There has recently been some speculation that life started on Mars, and then got blasted to earth by an asteroid or something. Molybdenum is very important to life (eukaryote evolution was delayed by 2 billion years because it was unavailable), and the origin of life is easier to explain if Molybdenum is available. The problem is that Molybdenum wasn't available in the right time frame on Earth, but it was on Mars.
Anyway, assuming this speculation is true, Mars had the best conditions for starting life, but Earth had the best conditions for life existing, and it is unlikely conscious life would have evolved without either of these planets being the way they are. Thus, this could be another part of the Great Filter.
Side note: I find it amusing that Molybdenum is very important in the origin/evolution of life, and is also element 42.
As someone pointed out to me when mentioning this to them, to be a candidate for Great Filter there would need to be something intrinsic about how planets are formed that cause these two types of environments to be mutually exclusive, else it seems like there isn't sufficient reduction in probability of their availability. Is this actually the case? Perhaps user:CellBioGuy can elucidate.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.