Around 400 million years ago, ancestors of the coelacanth began to creep out of the water onto the land. Many fish are able to wriggle along on land, but in order to lift their body clear of the ground ordinary fins are not strong enough. The relatives of the coelacanth had leg-like fins reinforced with bones, which allowed them to slither along like a modern crocodile even though they did not walk properly. They were still fish, however, and had to return to the water to breed. It is thought that these early four-legged animals, known as tetrapods, lived in shallow freshwater pools, like modern lungfish. They needed to develop lungs in order to breathe when oxygen levels in the stagnant water fell.
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