common for different species to collaborate through what is known as symbiosis. Today, humans have vital symbiotic relationships with wheat, rice, cattle, sheep, and many other species. But Margulis was talking about a much more radical type of symbiosis, one in which once independent bacteria, including the ancestors of modern mitochondria, ended up living inside a cell from the Archaea. Margulis called the mechanism endosymbiosis. At first, her idea seemed crazy, because it ran counter to some of the most fundamental concepts about evolution by natural selection. But most biologists now
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