The end of the Precambrian Eon is marked by the Cambrian Explosion, a period of rapid diversification of life that began approximately 541 million years ago.

Here's why this event is considered the boundary:

* Appearance of complex life: The Cambrian Explosion saw the emergence of a wide variety of multicellular organisms with hard parts, including shells, exoskeletons, and bones. This is a dramatic departure from the primarily single-celled and soft-bodied life forms that dominated the Precambrian.

* Fossil record: The Cambrian Explosion is well documented in the fossil record, which provides a wealth of information about the evolution of life during this time.

* Geological changes: The Cambrian Explosion coincided with significant geological changes, including the breakup of the supercontinent Rodinia and the rise of oxygen levels in the atmosphere. These changes likely played a role in the diversification of life.

While the Precambrian Eon was a long and important period in Earth's history, the Cambrian Explosion represents a major turning point in the evolution of life. It marked the beginning of the Phanerozoic Eon, which encompasses the vast diversity of life that we see today.