Amphibians are the oldest group of land - dwelling vertebrates on the planet. It is thought that they descended from fish between 420 and 360 million years ago.
There are about 7000 different species of amphibian on the planet although 90% of these are frogs.
The smallest amphibian on earth was only discovered in 2012 and is a miniature frog which is scientifically named Paedophryne amanuensis. It measures just 8 mm and is so tiny that is misses out a tadpole stage altogether, emerging from an egg as a tiny fully formed frog.
The biggest amphibian on earth is the chinese giant salamander. It can reach a maximum size of about 1.8 m long (6 ft).
All amphibians are currently under threat from the spread of a deadly fungal disease known as chytrid fungus. It is thought that the increase in this fungus is due to climate change.