Final answer:
The Cambrian Explosion occurred roughly 542 to 488 million years ago and led to a significant increase in animal diversity and complexity, with multiple theories suggesting environmental changes and genetic factors as possible causes.
Step-by-step explanation:
The Cambrian Explosion refers to a remarkable period in Earth's history, spanning from approximately 542 million years ago to 488 million years ago, during which there was a rapid diversification and evolution of animal species. This event is noted for the emergence of the majority of animal phyla that exist today. Despite earlier life forms present during the Ediacaran and possibly the Cryogenian periods, the Cambrian period is where the most significant burst in animal diversity and complexity occurred.
Two theories that attempt to explain the cause of the Cambrian explosion revolve around environmental and genetic factors. Firstly, rising levels of atmospheric oxygen and oceanic calcium may have afforded more complex animal life. Secondly, the evolution of Hox control genes might have increased the morphological variety of animals, facilitating an unprecedented scope of diversification. Despite ongoing debates and various hypotheses, the exact causes and triggers of the Cambrian explosion remain a subject of scientific intrigue.