Four hundred million years ago the most advanced forms of life on Earth, the fishes, lived in the water. Plants and insects alone occupied the land until the appearance of the amphibians more than 350 million years ago. Almost all amphibians have features that fall between those of fishes and those of reptiles. The most commonly known amphibians are frogs, toads, and salamanders. Although most have changed very little since they first began to breathe on land, some of the early amphibians were the ancestors of today's reptiles, birds, and mammals.
The word amphibian comes from the Greek amphi, meaning "both," and bios, meaning "life." It describes cold-blooded animals with backbones that pass their lives both in fresh water and on land. Because amphibians live in water and on land, their natural environments are shores, ponds, marshes, swamps, and low-lying meadows.