Amphibians are a class of animal that includes modern-day frogs, toads, salamanders, newts, and caecilians (limbless amphibians). Amphibians first evolved from lobe-finned fish and primitive tetrapods about 340 million years ago. Sometimes this date is incorrectly given as 400 or 380 million years ago, but amphibian fossils have not been unearthed from these periods.

Before there were any amphibians, about 380 million years ago, during the Devonian period, some fish began to evolve legs and digits. These early "tetrapodomorphs" lacked the chracteristics that define amphibians, hence they are classified as basal tetrapods rather than amphibians. Decades ago, they were classified as amphibians, though taxonomists have changed their view on the matter. This is why the origin of amphibians is sometimes incorrectly cited as 380 million years ago.

Some of the earliest tetrapods include Tiktaalik, among the earliest tetrapods with a weight-bearing wrist structure, and Acanthostega, which had eight digits on each foot. All of these early species would have been mostly aquatic, and used their limbs to navigate through swamps rather than taking extensive journeys over the land.

Between 380 and 360 million years ago is a period called "Romer's gap," in which barely any tetrapod fossils have been found, casting a cloud of mystery on the evolution of the first amphibians from the early basal tetrapods. Prior to the gap, no amphibian fossils are found, and the first known amphibian fossil appears shortly after the gap. After the gap, the world was in the Carboniferous period, where sea levels were high and the coasts were covered with flooded forests and swamps, which was a perfect environment for amphibians.

The first amphibians were temnospondyls, long-headed amphibians with a sprawling gait and distinctive look. These were the first truly terrestrial tetrapods, and would have eaten themselves silly by consuming insects that lacked specialized adaptations for defending against large vertebrate predators. The early temnospondyls were the size of large fish, ranging from about half a meter to 1.5 meters (1.6 - 5 ft) in length. The earliest temnospondyls had stubby feet, and probably couldn't move very fast.

Throughout the Carboniferous, temnospondyls grew in size and diversity until they occupied many of the predatory and herbivorous niches that terrestrial animals exploit today. By the late Permian, some even grew to 9 m (30 ft) in length, and resembled crocodiles. This animal, Prionosuchus is the largest amphibian known. In the Carboniferous, temnospondyls were joined by the diverse but less numerous lepospondyls. Lissamphibians, the group that includes all modern amphibians and their common ancestors, emerged about 300 million years ago, during the late Carboniferous.