search sets limits on what is going on out there, as compared to our ability to see it. We might make assumptions about what we expect, or don't expect to see. But once we look, we know what we saw, and what we were capable of seeing, had it been there. We may never know if there is any other intelligence out there, but I can hardly imagine that looking will do any harm.