To determine when invertebrates became common on Earth, we need to refer to the geologic time scale in the given table. According to the table, this event is marked in the Paleozoic era. This era includes several periods listed as follows:
- Permian
- Carboniferous
- Devonian
- Silurian
- Ordovician
- Cambrian
Among these periods, the Devonian period, which occurred approximately 410 million years ago, stands out as the period when invertebrates became common.
Hence, the correct answer is:
410 million years ago.
