The word "amphibian" comes from a Greek word that means "both lives." This is because frogs start their lives in the water and then live on land.
The earliest amphibians ("crown") evolved in the Carboniferous period from sarcopterygian fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land. They diversified and became dominant during the Carboniferous and Permian periods, but were later displaced by reptiles and other vertebrates. Over time, amphibians shrank in size and decreased in diversity, leaving only the modern subclass Lissamphibia.