Hydra vulgaris is a tiny cnidarian with a simple anatomy. Collaborative research led by Kyushu University found that, despite lacking a central nervous system, hydras exhibit characteristics at a molecular and genetic level associated with sleep in more evolved animals, indicating that sleep-related mechanisms may have been acquired before the evolution of the brain. Credit: Taichi Q. Itoh, Kyushu University

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Which came first, sleep or the brain?

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