The first vertebrates appeared in the Paleozoic Era, specifically the Cambrian period. The vertebrates were <span>jawless fish with backbones made out of cartilages. It was thought that the first fishes evolved during this period. The correct answer to this question is letter "A. Cambrian". I hope this helps.</span>
Oceans' acidity is increasing by this, and living creatures in the ocean would get affected by that
