Could the great filter be something like developing language? That's something that seems quite rare (only one species on Earth has it). If so, then discovering bacteria in the Solar System wouldn't be such a cause for worry.
> Could the great filter be something like developing language?
Perhaps it's a combination of factors?
Dolphins are social animals that appear to be capable of complex communications among themselves, but don't have hands to manipulate their environment the way we can.
Another interesting one I've heard is getting into space, period. It's possible that there are other technological civilizations out there, but all stuck under a few hundred km of ice or on a 2-earth-mass monster where it's impractical to reach orbit.
Earth may only be special in being in a sweet spot between Mars (too small to hold an atmosphere or protect from cosmic radiation, hence no life) and Gliese 832c (with its low-orbit velocity of something like 15km/s, hence much less practical to put stuff in space).
You can also get shielding with a thick enough atmosphere, or being under a solid ice/rock crust. But both of those also make it much harder to reach space.
