Amphibians

Amphibians are the only land animals that live part of their lives like fish. Just like their fish ancestors, amphibians lay their eggs in water. Their offspring, called tadpoles, are very similar to fish — at first. At birth, their eyes, skin, ears, tails and other body parts are more adapted to life in water than on land. They even breathe like fish through external gills.

Raised to Land: However, their fish-like state doesn't last long; shortly after hatching, tadpoles begin metamorphosizing into their adult form. Their water-breathing gills are replaced by air-breathing lungs; their eyes becomes adapted to vision outside water; they develop skin glands to avoid dehydration; their eardrum develops to lock the middle ear; most develop four legs to support their body on land; and frogs and toads lose their tail.

Then and Now: Amphibians are one of the oldest living classes of vertebrates, second only to fish. They evolved during the Devonian Period some 415 to 360 million years ago. Many types of amphibians evolved and went extinct over time, including some of the earliest land predators. Today's amphibians include frogs, toads, salamanders, newts and caecilians (earthworm-like creatures that live underground).

An Uncertain Future: Sadly, more amphibian species are under threat than any single animal group — a whopping 1,811 species according to the IUCN. This accounts for around 31 percent of all known amphibians. Habitat loss and pollution appear to be the major causes; others include climate change, over-exploitation, introduced species and diseases like chytridiomycosis.