Mitochondria Were Once Free-Living Bacteria That Were Absorbed by Cells
The endosymbiotic theory, confirmed by Lynn Margulis in the 1960s and now universally accepted, holds that mitochondria — the powerhouses of eukaryotic cells — were once independent alpha-proteobacteria that were engulfed by an ancestral cell roughly 1.5 billion years ago. Instead of being digested, they formed a mutually beneficial partnership. Evidence: mitochondria have their own circular DNA, divide by binary fission, and have double membranes — all hallmarks of bacterial ancestry.