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Approximately 540 million years ago, at the beginning of the Cambrian Period, the fossil record at locations across Earth is marked by the dramatic appearance of complex, diverse, multicellular organisms with hard parts. By the close of the Cambrian Period (490 million years ago), virtually every major animal group that exists today — excluding bryozoans — had appeared. Some scientists think the burst in diversity was rapid, perhaps in as little as 10 million years. |