The clams that fell behind, and what they can tell us about evolution and extinction

Every so often, life on Earth steps onto a nearly empty playing field and faces a spectacular opportunity. Something major changes—in the atmosphere or in the oceans, or in the organisms themselves —and the existing species begin to branch out into a brand-new world. Scientists are fascinated by this process, because it's a unique look into evolution at pivotal moments in the history of life.

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