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How does the beginning of the Cambrian Period also mark the beginning of the Phanerozoic Eon?

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Cambrian paleogeography — Cambrian Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Cambrian Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

The beginning of the Cambrian Period marks the start of the Phanerozoic Eon because it coincides with the Cambrian explosion—a period of rapid evolutionary diversification that resulted in the appearance of most major animal phyla.12

This transition is defined by a significant shift in the fossil record. While the preceding Precambrian time (or Cryptozoic Eon) is characterized by rocks containing mainly primitive organisms like bacteria and algae, the Phanerozoic Eon is distinguished by rocks containing conspicuous animal fossils. The evolution of hard body parts, such as calcium carbonate shells, allowed these organisms to fossilize more easily, providing geologists with a much more complete and recognizable record of life. Consequently, this remarkable adaptive radiation serves as the primary biological marker used to divide Earth's history into these two unequal eons.21