To determine when invertebrates became common on Earth, we need to refer to the geologic time scale provided.
From the table:
- The presence of invertebrates becoming common is stated to have occurred during the Ordovician period.
- Searching for the time frame of the Ordovician period, we find this information next to the key event mentioning invertebrates.
Upon examination, the precise time point for the Ordovician period shows that invertebrates became common around 410 million years ago.
Thus, the correct answer from the provided choices is:
- 410 million years ago
