How long ago did most major animal groups first appear in the fossil record?

• A. About 10,000 years ago
• B. About 65 million years ago
• C. About 2 billion years ago
• D. About 540 million years ago

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the context of the question. The problem is asking about the time period when most major animal groups first appeared in the fossil record. This event is known as the Cambrian Explosion.
2. Step 2: Recall that the Cambrian Explosion marks a significant evolutionary event in Earth's history, during which a large number of diverse animal groups appeared in a relatively short geological time frame.
3. Step 3: Recognize that the Cambrian Explosion occurred approximately 540 million years ago, based on fossil evidence and radiometric dating techniques.
4. Step 4: Eliminate the incorrect options by comparing their time frames to the Cambrian Explosion. For example, 10,000 years ago is far too recent, 65 million years ago corresponds to the extinction of dinosaurs, and 2 billion years ago predates multicellular life.
5. Step 5: Conclude that the correct answer is 'About 540 million years ago,' as this aligns with the scientific understanding of the Cambrian Explosion and the appearance of most major animal groups in the fossil record.