> Could they have remained as independent organisms and use their massive energy budget to evolve independently? There are independent mitochondrial relative.

> Could they have remained as independent organisms and use their massive energy budget to evolve independently? There are independent mitochondrial relative. They are mostly parasites that live inside cells. From https://en.wikipedia.org/wiki/Mitochondrion https://en.wikipedia.org/wiki/Mitochondrion > The proto-mitochondrion was probably closely related to Rickettsia. From https://en.wikipedia.org/wiki/Rickettsia https://en.wikipedia.org/wiki/Rickettsia > Being obligate intracellular bacteria, rickettsias depend on entry, growth, and replication within the cytoplasm of living eukaryotic host cells (typically endothelial cells). > Most notably, Rickettsia species are the pathogens responsible for typhus, rickettsialpox, boutonneuse fever, African tick-bite fever, Rocky Mountain spotted fever, Flinders Island spotted fever, and Queensland tick typhus (Australian tick typhus).