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DEVONIAN PERIOD

416-359 million years ago

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fish moving onto land

Although the transition from aquatic life to terrestrial life may seem dramatic, the fish ancestors of amphibians already possessed many of the novel features (such as lungs and limb bones) that amphibians would require. Although significant modifications were eventually made to accomodate this new lifestyle, many did not have to be present in the first amphibians and, instead, occurred over long periods of time.

For most of earth's history, there was no life on land. One reason was that, until photosynthesis had contributed enough oxygen to form an ozone layer, too much ultraviolet light from the sun bombarded the land, making life impossible. Plants and arthropods made the transition to land 100 million years before vertebrates made a similar transition. Although moving from aquatic to terrestrial life seems like an enormous change for a fish, a variety of modern ray-finned (actinopterygian) fish, such as mudskippers and walking catfish, have adapted to terrestrial life by crawling on their fins, adapting their gills for air breathing, and/or gulping air with a swim bladder.
Life on land presents new challenges to formerly aquatic animals:
a) breathing oxygen from air rather than water:


An air bladder capable of breathing atmospheric air evolved in the first bony fish (if not earlier) and lungs evolved in early sarcopterygians. Many amphibians possess gills during their larval stage (and some retain them as adults) which they use to breathe underwater.


b) locomotion

The first amphibians did not need to evolve new bones such as the scapula and clavicle of the shoulder, bones for the hip, the humerus, radius and ulna for the arm, or the femur, tibia, and fibula for the leg-these bones were already present in the fins of advanced sarcopterygian fish. Perhaps these fossil fish used their fins/limbs to leave water briefly to move from one body of water to another or to move along the bottom. Amphibians made a few modifications to the fins/limbs of rhipidistians. The pectoral girdle was separated from the bones of the skull, fingers and toes developed from cartilaginous fin rays, and the pelvis had to fuse to the spinal column for support. It is possible that the molecular changes which converted fins to limbs were not extensive. There is one set of genes (the Hox genes) which guide the development of both fins and tetrapod limbs. In fish, these genes are turned off after a short period of expression while in tetrapods their expression lasts longer.


c) more weight to bear

Water obviously provides buoyancy which helps support an animal's weight. Not only were the limbs strengthened and the pelvis attached to the vertebral column, the vertebral column itself had to become stronger. The adaptation of the vertebral column to bear weight on land occurred gradually over tens of millions of years as the parts of the vertebrae became larger, fused together, and replaced the notochord.


d) resist water loss (dessication)

Tetrapods breathe through their nose rather than mouth in order to limit water loss. The first tetrapods would have been partially protected by the scales of their fish ancestors. Reptiles developed keratin scales in the skin to prevent water loss. Amphibians could not do this since their lungs, while functional, are still inefficient. Modern amphibians depend on exchanging oxygen and carbon dioxide through their skin. In fact, a very successful family of salamanders in North America has actually lost its lungs and respires only through the skin.


e) change in sense organs

The lateral line system, involving a system of grooves in the skull, was used by sarcopterygians to locate animals and objects in water. Although it was still present in the most primitive amphibians, it was not useful on land. The eye and olfactory epithelium would have to be moistened to function in air. Rhipidistians developed nasolacrimal duct to accomplish this. Part of the jaw apparatus (the hyomandibular) began to conduct sound to the inner ear and became the stapes. Although its primary function in later tetrapods would be the transmission of sound waves, in early tetrapods its primary function was to support the braincase. It remained a solid, stout bone for quite some time and probably did not transmit sound well until early reptiles.


In conclusion, although the transition to land involves a number of anatomical and physiological changes, they are perhaps not as great as they might seem. Although breathing oxygen from the air is essential, the ancestors of amphibians had already accomplished this. Although fins cannot provide the same locomotion styles as limbs, a number of modern fish have adapted their fins for some degree of terrestrial locomotion. The sarcopterygian fish of the past would have been even better prepared for this transition, given the stout bones in their fins-bones which are homologous to the bones in tetrapod limbs. Other adaptations to life on land (such as a strong vertebral column and the modification of sensory structures) would occur gradually and were not complete in the first amphibians.
What promoted the evolution of sarcopterygian fish into amphibians? First of all, any fish which can breathe air can survive in stagnant water while other fish die. This is clearly an advantage. The ability to move onto land, even temporarily could help a fish evade a predator or move from a pool of water which was drying out. Finally, it should be remembered that when the first fish began their transition onto land, terrestrial environments were full of insects (many of them wingless) which had no vertebrate predators.