Recently they have contributed much to science from studies of ecology and evolution, behavior, and general physiology to genetics, endocrinology, and cell structure and function. Salamanders have also become increasingly important in environmental studies as indicators of problem areas as their fragile systems can not withstand the phlethora of manmade pollutants. The class Amphibia includes frogs, toads, salamanders, newts, and caecilians. They are active at optimal temperatures and withdraw from more extreme heat or cold. Voice production is largely an attribute of frogs.
Most amphibians also have four limbs.
Limbs and lungs are adaptations for life on land. The limbs evolved from the ancestral fishes lobed fins. Amphibians are thought to have arisen from lobe-finned crossopterygian fishes. These fish had fins supported by bones, a well-ossified skeleton, lungs, and, in some species, internal openings to nasal passages. Considerable conjecture exists, however, as to whether amphibians actually arose from several lineages.