Final answer:
The relatively short time when many new life forms appeared on Earth, marking the most rapid evolution and diversification of life, is called the Cambrian explosion. It signified the start of the Paleozoic Era around 542 million years ago.
Step-by-step explanation:
Paleontologists refer to the relatively short time when many new life forms appeared on Earth as the Cambrian explosion. This event occurred at the start of the Paleozoic Era, about 542 million years ago, marking the most rapid evolution of new animal phyla and unprecedented diversification of animals in Earth's history. Notable amongst the diversity were organisms such as primitive sponges, trilobites, echinoderms, mollusks, worms, arthropods, and chordates.
During the Cambrian explosion, there was a significant increase in the complexity and variety of life forms, with most of the animal phyla we know today making their first appearance. The rise of these many new species and the evolutionary changes they underwent, happened relatively quickly in geological terms. This phenomenon remains a subject of intense study and fascination in paleontology.