Final answer:
Fish began crawling on their fins when they transitioned from water to land. This transition involved structural changes that allowed them to survive on land. The limbs of fish evolved into the limbs of vertebrates for movement on land.
Step-by-step explanation:
When fish began crawling on their fins, it was during the transition from water to land for vertebrates. This transition involved various structural changes that allowed them to resist drying out and breathe atmospheric oxygen. The limbs of fish evolved into the five-toed limbs of vertebrates, enabling them to move on land.