Amphibians

Biological Environment > Vertebrates > Amphibians

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Once the plants and arthropods were living on land there was plenty of food for any vertebrate which could manage to come out of the water. Some fish lived in ponds which dried up in summer. Their swim bladders evolved into lungs which they used to breathe air. They used their fins to crawl from one pond to another and these evolved into legs, two at the front and two at the back.

The vertebrates which emerged from the water and became land animals around 350 million years ago we call amphibians "am-fib-ee-ans". Their name means "both lives" because they lived both in water and on land at different times in their lives.
Leaving the water was one of the greatest steps ever taken by our ancestors. It needed changes in every part of the body. The most obvious changes were the appearance of legs and the ability to breathe. Other changes were not so obvious but were just as important. For example the way the blood flowed round the body had to change.

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Amphibians were still not totally free from the water. They needed to return to it to reproduce (like the ferns and the arthropods before them). Their eggs were laid and fertilized in water and the young developed in the water just like their fish ancestors. But when amphibians grew up they left the water to live on the land. Most frogs and newts are still at this stage of evolution.