The oldest known animal fossils, identified as eggs and embryos, had been expected to reveal secrets from a period of great evolutionary change. Will the latest theory about the fossils' origins confound these hopes? The origin of animals is almost as much a mystery as the origin of life itself. An abundant fossil record extends back 542 million years to the beginning of the Cambrian period, testifying to the establishment of all of today's main groups of animals by this time. However, the degree to which animal evolutionary history extends beyond the Cambrian is a controversy rich in speculation but sparse in evidence.
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