[nnq]

What intuition makes you believe that once you have microbial live + some kind of stability (low levels of ionizing radiation + a decent level of chemical stability, so not too hot), evolution to advanced multicellular organisms is not pretty much the default?

I'd be more or less inclined to believe that once you have the first microbes advanced enough to support cell2cell signaling, and some kind of "condensed blueprint information mechanism" (what DNA is on Earth), the rest of the ring would pretty much be unavoidable, though slow.

Human-level intelligence might be rarer, in that you'd probably need a quite few evolutions to plateau + extinction cycles (think cambrian extinction, dinosaurs' extinction and probably a few others), most likely more than we had on Earth (I's assume we kind of got lucky), probably since what you get to something like "monkey level intelligence", most increases in intelligence would be disadvantageous for survival, unless they happen in a species with a tool-usage-friendly body plan + some pack/social tendency + environment change to challenge them to evolve but not to extinct + "construction friendly solid-ish environment" (I imagine dolphins, squids and octopuses, and most monkeys plateaued because they lacked at least one ingredient of the magic combination, and this tends to be the rule...), but this should pretty much happen.

And then you have the fact that our galaxy should have a few billions or tens of billions of Earth like planets based on what we know (probably more "life friendly" planets if you allow that other chemistries than "carbon + liquid water" could support life), it's pretty much unavoidable that there are quite a few human level species in the gallaxy.

The scary bit it that there should be quite a few above that, that probably went past their first singularity/transcendence event and are no longer constrained by biology and individual mortality and probably dumped their biological bodies long ago... scary being the fact that we don't see any sign of their presence.

[Symmetry]

The problem is that basic microbes only resperate over their surfaces so they have big square/cubed law limits on how large they can be and, by extension, on how complex their genomes can be. You need a hopeful monster that can run its metabolism throughout the cell to develop large genomes of the type that can support differentiated cells in multi-cellular creatures.

[nnq]

> You need a hopeful monster that can run its metabolism throughout the cell to develop large genomes of the type that can support differentiated cells in multi-cellular creatures

There are a quite a few creatures sitting at the boundary between unicellular and multicellular like colonial algae (https://en.wikipedia.org/wiki/Volvox) and hydra (https://en.wikipedia.org/wiki/Hydra_(genus) - I call this one "at the boundary" because you cans simply cut a piece of an adult and have the pieces heal intro 2 live adults, so "being multicellular" happened before having proper genomic mechanisms for encoding body plan and body plan related cell differentiation for this organism, for example, information pattern is all emergent based on relative position of cells - there are experiments to prove this, it's not all inference from what I said above). So there's no missing link here, you can have a pretty smooth continuum from unicellular to "proper multicellular" (like body plan with cell types differentiation encoded in genome so it can "boot up" from just one fertilized cell). Also yeasts and unicellular algae have pretty huge genomes with tons of redundancy / duplication / room to spare / room to accommodate multiple cloned copies etc..

Yeah, biologists do like to point out that there are "very clear differences" between the organisms I mentioned above, and that they are clearly separate things, because they do love their beloved classifications of things :) (god forbid one puts an organism on wrong shelf of their library)... but if you look at things more computationally/informationally, there's a lot of flexibility and no clear gaps/boundaries "down at the bottom of the tree of life"... really can't see anything that evolution would have a problem searching through, no big ridges to block gradient descent in this problem space (like there are before proper-unicellular-life-with-genetic code, despite all "RNA world" theories... that's why I'd be more inclined to think that there may be planets with life supporting condition where microbial life has not had the chance to start yet despite the requirements being there... rather than life "getting stuck" at the microbial stage)

[Symmetry]

I'm not talking about the difference between unicellular and multicellular life. I'm talking about the difference between small cells that resperate over their cell walls (prokaryotes and archaea) and cells that resperatory organelles (eukaryotes). Both algae and hydra are already over this barrier and I agree that once you're a eukaryote or the alien equivalent there's an easy path from there to complex multi-cellular interactions.

So certainly there's no missing link there. Heck, you've even got prokaryotes forming bacterial mats. The clear boundary is, again, between small cells that resperate over their surfaces and more complex cells that can resperate using structures throughout their interiors.