Question

why did endosymbiosis not occur by a ”proto-eukaryotic” cell phagocytizing another cell?

Answer 1

Endosymbiosis not occurs by a proto-eukaryotic cell because already it becomes. Simply as we know the theory of "origin of eukaryotes". And how it compartmentization every organelles which present in it. The Emergence of eukaryotes have been considered as a major transition in evolution of life on earth. This transition not only a highly sophisticated subcellular compartmentization and a tight control of metabolism compartmentlization, but also a high level of control in gene expression. The endosymbiotic origin of mitochondria and plastid has prominent position in eukaryotes.

Nowadays we know that the intracellular symbiotic association involving a host cell and a symbiont has been one of the important evolutionary forces in evolution, and has been key in major innovations in Eukaryotes. The origin of mitochondria and plastids, which introduced aerobic respiration and photosynthesis to Eukaryotes, respectively, are the two most significant examples, but there are many other endosymbioses that occured more recently across the Eukaryotic Tree of Life. The conversion of a free-living organism to an organelle, involves a high degree of metabolic integration, the transfer of genes from the symbiont to the host, a process called endosymbiotic gene transfer (EGT), and targeting and transport systems for re-localizing the proteins back to the endosymbiotic organelle (Cavalier-Smith and Lee, 1985). How this integration happens is an active research topic, with major implications for our understanding of eukaryogenesis.