[simonh]

The thing that only happened once on Earth and that's a prerequisite to developing complex life forms is not endosymbiosis, it's life going multi-cellular. They are not the same thing.

Endosymbiosis is not identical with going multi-cellular, and it seems that all but perhaps one of the known instances of endosymbiosis didn't play any role in us going multi-cellular anyway. In fact this article makes the case that it may not have been critical at all.

[gus_massa]

Multicellularity looks relatively easy. From https://en.wikipedia.org/wiki/Multicellular_organism#Occurre...

> Multicellularity has evolved independently at least 25 times in eukaryotes, and also in some prokaryotes, like cyanobacteria, myxobacteria, actinomycetes, Magnetoglobus multicellularis or Methanosarcina. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, symbiomycotan fungi, brown algae, red algae, green algae, and land plants. It evolved repeatedly for Chloroplastida (green algae and land plants), once for animals, once for brown algae, three times in the fungi (chytrids, ascomycetes and basidiomycetes) and perhaps several times for slime molds and red algae.

[simonh]

It’s relatively easy once you have evolved the biochemical infrastructure to support it, but on Earth that took several billions of years to achieve. There’s no way around it, no amount of hand waving how easy it is negates the fact it took billion years of evolution to do it.

Also most of those forms of multicellularity are extremely basic, little more than tangles or sheets of cells, even after hundreds of millions of years of further evolution. That’s not likely to get to intelligent life.

[gus_massa]

I think we agree. One you have prokaryote it's probably easy to get multi cellular prokaryotes.

My guess is that the transition form eukaryotes to prokaryotes in the hard step.

Also, photosynthesis seams to be more complicated than what I expected. Perhaps that is the hardest step. (It's an indirect step to intelligent life, but perhaps a lot of free oxygen to burn food efficiently is necessary for intelligent life.)

[sterlind]

it took at most a billion years. there may have been predecessors that were lost - the only evidence we have from that era is what made it into the DNA of surviving ancestors.

I guess I still feel like abiogenesis should be the bigger filter. we have ideas and suggestive experiments about how it happened, but nothing's come close to convincingly demonstrating how fully self-replicating life can evolve through simple steps. whereas endosymbiosis just seems to require two prokaryotes from different trees surviving in the same membrane, and it's not difficult for me to imagine a plasmid slipping in and making copies of enough enzymes to get by.

[simonh]

As I pointed out though, endosymbiosis isn’t by itself enough. It can provide many benefits, one of which might be the ability to develop complex sophisticated multicellular organisms, but it doesn’t guarantee that capability.

This sub thread is specifically about developing intelligent life, and of all the branches of multicellular life on earth only one has achieved that capacity, animals. None of the others seem to be anywhere close, or ever likely to be, for all their fancy biochemical tricks. So it seems like the vast majority of endosymbiotic events really don’t help much towards that outcome.