Abundant fossil evidence first appeared in the Cambrian Period, which began about 541 million years ago. This period marks the beginning of the Paleozoic Era and is often referred to as the "Cambrian Explosion" due to the sudden and dramatic increase in the diversity and abundance of fossilized life forms.

Before the Cambrian, life was primarily single-celled and soft-bodied, leaving very few traces in the fossil record. However, during the Cambrian, hard-shelled creatures like trilobites, brachiopods, and early chordates evolved, allowing for their preservation as fossils. This period represents a turning point in the history of life on Earth, providing crucial evidence for the development of complex multicellular organisms.