Evolution of the Dinosaurs
What were some of the problems amphibians faced in moving from water to land?
The early amphibians’ main problem was support. In the water, a body is virtually “weightless” because it is supported by the buoyancy of water. But on land, an amphibian’s body had to be held up from the ground, and the internal organs needed to be protected from being crushed by gravity; thus, a strong ribcage was essential. The backbone, ligaments, and muscles also had to be strong, supporting not only the weight of the body between the front and hind legs, but also the head. The limbs and limb muscles had to change design to allow walking. Hind limbs became attached to a supportive pelvis, and the skeleton as a whole was made stronger.
Another problem was adapting to breathing on land. Early amphibians had to modify their respiratory system (changing from gills to lungs), as lungs took over more and more of the breathing. The reproductive system, water balance, and senses also had to adapt to the new life in and out of the water. For example, the first amphibians probably spent much of their time in the water, giving birth to totally aquatic young (tadpoles) that would eventually be able to live both in and out of water. Amphibians’ water dependency adapted to allow them to live out of water as long as they at least stayed damp. Their senses also had to adapt—their sight, smell, and hearing taking on more important roles. For instance, amphibian eardrums developed to enable the semi-land dwelling animals to hear sounds in the air. Their eyes had to modify in order to see in air instead of water; protective eyelids developed; and tear ducts evolved, allowing their eyes to be continually moistened with tears.
Even after all these changes, amphibians still were tied to ponds, lakes, or the edges of the oceans, especially since the eggs still had to be laid and hatched in water. Evolution did not change the amphibians too much—modern amphibians are still tied to water.