Amphibians (the group of organisms to which frogs belong) are believed to have been the first vertebrates to leave the water and live on land. Approximately 360 million years ago, amphibians left the sea and were successful on the land because of the lack of other vertebrates to hunt them. Over the next 125 million years, 15 major groups of amphibians, including many species of frogs, evolved. However, once birds and other similar predators arrived, this number declined to the three orders of amphibians that we have today: frogs and toads under the order Anura, salamanders (Caudata) and the order Gymnophiona which includes a group of worm-like tropical organisms known as caecilians.
As the frogs and other amphibians moved to the land, they had to adapt in many ways: they needed new limbs that could let them travel on the ground, lungs and moist skins for dealing with the air, skin that would not let water evaporate very easily, and eggs that wouldn't dry out and die in the air. While frogs did make many of these adaptations, they were not entirely successful. Frogs still need water or at least a moist area to breed in because the eggs still dry out easily. Their skins do not keep all of the water in, so many species spend most, if not almost all, of their lives in the water. These missed adaptations are what keep frogs from living in extreme environments like deserts.
Like all amphibians, frogs are cold-blooded, which means that they cannot maintain their own body temperature as we humans can. Frogs must maintain their body temperature by using their environment. Frogs also have to be careful of predators, including many fish, birds, reptiles, and mammals.
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