The origin of all animals can be traced to the understanding of how a single celled organism, develops into a multiple celled organism. Using the simple concept that an environmental mutation of such an organism, when beneficial, will survive and eventually dominate over it predecessor makes it possible to understand how vertebrates, and animals came to exist. Although the origin of the first animals, as they are classified, were undoubtedly aquatic, a true case study of the origin of the multitude of animals we see today can be observed when looking at the insurmountable odds that face the first land animals. From there first breathe of true air, to the running, jumping and climbing animals we see today, it is a magnificent task.
It is commonly accepted scientific fact within it's respected community that terrestrial animals have evolved from aquatic life. The discovered empirical evidence combined with paleontological study and improved observed phylogenetic evidence has lead to this conclusion. But how did this transformation occur? The question is black and white but the answer will most likely always remain theoretically gray. The exclusive environment needed for this transformation coupled with multiple adaptations for this environment that must be developed is staggering. Not only must the correct transitional environment exist, but it also must exist long enough for an aquatic organism to exist and adapt for a land environment. The slightest interference or misstep of such an environment or adaptation could have lead to a collapse of the entire transformation. Although the relative harmony of all evolutionary advances is based upon a magnitude of generations combined with luck and success, this transformation into an entirely new environment seems especially tolling. Compare such advancement to the idea of humans evolving for flight and a life in the air and the picture becomes clear.
The Perfect Environment, That must have Existed at Some Point
Such an environment must have existed and possessed very unique characteristics that must be ideal for our aquatic land dreamer.
1) It must have been transitional, meaning it must have possessed both aquatic and terrestrial properties.
2) The environment must have been on the border of uninhabitable by most aquatic life
This was to prevent further aquatic adaptations and promote a move to land for the fish. A present day organism that possesses both aquatic and terrestrial traits is common amphibians.
3) It was a fresh water and shallow environment.
Amphibians are found entirely in fresh water and its surrounding area. Thus, it would be logical to assume that the environment was fresh water. It must also be shallow which is noticeably warmer than deeper waters due to the vastly superior penetration of sunlight, colder and deeper water would promote aquatic life, warm shallow water provides an ideal habitat for the changing fish due to its chain of organisms that lead to abundant food and protection against outside predatory intervention.
4) It was Warm
To coincide with shallow depth, the water must be warm in general; it cannot be located in an area of colder air climates or one that does receive sufficient sunlight. Essentially, the habitat must be warm and shallow in order for the fish to thrive. Warm water also holds significantly less oxygen than cold water which becomes relevant for the environmental protection of the fish.
5) Plant life was flourishing
It must have vast emergent plant life. Plant life is the beginning of our habitats food chain and eventually leads to our fish’s food source. The abundance of such vegetation within the habitat would allow for the abundance of nutrients given by this life and create a richer environment. The organic material left by this vegetation would allow for a nutrient rich environment which would also be necessary.
6) Bacteria and Fungi had to get there first
The warm water combined with plentiful debris from vegetation would allow bacteria and fungi to thrive and decompose the organic material. The smaller organisms like bacteria would need to act as a food source for insects and small fish, which would additionally need to be abundant because they juvenile fish and eggs would provide a reliable food source for our transitional fish. The shallow water of this habitat would make it easy for smaller fish to thrive because larger fish would be reluctant to move to the shallows where there is less mobility.
The Perfect Animal for Adaption
Now that the ideal environment has been created to stimulate such a transformation, it is logical to now look at the ideal organism and its ideal adaptations for this environment.
1) It was predatory
The fish must have been predatory, while vegetation as a food source is just fine in a fully aquatic life, vegetative feeding mechanism cannot be as easily transitioned into terrestrial life.
2) It Got Flatter
The fish must have adapted its entire structure to be more flattened compared to its brethren. The flattening of its entire framework would allow the fish to enter the shallowest of these waters in search of food.
3) It developed Toes and Claws
The fish would need to adapt toe or claw like structures. It would accomplish this by losing the webbing of its fins in the pectoral and pelvic regions. The formation of small appendages would be beneficial to move across the shallow floor of the habitat.
4) It developed a Lung, of Sorts
Air contains 35 times more oxygen than water. While this fish already lives in a low oxygen environment, it would be extremely beneficial for the fish to have the capability to use its swim bladder as a lung. 35 times the oxygen concentration would lead to 35 times the efficiency of intake. This would allow the fish to remain stationary for long periods of time in wait for prey.
5) It Developed a New Nose and Neck
The nostrils of the fish would need to adapt to create a separate air passage way for air intake. While the ability to use air for oxygen is profoundly beneficial, the development of a dorsal nostril separate air path would allow the fish to breath air without using its mouth and possible flooding its lung.
Additionally, the fish must adapt to be able to tilt its head upward for air to make use of these new breathing mechanisms.
The snout, jaws and entire head region would need to be elongated and enlarged preferably which eliminate the need for strict accuracy in hunting prey and maximize the adapting fish’s success rate.
6) It developed a new Tail Area
An adaptation for an elongated tail would be useful to propel the fish toward prey. It would also allow for increased mobility of the fish without forced movement of the main body of the fish.
With these adaptations and our ideal environment, land animals were born and the diversity that we now see in both land and aquatic life will continue their separate yet once related development.