Biology
Mitochondria were once bacteria
Yes: the mitochondria that power almost every cell in your body were once free-living bacteria, swallowed by another cell roughly a billion years ago and never digested. The idea was largely dismissed until biologist Lynn Margulis revived it in her 1967 paper, now the accepted explanation for why mitochondria still carry their own separate DNA.
Sources & further reading
NCBI Bookshelf, National Library of Medicine (NIH)The Origin and Evolution of Cells (The Cell, 2nd edition)The textbook account of how mitochondria and chloroplasts began as free-living bacteria taken up by an early cell.Arizona State University, Embryo Project EncyclopediaOn the Origin of Mitosing Cells (1967), by Lynn SaganThe history of how the once-fringe idea that mitochondria were captured bacteria became accepted science.
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