Complete information on the anatomical and palaeontological evidences of evolution

(1) Anatomical Evidences (Morphological Evidences):

The external, internal as well as comparative anatomy of vertebrates indicates the occurrence of organic evolution.

(a) Homologous Organs:

Organs which are similar in basic structure and development whatever varied functions they may be performing are called homologous organs. Homology is observed in the limbs, hearts, brain of vertebrates, appendages of arthropods, etc. For example, the fore limbs are adapted for leaping in the frogs, for crawling in lizards, for flying in birds and bats, for walking in cows, for running in horses, for swimming in whales, for climbing in monkeys and for grasping in man. For their varied functions, these limbs have different shapes, which may be partially or totally different from the normal ancestral pentadactyle foot pattern. In all these fore limbs, the arrangements of bony elements are essentially similar. Hence, it proves their origin form the same ancestor.

Analogous organs- Some organs exhibit similarity in structure and function such as wings of butterfly, birds and bats. But they are different in origin and development. Such organs are called analogous organs. Do not show common ancestry rather they establish convergent evolution of different groups of organisms due to common habitat and adaptation.

(b) Vestigial Organs:

The ear and belly muscles, appendix, tail vertebrae and hair etc. of men are all degenerate and non-functional. It is supposed that these were fully developed and functional in the ancestors of man. There are also many such vestigial organs present in different organisms proving their origin from the ancestors in whom these were fully functional.

(c) The Connecting Links:

The organisms, which present dual features of two related groups, are called connecting links. For example, Peripatus is an annelid in some of its characters and in other, it is an arthropod.Hence, it is intermediate transitional animal and connects arthropods with annelids. Similarly, Archaeopteryx, this fossil bird connects birds with reptiles and Ornithorhynchus as well as Echidna connects mammals with reptiles.

From this, it becomes clear that the higher groups of animals have evolved from the lower ones and have passed through some transitional stages in order to reach their present status.

(d) Classification:

The present day classification of animals and plants are based on increasing complexities of structure of different groups of organisms. In the taxonomy step-by-step advancement of different life forms have been considered to make it a natural one. The lowly and early-evolved ones are protozoans and are ranked in the first phylum. Then the sponges, coelenterates, annelids and so on are ranked as second, third phyla according to their degrees of complexity. So taxonomy is one of the best evidences of organic evolutions.

(2) Palaeontological Evidences:

Palaeontology is the branch of science that deals with fossils. The fossils are the remains of past plants and animals in the form of rocks in the earth’ this crust. Fossils were formed when the earth containing buried organisms changed into rocks. Alongwith the soils the organisms were also converted into rocks. The period of fossilization or petrification can be studied by different methods and thus, the time of those past organisms can be ascertained.

The first fossils were discovered of about 900 million years old. Those were unicellular plants and animals. The fossils of plants and animals are excavated from the different layers of earth’s crust. Studying these fossils, it was noted that gradual complexity in their organization took place with the passing of time. The invertebrate fossils of 460 million years old were found above those of unicellular ones. Fossils of fish-like vertebrates were found above the invertebrate bed.

This was followed in successive layers by amphibians (350-300 million years), reptiles (300-150 million years), mammals (180-150 million years), birds (150-120 million years) and man (1 million years).

By analyzing the fossils from different time, the gradual evolutionary changes can be noted. It is possible to trace the origin and successive stages of evolution of present organisms like camel, horse, man, etc.

The palaenotology provides us the scenes of life-drama which was staged on this earth on the past.

Most of the characters have become extinct now. The Golden Age of dinosaurs was full of dramatic episodes, when the reptiles radiated and give rise to numerous varieties of vertebrates. From the reptiles, the present day birds and mammals took their origin. To relate extinct reptiles and birds was a difficult problem. But due to the the discovery of fossils bird Archaeopteryx had both reptilian and avian features and is regarded to be a transitional form through which the avian evolution might have passed.

It has been established that modern horse Equus evolved from a pentadactyle ungulate Eohippus in Eocene (60 million years ago) passed through the stages like

Mesohippus – Miohippus – Merychippus and Pliohippus.

Thus, we conceive the following ideas offered by palaeontology on perennial process of evolution.

(i) The animals and plants were changing in the past ages.

(ii) The earliest organisms were the simplest and complex organisms appeared progressively in successive years.

(iii) There were transitional forms- in between two great groups. Its occurrence gives a stepwise proof of the progress evolution.