Final answer:
The Cambrian explosion is D) a dramatic diversification of life that began about 540 million years ago, leading to the most rapid evolution of animal species seen in Earth's history. It resulted in the emergence of most animal phyla and significantly altered life on Earth.
Step-by-step explanation:
The Cambrian explosion refers to D) a dramatic diversification of life that began about 540 million years ago. This event is characterized by the most rapid documented diversification and evolution of animal species in Earth's history. Prior to this period, life was much simpler, but during the Cambrian, the majority of animal phyla we recognize today emerged, representing a significant milestone in the history of life on Earth.
Several theories attempt to explain the cause of the Cambrian explosion. These include environmental changes like rising atmospheric oxygen levels and increased oceanic calcium concentrations that might have fostered more complex life forms. Additionally, the evolution of Hox genes, which control the body plan of an embryo along the head-tail axis, could have provided the morphological flexibility necessary for the diversification observed during this period.
The Cambrian explosion's significance is underscored by the vast array of complex life forms that appeared in a relatively short geological timeframe, fundamentally altering the trajectory of life on Earth and setting the stage for future evolutionary developments.