There doesn’t actually have to be one great filter. If there are 40 “little filters” between abiogenesis and “a space-faring intelligence spreading throughout the galaxy”, and at each stage life has a 50% chance of moving past the little filter, then the odds of any one potentially life-supporting planet getting through all 40 filters is only 1 in 2^40, or about one in a trillion, and we probably wouldn’t see any others in our galaxy. Perhaps half of all self-replicating RNA gets to the DNA stage, half of the time that gets up to the prokaryote stage, half of the time that gets to the eukaryote stage, and so on, all the way up through things like “intelligent life form comes up with the idea of science” or “intelligent life form passes through an industrial revolution”. None of the steps have to be all that improbable in an absolute sense, if there are enough of them.
The “little filters” wouldn’t necessarily have to be as devastating as we usually think of in terms of great filters; anything that could knock either evolution or a civilization back so that it had to repeat a couple of other “little filters” would usually be enough. For example, “a civilization getting through it’s first 50 years after the invention of the bomb without a nuclear war” could be a little filter, because even though it might not cause the extinction of the species, it might require a civilization to pass through some other little filters again to get back to that level of technology again, and some percentage might never do that. Same with asteroid strikes, drastic ice ages, ect; anything that sets the clock back on evolution for a while.
There doesn’t actually have to be one great filter. If there are 40 “little filters” between abiogenesis and “a space-faring intelligence spreading throughout the galaxy”, and at each stage life has a 50% chance of moving past the little filter, then the odds of any one potentially life-supporting planet getting through all 40 filters is only 1 in 2^40, or about one in a trillion, and we probably wouldn’t see any others in our galaxy. Perhaps half of all self-replicating RNA gets to the DNA stage, half of the time that gets up to the prokaryote stage, half of the time that gets to the eukaryote stage, and so on, all the way up through things like “intelligent life form comes up with the idea of science” or “intelligent life form passes through an industrial revolution”. None of the steps have to be all that improbable in an absolute sense, if there are enough of them.
The “little filters” wouldn’t necessarily have to be as devastating as we usually think of in terms of great filters; anything that could knock either evolution or a civilization back so that it had to repeat a couple of other “little filters” would usually be enough. For example, “a civilization getting through it’s first 50 years after the invention of the bomb without a nuclear war” could be a little filter, because even though it might not cause the extinction of the species, it might require a civilization to pass through some other little filters again to get back to that level of technology again, and some percentage might never do that. Same with asteroid strikes, drastic ice ages, ect; anything that sets the clock back on evolution for a while.
If that was true, we’d expect to find microbial life on a nontrivial number of planets. That’ll be testable in a few years.