Earth's biosphere is brimming with symbiotic relationships: from bacteria that became our cells' mitochondria, to mycorrhizal fungi that help plants grow, to the myriad mites, wasps, worms, and flies that make a living by parasitizing other animals. Symbiosis often appears to be a one-way street: When it evolves, there is no going back as symbionts become irreversibly locked into relationships with their hosts or partner species. Such "obligate symbioses" are some of the most ancient ecological