[khafra]

The probabilistic evidence is a bit more indirect, and depends on anthropics:

Since we don't see galaxy-spanning, highly advanced civilizations, there seems to be some combination of factors that prevents planet-bound dirt from turning into them. If those factors are primarily after our stage of development between dirt and galactic civilization, that's Very Bad News, because we shouldn't expect to get extraordinarily lucky. If those factors are primarily before our stage of development, that's reasonably good news, because we've already made it past the hard part.

Seeing bacteria isn't as bad as seeing the ruins of a civilization just past our stage of development. But it does move the probability-mass forward more than if we just found dirt.

[kalms]

There's so many things that can go wrong, during the period needed for advanced intelligent life to evolve. Add to that the inherent difficulty of moving between solar systems. Space is vast, and we're not really making that much noise. I don't see a big paradox here?