Mitochondria certainly count: as we’ve seen, these cellular batteries were once free-living bacteria that became permanently enclosed within a larger cell. This process, known as endosymbiosis, was first proposed in the early twentieth century, but it only became accepted several decades later, largely thanks to the outspoken American biologist Lynn Margulis. She turned endosymbiosis into a coherent theory, which she expounded in a genre-hopping paper that contained an impressive mix of evidence from cell biology, microbiology, genetics, geology, paelaeontology, and ecology. It was a bravura
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