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Lynn Margulis, an American biologist, was a vocal critic of neo-Darwinism, the prevailing theory of evolution. She argued that neo-Darwinism overemphasized the role of natural selection and competition, neglecting the importance of cooperation and symbiosis in evolutionary processes. Margulis proposed an alternative theory, known as the endosymbiotic theory, which suggests that eukaryotic cells evolved from the symbiotic relationship between different types of bacteria.