How difficult is the transition to multi-cellular life?

[iancampbell]

As I see it there were three events that led to eukaryotic life as we know it; the evolution of the cell nucleus, followed by the inclusion of endosymbiotic mitochondria, followed by the inclusion of endosymbiotic chloroplasts.  There is some doubt about the order in which these events occurred, but here is Wikipedia's version.

It should be noted that this need not be the way that life has developed on other worlds; in fact I would go as far as saying that eukaryotes as such can only be found on Earth, and any instances of a similar arrangement which evolves elsewhere would need to be classified separately. Maybe the nucleus is not necessary as a way of separating out the nuclear DNA, or maybe there are several different nuclei, or maybe there are different endosymbiotes, or none.

Multicellular life has evolved several times on Earth, and maybe most forms of multicellular life in the universe are colonial organisms rather than Earth-like cellular organisms. We have at least one non-cellular species of macroscopic organism in OA (the Soft Ones) and these might be representative of a life-type that is at least moderately common.

Hi Steve, thanks for the answer. My question about eukaryotes was asked in the context of the great filter. I.E., the idea that there is some evolutionary barrier that stops 99.9999% of biospheres from producing intelligent lifeforms (that can spread through space).

In your opinion, how likely is it that an alien biosphere composed solely of prokaryotes could evolve into eukaryotes? In his book, Nick Lane contends that the symbiotic relationship between prokaryote and mitochondria was an exceedingly unlikely event.

Another question, IMHO, is whether multicellular prokaryotes are possible.