Check out the abstract of this paper: https://www.pnas.org/content/99/18/11558 It takes for granted not only the long-established belief that chloroplasts were once free-living blue-green bacteria, but a number of eukaryote-within-eukaryote endosymbioses. As for multi-cellular life, plants, animals, fungi, and {brown,red,green} algae independently developed it: https://en.wikipedia.org/wiki/Multicellular_organism
This seems like a good article that describes the multicellularity problem in general, several interesting borderline cases, experiments in inducing the evolution of simple multicellularity (in one case, just by introducing predators into a culture), and ideas as to why it took as long as it did: https://www.sciencemag.org/news/2018/06/momentous-transition...
This seems like a good article that describes the multicellularity problem in general, several interesting borderline cases, experiments in inducing the evolution of simple multicellularity (in one case, just by introducing predators into a culture), and ideas as to why it took as long as it did: https://www.sciencemag.org/news/2018/06/momentous-transition...