To the fish this is no problem as it is continually bathed by the fluid in which it lives, but to land-dwelling creatures it is a major problem. The amphibians are faced with the necessity of retaining their body fluids while no longer immersed in the water. It can be inferred that the earliest amphibians, Ichthyostega for example, never ventured very far away from lakes and rivers to which they returned at frequent intervals, a habit that has followed through to most of today’s amphibians. Certain branches of these early amphibians evolved more and more efficient ways of conserving water in the body, however, and these were the precursors of the reptiles, from which the birds and the mammals ultimately arose.
Another problem that arises from life on land is the influence of gravity. Whereas fishes are supported by the dense water in which they live and have developed a system of locomotion through body movements and fin stabilization, the first land-dwelling vertebrates had to evolve a system of moving over the land and supporting the body in the relatively thin atmosphere. A strong backbone and sturdy limbs were developed in the earliest stages of their evolution. The vertebral column was supported by a pectoral girdle at the front (which in turn was supported by the arms and the hands) and a pelvic girdle at the rear part of the body (which was supported by the legs and the feet). These limbs served not only to hold the body in counter-reaction to gravitational pull, but were the means by which the animal could propel itself over the land surface. The early amphibians retained a tail, which probably functioned as an organ of balance. In addition, the tail could still be used as an organ of propulsion when the animal returned to a watery environment. Although amphibians made a major contribution to life on land, they never really found a satisfactory solution to terrestrial reproduction. Throughout their history these animals have had to return to water or at least damp areas in order to lay their eggs.
From the original ichthyostegids a number of lines evolved that not only eventually led to our modern amphibians but also radiated into the ancestral lines of reptiles, birds, and mammals. Most of the earlier amphibians were large salamander-like creatures.
