This theory is actually proposed by Nick Lane et al - it says that microbial life is probably ubiqutuous and that complex life is not and was a happy accident on Earth, which means its very improbable on other planets. The prereqs for it happened only once on Earth, when one microbe swallowed another and the smaller one became mitochondria enabling complex cell structure and shape. Given that microbial life exist in extreme environments on Earth and that its prereq is just a hot molecular soup, its probable it exists everywhere. On the other hand, the 'happy incident' didn't repeat on Earth to create another complex cell form - all higher life uses the same cell structure.
That's what the great filter theory means when they talk about it being behind us -- we don't see life in other places because developing to where we are now is almost impossible.
I don't buy it though -- we're in an average planet on an average star, nothing about our environment seems to be particularly rare.
I think the statement that we are on an average planet deserves more scrutiny. We have yet to observe another planet like ours.
There are a number of Earth's attributes that we don't understand and have poor extra-solar data on. Particularly, the following:
Technically active plates after 4.5 Byrs.
Effective magnetic core after 4.5 Byrs.
Large amounts of surface water, presumably from our late heavy bombardment.
Small enough to leave with a chemical rocket while meeting the above.
I tend to agree with both your points, but was addressing the specific claim that Earth-like conditions are average, or at least not particularly rare.
We simply don’t have data to support that claim, and the parameters required to make earth habitable for life (as we know it) are vastly more numerous than most people acknowledge.
Lets start with Oscam Razor then: its wast universe. Even on Earth, there are untouched places. All other theories compared to this one, which is easy to understand and also already known to be in effect on this planet, are as close to the truth as theory that great filter are evil pink elephants in the center of the galaxy.
