All amphibians are vertebrates, which means they have backbones. The other traits all amphibians share are a life cycle in which part of the animals life is spent on land and part in the water, permeable skin without scales, gills, and an inability to regulate body temperature internally.
Major classes of amphibians include frogs, toads, newts and salamanders. Amphibians first evolved 370 million years ago, and they are the common ancestor of all modern mammals, reptiles, birds and other land animals. Early amphibians used fins to hoist themselves out of the water and move around on land. It was not until 120 million years later that amphibians developed legs and began to spend significant time on land.