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Notes -
I’ve seen this posited as well - that the primary endosymbiosis that formed eukaryotes, where a free-living bacterial cell got incorporated into a host cell to create mitochondria, expanding the cellular energy budget, is an extremely rare event that might limit the prevalence of advanced life throughout the galaxy.
That hypothesis falls apart a lot though when you consider that this has actually occurred multiple times; e.g. chloroplasts are also the result of endosymbiosis and in fact they have developed independently in Archaeplastida and Paulinella alike, which certainly speaks against the idea of it being a rare occurrence. And then there are the nitrogen fixing organelles in certain types of marine alga, which appear to have been a result of this same process. For primary endosymbiosis to have happened at least four times indicates the mechanism itself is not rare, and significantly weakens its ability to explain the Fermi paradox.
Yea but all of those symbioses happened after the initial mitochondrial endosymbiosis, which drove adaptions that made those subsequent acquisitions much easier. I think there’s something special about that first endosymbiosis that makes it especially difficult.
I just looked through my sheaf of saved papers to evaluate this and interestingly enough there are further examples of symbioses occurring without the initial mitochondrial endosymbiosis (albeit not well publicised ones) - for example mealybugs possess specialised cells containing endosymbiotic β-proteobacteria which themselves contain endosymbiotic γ-proteobacteria, essentially forming a set of nested obligate endosymbionts. The authors note this probably occurred multiple times, with different precursors of the ancestral γ-proteobacteria independently infecting the ancestral β-proteobacteria on approximately four occasions.
There's also the fact that it's hard to gauge how likely endosymbiosis would have been on early Earth; mitochondrial development likely hindered the development of further endosymbiosis along those lines to some degree because the already established eukaryotic lineage filled available niches.
Huh cool, I didn't know about this serial endosymbiosis!
I will say that even in that case the mitochondrial origin event is still a little different, as it was the fusion of two very different clades of life (Asgard archae and alpha-proteobacter).
In Lane's book (The vital question) there was also some evidence for a proto eukaryote found via deep sea trawling. So you may be right.
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