It is now generally accepted that the mitochondria in our cells are of bacterial origin, specifically proteobacteria. Without plunging deeply into the biology, the current endosymbiotic hypothesis suggests that bacteria were taken up by a cell (endocytosis- a common phenomona) and subsequently not only survived but reproduced within the cells as they divided.
That is enough science for now. I suggest you read Not Just Kingdoms But Communities. Just one page long, it is rich in insights. Part bacteria, part virus, you may be what your cells ate- billions of years ago.
* North American Mycological Association