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.
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