I've heard that the theory is we originally got mitochondria in our cells because of phagocytosis. Mitochondria are quite strange, because even though they seem to be a necessary part of most animal cells, they are also quite distinct and have their own kind of DNA (which is passed down differently from our own DNA. That's why it can be used in some kinds of research where normal DNA can't be).
They think it was originally a bacteria that got absorbed by phagocytosis and entered into a symbiotic relationship with the devouring cell.
I think that's so intriguing.