I know this is not related to the topic discussed, but I think the OP would be interested in this. There is an theory that mitochondria was an independent prokryotic cells that merged with another single cell organism called Symbiogenesis. Basically, 1.5 billion years ago, the ancestor of eukrayotic cells had engulfed an protebacteria ( close relative to rickettsiales which cause Q-fever) and instead of destroying by fusing it with an lysosomes vesicle containing enzymes (exocytosis), it forms an Endosymbiont relationship. The fusing of this protebactria and the single cell organism resulted in an acceslation of evolution. This is because the proto-mitochondria gave the cell more ATP synthesis capacity and energy production. When complex eukroyotes evolved sexual reproduction, the mDNA is passed on from the Females side. This is how we track our paternal ancestry.