Abundant fossil evidence did not appear in the geologic record until about 540 million years ago, marking the beginning of the Cambrian period. This period is often referred to as the "Cambrian Explosion" due to the rapid diversification of life forms that occurred during this time.

Before the Cambrian period, life was primarily composed of simple, single-celled organisms and small, soft-bodied multicellular organisms that rarely fossilized well. The Cambrian Explosion saw the emergence of a wide variety of complex life forms, including trilobites, brachiopods, and early vertebrates, which left behind a wealth of fossil evidence.