Your cells run on captured bacteria

Mitochondria — the power plants in every one of your cells — were once free-living bacteria that got engulfed by another cell around two billion years ago and never left. They still carry their own separate DNA. Lynn Margulis was ridiculed for proposing this; she was right.

— Lynn Margulis (Sagan), On the Origin of Mitosing Cells — Journal of Theoretical Biology, 1967