[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.