The Cambrian explosion refers to a rapid diversification of animals that occurred more than 500 million years ago. Which statement best describes this event?

• A. It marked the rapid appearance of many major animal groups.
• B. It occurred after the dinosaurs went extinct.
• C. It was a period of increasing astrochemical complexity.
• D. It describes the emergence of simple single-celled organisms.

Answer: (A)

A rapid diversification of multicellular animals early in animal evolution is what the Cambrian explosion represents. In a geologically brief window, many major animal lineages appear in the fossil record for the first time, and you start to see a variety of body plans, hard shells, and complex ecological roles that signaling a modern-looking animal world. This burst stands out because it shows a sudden expansion of animal life rather than a slow, gradual buildup of simple organisms.

That’s why the statement about the rapid appearance of many major animal groups is the best description. The other ideas don’t fit: it didn’t occur after dinosaurs went extinct (dinosaurs came much later), it isn’t about astrochemical complexity, and it isn’t about the emergence of simple single-celled organisms (that happened long before the Cambrian).