How difficult is the transition to multi-cellular life?

[Drashner1]

256 garden worlds and 50 xenosophont species means that one in five garden worlds develops a civilisation, which seems a bit too high. However most of the xenosophonts we have described have not emerged on Earth-like worlds.

Couple of thoughts here:

a) This might be an argument for the idea that a 'garden world' is any planet with a complex, naturally evolved, biosphere - regardless of whether or not baseline humans would find it shirtsleeve habitable. Given the sheer environmental diversity that Y11k human descended beings enjoy, the definition of 'shirtsleeve habitable' being limited to humans like us feels a bit...odd.

b) Regarding the number of xenosophont races - are we counting only races currently extant or those that are currently extinct? Also, some of the currently extant xenosophonts are extremely old (the Muuh, the Soft Ones). If we are including extinct and/or ancient races in this count, would stellar drift and/or galactic rotation play a factor here? Basically, would it be inappropriate to include some of these races since they they originally evolved far from the Terragen Bubble and have only had their worlds or artifacts come into Terragen space relatively recently, cosmically speaking?

If we stick with the figure of 256 Earth-like garden worlds, but introduce my suggestion in post #14 that would mean ~2500 worlds with macroscopic life in total, that is to say 10 times as many (but only 1 in 10 are Earth-like).  However the figure of 256 was established in the early years of the scenario, before we expanded the territory somewhat; it could easily be twice that now.

True - although we could just as readily lower it if we found that to fit in better with our current take on things. Or we could could non-baseline human habitable, but possessed of a complex ecosystem worlds within that 256 number.

Not overly attached to the 256 number (not even totally sure I'm remembering that number accurately) but would like us to come to some sort of standard for the project in terms of both garden world numbers and what exactly is meant by that term.

Todd