Question

what important event in animal evolution marks the beginning of the Cambrian period

Answer 1

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This period lasted about 53 million years and marked a dramatic burst of evolutionary changes in life on Earth, known as the "Cambrian Explosion." Among the animals that evolved during this period were the chordates, animals with a dorsal nerve cord; hard-bodied brachiopods, which resembled clams; and arthropods, ancestors of spiders, insects and crustaceans.

http://www.livescience.com/28098-cambrian-period.html