Our cells are packed with various protein-stuffed sacs, each dedicated to carrying out essential tasks. One kind of organelle is peculiar, though. Mitochondria are jellybean-shaped structures whose jobs include making the fuel that our cells use to power everything they do. What makes mitochondria strange is that they carry their own DNA. It’s not a lot of DNA–just 37 genes–but mitochondria can make extra copies of it as they grow and divide. In other words, they act an awful lot like bacteria.

About a century ago, Russian biologists proposed that mitochondria actually started out as bacteria, which set up house in our single-celled ancestors. In the 1960s, University of Massachusetts biologist Lynn Margulis resurrected the idea, pointing to certain features in mitochondria,