To determine when invertebrates became common on Earth, we need to look at the geologic time scale associated with this event.
1. According to the geologic time scale provided, invertebrates became common during the Silurian period.
2. Next, we locate the Silurian period within the broader framework of geological time. Based on the time scale, we note that the Silurian period is part of the Paleozoic era.
3. The Silurian period occurred approximately between 443 million years ago and 419 million years ago.
4. Since invertebrates became common sometime within the Silurian period, we take an average of the start and end dates of this period to provide an estimate.
Therefore, the average number of years ago when invertebrates became common is:
[tex]\[ \frac{443 + 419}{2} = 431 \][/tex]
Hence, invertebrates became common around 431 million years ago.
So, the correct answer is not explicitly listed in the given options:
- 1.6 million years ago (Incorrect)
- 132 million years ago (Incorrect)
- 410 million years ago (Close but Incorrect)
- 520 million years ago (Incorrect)
Therefore, the correct answer based on the provided geologic time is approximately 431 million years ago.
