Final answer:
The Cambrian Explosion occurred during the Paleozoic Era, in the Cambrian period, 541 to 485 million years ago, and is characterized by rapid diversification and evolution of animals, including the rise of most phyla known today.
Step-by-step explanation:
The Cambrian Explosion took place during the Paleozoic Era, specifically in the Cambrian period, which is the first period in the Paleozoic era, occurring between about 541 to 485 million years ago (Ma). This event marks a significant period in Earth's history, known for the rapid diversification and evolution of animal species. Prior to the Cambrian period, during the Ediacaran and Cryogenian periods, life forms were simpler and not as diverse. The Cambrian period, however, witnessed the emergence of most of the animal phyla known today, and it is characterized by a remarkable increase in animal complexity and the appearance of many new body forms, including those similar to current echinoderms, mollusks, worms, arthropods, and chordates.
Theories attempt to explain the cause of the Cambrian explosion by considering various factors such as rising atmospheric oxygen levels, increases in oceanic calcium concentrations, and ecological dynamics like predator-prey relationships. Genetic and developmental changes, including the evolution of Hox control genes, may also have provided the morphological flexibility necessary for the proliferation of new animal forms. The Cambrian explosion remains a major topic of interest and debate in the scientific community due to its profound impact on the trajectory of life on Earth.