1) Evidence from comparative anatomy

Homologous structures can be used to explain the evidence of comparative anatomy. Homologous structures are those structures which are built in the same common plan but yet modified to perform different functions. For example the forelimbs of frog, bird, horse, whale and man are in the pentadactyl plan but yet perform different functions.

Thus, the fore-limb of frog is modified for landing; that of the birds is modified as wing for flying, the fore-limb of horse has only one digit (finger) for fast running, that of whale is used for swimming while in man, the fore-limb could be rotated and has five fingers for holding and grasping. These different functions are brought about due to differences in environment. The only reasonable explanation to this common plan of the forelimbs of the vertebrates indicate a genetic relationship and common ancestor.



2) Evidence from vestigial organs

Other convincing aspects of comparative anatomy are the vestigial organs. These are the rudimentary or useless organs still found in animals and plants. A good example is the appendix of man. This organ has no function in man but it is notorious as seat of disease. In herbivorous mammals such as cow and goat, this organ is large and performs useful job. Bacteria are present in it which help to break down cellulose of cell walls found in a vegetable diet. The appendix of these mammals also allows extra time for food digestion. Thus, the appendix of man can only be explained as a degenerative organ inherited from his ancestor with a functioning appendix and vegetable diet, if not, it is very hard to explain why such a functionless and useless organ should be created to be a source of disease to man.

The caudal vertebrae (coccyx) of a man have become vestigial. They have become fused together and reduced to small sized bones called coccyx. The caudal vertebrae have no function in man but in other mammals they perform useful function. The caudal vertebrae have developed into tails in most mammals. In monkeys the tail is used for balancing. In goats, sheep and cattle, the tails are used for driving insect pests away from the posterior part. Again the only possible explanation to the possession of a tail by man is that man and mammals possessing tails descended from a common ancestor that once used tail. Some other vestigial organs of man are pyramidalis nasi muscle of the forehead, and posterior auricular muscles of the ear.

The pyramidalis nasi (extrinsic) muscles in man is functionless but other mammals use it in moving the scalp to drive away insect pests.

The posterior auricular muscle in man is functionless but used by other mammals in moving the pinnae (outer ears) towards the direction of sound and driving away insect pests.


3) Evidence from physiology

Enzymes and hormones are similar in many groups of animals. Trypsin is present right from protozoa to man. Amylase is also found in the majority of animals. The actions of these two enzymes are similar in all animals. Thyroid hormone is present in all vertebrates. It has been proved that the thyroid hormone of vertebrates is exchangeable among them. For example, the thyroid hormone from cattle has been used successfully to treat human thyroid deficiencies. This proves that all vertebrates descended from a common ancestor.


4) Evidence from fossils

Fossils are long-preserved remains or imprint of organisms that once lived.

The evolution of the modern horse can be traced from its ancestors through fossil evidence.

The most ancient horse called Eohippus lived during the Eocene period about 55,000,000 years ago. The Eohippus was about a medium sized dog. Its teeth were short-crowned. The forelimbs had four functional toes with the rudiment of the fifth while the hindlimbs had four toes. In this Eocene period, the Eohippus lived on abundant vegetation bordering streams. Therefore, the short-crowned teeth were used for browsing than grazing. The forelimbs and hindlimbs were suitable for support on the soft ground. Height was about 0.4m.

In Oligocene period, about 35,000,000 years ago, another form called Mesohippus evolved. Mesohippus had three toes in both fore and hind limbs and long-crowned teeth. In Oligocene period, there were fewer streams and the land was becoming drier. Grasslands and forests were becoming dominant. Under this condition, the need to escape from enemies became important hence the reduction of toes to three for speed. The third toe was enlarged. The high crowned teeth were adapted for grazing than browsing. Height was 0.6m.

In Miocene about 26 million years ago, Merychippus evolved. The weather was very dry. Grass lands were dominant. The need for speed to escape from predators became very important. The second and fourth toes became reduced and the third toe enlarged. Teeth became longer with cemented crown. Height was 1m.

In Pliocene period about 7 million years ago, Pliohippus evolved. There was still greater need for speed to escape from predators as the weather was still very dry. The second and fourth toes became highly reduced and third toe enlarged with hoof. High crowned teeth for eating grass became prominent. Height was 1m.

In Pleistocene, about 1 million years ago, the modern horse called Equus evolved. The second and fourth toes disapeared. The third toe became enlarged with hoof. Teeth now have large surface area. Only molar teeth were present.


5) Evidence from embryology

The developments of vertebrate embryos are similar. The law of biogenesis states that during the development of an individual, it passes through many embryonic stages that its ancestor underwent simply because the mechanism of development was inherited from a common ancestor.

In other words, ontogeny recapitulates phylogeny.

In the development of human embryo many primitive features appear and disappear. The appearance of gill slits like those of fish, two chambered heart like those of amphibians, possession of tail like those of reptiles and coat of hair like those of monkeys are obvious facts to show that human being evolved through the fish, amphibian, reptile and mammalian line.

The whalebone whales have no teeth in the adult stage. They feed on minute organisms of the sea. But it is surprising to note that their embryos have tooth buds which are absorbed as development continues. Birds have no teeth, yet their embryos have teeth buds.

Thus, the presence of tooth buds in whales and birds prove that they have descended from ancestors that had teeth.


6) Evidence from cytology

The electron microscope has revealed that the cell structures (membranes, mitochondria, Golgi apparatus, ribosomes, lysosomes e.t.c.) are all similar in all living organisms. This shows that all living organisms evolved from a common ancestor with similar cell structures.

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