In answering this question, we will utilize the clues provided within the geologic time scale and our understanding of key events in Earth's history. The question asks, "How long ago did invertebrates become common on Earth?"
Step-by-step, let's break this down:
1. Identify the Relevant Era:
- The table lists different geologic eras: Cenozoic, Mesozoic, Paleozoic, Proterozoic, and two earlier ones.
- The era we are interested in is the Paleozoic, as the question pertains to the time period when invertebrates became common.
2. Look at the Key Events:
- According to the provided table, invertebrates became common sometime during the Paleozoic era, which is mentioned alongside the "Permian mass extinction".
3. Focus on Specific Periods:
- The Paleozoic era consists of several periods: Permian, Carboniferous, Devonian, Silurian, Ordovician, and Cambrian.
- Important to note, the Cambrian period is often known for the "Cambrian Explosion," a significant event where a large number of life forms, particularly invertebrates, rapidly diversified.
4. Determine the Time Frame:
- The Cambrian period is part of the Paleozoic era.
- It is crucial to note the time frame of the Cambrian period: it began approximately 541 million years ago and ended around 485 million years ago.
- Therefore, the time when invertebrates became common would fall within this period.
5. Select the Closest Provided Option:
- Among the given options: 1.6 million years ago, 132 million years ago, 410 million years ago, and 520 million years ago, we need to find the one that fits within the Cambrian period range (between 541-485 million years ago).
Based on this analysis:
- 1.6 million years ago is far too recent.
- 132 million years ago is during the Mesozoic era, again too recent.
- 410 million years ago places us in the Devonian period, which is later than the Paleozoic era's Cambrian period.
- 520 million years ago is the most likely option, as it falls well within the Cambrian period range.
So, the answer is:
520 million years ago
