Amphibians possess many characteristics that make them unique in the animal world. There are over 6,000 living species of amphibians throughout the world. They are estimated to have been around for 350 million years, with the earliest known frog appearing about 190 million years ago during the late Jurassic period.
Amphibians (with a couple of exceptions) are biphasic; they have an aquatic and a terrestrial stage at some point throughout their life. Amphibian eggs are shell-less in most species and are deposited in the water. The eggs rely on the moisture from the surrounding water, unlike amniotic eggs which are enclosed in their own fluids. Tiny larvae are hatched from the eggs, and the metamorphosis process begins. Depending on climatic conditions, the larvae can remain aquatic for months or even years until conditions are suitable for metamorphosis. During metamorphosis, the larvae transitions from aquatic to terrestrial, gills are replaced by respiratory organs, limbs and other adult features begin to grow, the skin changes developing glands to avoid dehydration, they eyes develop eyelids and adapt to vision on land, the eardrum is developed, and the tail disappears in frogs and toads.
Amphibian skin is thin and highly glandular; there is no hair, feathers, or scales. The skin is permeable which allows for the exchange of gasses. Their glandular skin allows for the secretion of toxins which wards off predators as it generally tastes bad. Some species have bright and colorful skin, while others can blend in with their surroundings and camouflage themselves. Some of the toxins produced by amphibians are used in human medicines.
Another unique characteristic of amphibians is that they are bio-indicators. Amphibians offer visible warnings that changes are taking place within the environment. Because of their sensitivity to environmental changes, amphibian populations in an area can indicate that the environment is changing such as the quality of the air or water. Amphibians are not only unique in their evolution; they are also unique in that they benefit humanity.