The earliest tetrapods - four footed vertebrates adapted for life on land, such as this icthyostega - are thought to have appeared during the Devonian. All life on land faced the following problems:

• without the buoyancy of water, the body weight of the animal can crush internal organs.

• the extreme temperature changes out of water require homeostatic mechanisms.

Strong bones and sophisticated metabolisms were among many of the new evolutionary innovations which enabled vertebrates to colonize the land.