The oldest known fossils of fish are from the Ordovician period, in the Palaeozoic era, 470 million years ago approximately, and some of these fossils have remained almost equal throughout our planet’s history.
In the Devonian period, 400 million years ago approximately, the fish developed new limbs to transport themselves in deeper waters. The fish used gills to take oxygen from the water.
The first fish with these characteristics were the Rhizodonts, their fins’ external structure become to seem to that of the fingers.
Most of the fish had an organ called air bladder (also called: gas bladder, fish maw and swim bladder). It is a bag with flexible walls, full of gases, located below the vertebral column and over the digestive tube, it controls the floatability through a complex system of blood gaseous exchange, and allows the fish rise or descend in the water, without the needing of using the musculature.
The next new mutants which appeared were the lungfish, which were sarcopterygii fish. They had ovulated fins and lungs which derived from the air bladder, as well as nostrils opened to the exterior, which allowed them to breathe air in the surface of the water without the participation of the mouth. The first lungfish were the Eusthenopteron.
The lung of the organisms which breathe air and the air bladder of almost all the actual fish have evolved from the double air sacs of the primitive fish. In these, as the air bladder in the actual fish, the air sacs inflated and deflated to determine the depth at which the fish was swimming. In other groups of fish they transformed in primitive lungs, provided with abundant folds to maximize the absorption of oxygen in a medium poor in this element. Both kinds of fish evolved from a previous adaptation, but lead a group of organisms very different.
The next fish which appears is the Panderichthys. The limbs seem more to the tail of the first tetrapods, instead of a fin. It head seems more to the head of the amphibians.
The next species are the new tetrapods. The new modern theories on the jump to the land of the first tetrapods say, as I said before, that the fish adapted their limbs to swim in deeper waters. The new tetrapods were similar to the Ichthyostega, one of the first amphibians. Supposedly, these creatures surrounded the calm and shallow waters. The primitive legs helped these animals to submerge their heads under the water. Their hands will be looking out, instead of looking at the front. They had a flat head, as the Ichthyostega.
It is demonstrated that amphibians had evolved from fish, so, why the amphibians have lungs, if their ancestors had gills? The answer is very simple. The fish suffered mutations, but, inevitably, some of the mutations were not favorable, for example, the amphibian lung.
The features of amphibians are quite similar to that of fish. The amphibian heart consists of two atriums (left and right) and one ventricle. However, oxygenated and deoxygenated blood mixes in the ventricle, making the amphibian respiratory system quite inefficient. So, to compensate, they absorb oxygen through their skin and the walls of their mouths, as well as through their lungs. However, the way they breathe through their skin works in much the same way as gills, so they have to keep themselves moist.
Amphibians also have a special breeding habit. The larvae hatch in water, and when they become adolescents, they lose their gills and start breathing with their amphibian lungs. This allows them to become terrestrial to search for food. Once they have matured, amphibians only return to water to breed. They court in the water and the female lays eggs onto water plants’ branches.
However, an exception to this rule is the Axolotl, or Mexican Walking Fish, which remains in its larvae stage during its whole life. It lives in water and breathes through gills and surfaces occasionally to take a gulp of air into its lungs. Axolotls have four legs, but they are not strong enough to move the axolotls’ body weight efficiently on land, so it generally resides in water.
There are always some people who can‘t, or won‘t, believe that evolution is a fact, despite the extensive amount of scientific research and evidence that provides logical explanations. Creationists support the biblical theory that an intelligent being designed and created every species of plant and animal that ever lived on Earth. They attack Darwin‘s theories and claim that scientists have never found transitional species. Scientists have gained much evidence since Darwin made his claims, and can now say that his theories are old and flawed. Also, scientists have found many fossils of transitional species. Finally, we thought that if we put a video on our work it would be more interesting, so, enjoy yourselves!:
Well, so that's all! We hope that you like our work about evolution! Bye-Bye! :)