Not to be too pessimistic, but there's no reason that there should only be one "Great Filter". There could easily be multiple astronomically improbable events barring interstellar civilizations from bearing fruition. It might be just as improbable for unicellular life to form as it is for unicellular life to beget multicellular life, so on and so forth.

Or even reason to assume that exobiotic life is cellular, and needed to jump from singular to multi-cellular structures like ours did on this planet.

I mean, in principle, but that's probably like assuming that exobiology is carbon based. Carbon and cells are both really super useful.

The great filter exists at all times as cited by the fact that the longevity of a species does not guarantee their future survival. All life may die off at any time.

The jump to the start of unicellular life seems like the big stumbling block. It might be so incredibly rare that it's lucky for it to even happen once in the universe.

Evolution is a process rooted in randomness. The more faces a dice has, the longer you need to throw it to get the highest number. Or, reversely: the longer you have to throw your dice to get the highest number, the more faces it generally has.

It took "only" < 400 million years for life to develop, meaning unicellular life, but it took 3 billion years for multicellular life to develop. If we apply this basic rule, multicellular life seems the hard thing. Of course though, statistics wise this is pretty crappy ground to stand on because we only have one sample :).

The hard part is (probably) the transition from Prokaryotes to Eukaryotes. The transition from unicelular Eukaryotes to multicelular Eukaryotes is (probably) easy [1] .

Edit: [1] From https://en.wikipedia.org/wiki/Multicellular_organism#Occurre...

> Multicellularity has evolved independently at least 46 times in eukaryotes, and also in some prokaryotes, [...] . However, complex multicellular organisms evolved only in six eukaryotic groups [...]

One sample is hard to read because even low odds can happen. Picture the following statement: "The chance of life forming anywhere in the universe over a 100 billion year period is equal to 1 in Graham's number".

If this statement is true then it is incredibly unlikely life forms in our universe at our current stage exist. Low odds can occur and the only reason anyone is here to observe them is because they did occur. Since we only know of life forming once we cannot make assumptions about the odds of it happening.

"It took "only" < 400 million years for life to develop"

Yes, that's what people assume, but the universe existed for billions of years before that, so what if life might have used all that time to evolve before it landed on Earth deep inside some rocks?

Unicellular in the form of bacterium may in fact not be that much of a jump. It may even have happened twice in the form of archaea and bacterium.

The jump from prokaryotic to eukaryotic is far more intimidating.

That's actually happened a bunch of times, too. I'm just going to link to a comment I made a while back: https://news.ycombinator.com/item?id=18456049

"Diatom-derived plastids" blew my mind.

You need heredity, differential reproduction and selection. We really just need to start with a single case of a self-replicating system. Hard yes, but with countless trials running in parallel for millions of years it seems intuitively possible

If the universe exists for eternity, and in approximately the same form, then even very small odds may approach near certainty.