Tetrapods evolved from fish through the “fish–tetrapod transition” which usually refers to the origin, from their fishy ancestors, of creatures with four legs bearing digits (fingers and toes), and with joints that permit the animals to walk on land.
The bottom fins of lobe-finned fishes are arranged in pairs and supported by internal bones the necessary conditions for these fins to evolve into primitive legs.
Lobe-finned fishes of the Devonian period were already able to breathe air, when necessary, via "spiracles" in their skulls.
Amphibians, reptiles, mammals, and birds evolved after fish.
The first amphibians evolved from a lobe-finned fish ancestor about 365 million years ago.
They were the first vertebrates to live on land, but they had to return to water to reproduce.
This meant they had to live near bodies of water.
Tetrapods first appeared around 380 to 385 million years ago.
Tetrapods are a group of vertebrates that includes amphibians, reptiles, birds, and mammals.
Tetrapods include all living land vertebrates as well as some former land vertebrates that have since adopted an aquatic lifestyle (such as whales, dolphins, seals, sea lions, sea turtles, and sea snakes).
Tetrapods include all land-living vertebrates, such as frogs, turtles, hawks, and lions.
The group also includes a number of animals that have returned to life in the water, such as sea turtles, sea snakes, whales and dolphins, seals and sea lions, and extinct groups such as plesiosaurs, ichthyosaurs, and mosasaurs.
The animals responsible for giving rise to this huge array are known as “stem tetrapods”.
These brave beasts first ventured out of the water and onto land more than 360 million years ago.
During the Devonian, two major animal groups colonized the land.
The first tetrapods land-living vertebrates appeared during the Devonian, as did the first terrestrial arthropods, including wingless insects and the earliest arachnids.
And birds and humans are tetrapods even though they only walk on two legs.
All these animals are tetrapods because they descend from the tetrapod ancestor described above, even if they have secondarily lost their “four feet.”
One of the key characteristics of tetrapods is that they have four limbs or, if they lack four limbs, their ancestors had four limbs.