12th Class Biology Origin of Life Evidence of Organic Evolution

Evidence of Organic Evolution

Category : 12th Class

The following are the evidences in favour of Organic Evolution :

(i) Evidences from Classification : All the known living animals and plants have been classified into various species, genera, families, order, classes, phyla and kingdoms. The classification of a particular animal is attempted only after its extensive study. It is seen that every living being is related with other living being. The relations may be very close or may be quite apart. On their relationship, they are put under various orders, classes, phyla etc. On the superficial examination one can hardly believe that they are interrelated. But after their careful study they can be arranged in definite order, Protozoa (acellular) being at the base while Chordata at the top. No doubt, there is some sort of gap between chordates and non-chordates today but who known that this gap may be filled some day by some further discoveries. Moreover, the present-day types represent only the terminal twigs of a vast phylogenetic tree and for establishing relationship we should focus our attention on the main trunk. The animals can be classified as described above in a systematic order.

(ii) Evidences from Comparative Morphology and Anatomy : In all the living animals, the basic substance of life is Protoplasm. If the species had been created separately, then there should be no relationship in the various organs and systems of animals. But on the contrary, we see that large number of animals although unlike in appearance show most of the systems and organs made on the same plan. The resemblance are very close in the members of the same group. For example –

(a) Analogy and Homology : While examining the various structures in the bodies of different animals, one may come across certain organs of same origin but of different functions. For example, the forelimbs of salamander, crocodile, bird, bat, whale, and man, all have the same origin and essential structures but different functions to perform. Such structures are called homologous. On the other hand, the wings of insect, pterodactyle, bird and bat perform the same function, though they have different origin and entirely different structures. Such structures are termed analogous.

The homologous structures give us the answer that how the two different types of animals or organs have the same origin and functions to perform; this shows that they have changed themselves according to their different needs.

  • Analogous organs are those, which are functionally similar.
  • Homologous organs explains divergent evolution.
  • Similarity developed in distantly related groups as an adaptation to the same function is called convergent evolution.

• The wings of an insect and a bat exhibit analogy.




(b) Vestigeal organs : There are present in the body of animals certain structures which have no function and are very much reduced. These structures are quite developed and functional in allied animals. Such useless organs are termed as vestigeal and are of frequent occurrence. In human body alone, there are as many as ninety such organs. For example, vermiform appendix is vestigeal in man but functional in rodents, horse and other herbivorous animals.



  • Hind limbs is a vestigeal organ of python.
  • Muscles of ear pinna is vestigial organ of man.
  • Wisdom teeth is vestigeal organ of man.

(c) Atavism : Atavism or reversion is the sudden reappearance of some ancestral features which was either vestegial or altogether absent in parents. Such as Large canines, thick body hairs, short temporary tails, ability to move pinnae.

(d) Connecting links :Intermediate or intergrading forms between two groups of organisms :


Connecting link between


Living and nonliving

 Euglena (Protozoa)

Plants and animals

Proterospongia (Protozoa)

Protozoa and Porifera

 Peripatus  (Arthropoda)

Annelida and Arthropoda

 Neopilina (Mollusca)

Annelida and Mollusca

 Balanoglossus (Chordata)

Nonchordata and Chordata

 Dipnoi (Lungfish)

Pisces and Amphibia

 Archaeopteryx (Aves)

Reptiles and Birds

 Prototheria (Mammalia)

Reptiles and Mammals


Missing links : These are the fossil forms transitional between two present day groups (taxa) of organisms.

E.g., Stegocephalous  - between fishes and amphibians

Archaeopteryx – between reptiles and birds

Archaeornis - between reptiles and birds

Inostracevia - between reptiles and mammals

Manchurosaurus - between reptiles and mammals

(iii) Evidences from Physiology/Biochemistry : Various types of chemical tests exhibit many basic similarities in physiological and chemical properties that show a physiological relationship among animals.

Take the example of thyroxin hormones, which are similar in all vertebrates. The thyroid of human beings can be very easily replaced by the thyroid of cattle without any ill effects and so on.

Evidences from Serology : This is a method by which the reactions of blood serum is observed. From the blood are also extracted the crystals of Oxyhaemoglobin. The structure differs in different vertebrates, but in a definite order. The reaction is nearly identical in man and anthropoid monkeys, but slightly less identical with other mammals.

(iv) Evidence from Embryology : Van Baer (Father of modern embryology) put forward :

Germ layer theory : Various body structures arises from the same germ layers in different species of animals.

Baer's Law : It revealed the fact about the sequence of development of structures in organisms (i.e., less general features ® more general features ® more special features).

Muller and Haeckel in year 1864 reinterpreted the Baer's law in light of evolutionary theory as Biogenetic law. According to this law 'structure of ancient origin develop earlier than structure of newer origin' or 'development of structures in an organism follow the same sequence as they evolved in is ancestors' or Ontogeny (life history of an individual) repeats phylogeny (evolutionary history of race).




Age  million years



Dominated Fauna

Cenozoic (Age of Modern life)


















Modern man, mammals, birds, fishes, insects.


Extinction of great mammals. Primitive man common. Evolution of human society & culture.








Evolution of primitive man?like forms from man like apes. Formation of modern mammals.


Mammals at peak. Evolution of man?like apes.








Rise of monkey, apes and monocots


Diversification of placental mammals birds, origin of horse


Origin of primates, placental mammals, angiosperm forests

Mesozoic (Era of Medieval life)



Extinction of dinosaurs and toothed birds. Rise of first modern birds.


Jurassic (Age of Giant Reptiles)

Origin of advanced lizards, crocodiles, alligators, marsupials toothed birds. Dinosaurs became large. Reptiles dominant.



Origin of dinosaurs & primitive mammals (egg?laying mammals). Extinction of primitive amphibians.

Paleozoic (Era of Ancient life)



Extinction of many marine invertebrates like trilobites. Rise of modern insects. Evolution of mammals like reptiles.



(Age of Amphibians)

Origin of reptiles and winged insects.



(Age of fishes)

Origin of first land vertebrates (amphibians) Fishes abundant.



Origin of jawed fishes and wingless insects.



(Age of invertebrates)

Origin of chordates with first jaw? less fishes (origin of vertebrates). Invertebrates abundant.



All invertebrate phyla established. Trilobites (swimming crustaceans which do not exist today) dominant










(Era of early life)


Origin of marine metazoans including sponges, cnidarians, annelids, molluscs and arthropods.

Scanty fossils.

Origin of prokaryotes (Monera) and Eukaryotes (Protista).



Origin of life.




No life?



(v) Evidences from Palaeontology : The study of fossils and their interpretation forms one of the great evidences of evolution. An Italian scientist, Leonardo da Vinci, was the first person to recognize their importance and said they were either remains of organisms of their impressions on some sort of clay or rock.

A number of fossils have been discovered from time to time. Some of these fossils are very prefect in their state of preservation. For example, the fossils Archaeopteryx shows the characters of reptile on the one hand, and the characters of birds on the other hand, meaning that the birds have evolved from reptiles, the Archaeopteryx being a connecting link.

In the same way all the evolutionary stages of horse, elephant, camel and man etc., can be constructed. The earliest horse was known as Eohippus. It was eleven inches in height and made its appearance in Eocene time from some unknown five-toed ancestor. The Eohippus, after various evolutionary stages, transformed itself into the present-day horse Equus, which shown many dissimilarities from its great grand, grand ancestor. The various stages in its evolutionary life had been preserved as fossils. The same is the case with camel, elephant and man, etc. So this science of paleontology helps in a great deal in understanding the process of evolution.

  • Wallace gave a theory very similar to that of Darwin.
  • In Cenozoic era mammals and birds were evolved.
  • There was no life in Azoic era.
  • Fossils are remains of organisms present in the rocks.
  • Ruling reptiles were dominant during Mesozoic era.

Fossils :

(a) Direct evidences of organic evolution are provided by fossils (L. fossil, dug up).

(b) The science of discovering and studying fossil record is called Palaeontology (Gr. palaeo, ancient;  logy, study of).

(c) Fossils are remnants, models and impressions of extinct organisms.

(d) Fossils are found preserved in earth's sedimentary rocks.

(e) Fossil may be an entire organism buried in sediment or snow, small part of ancient organism or impression of ancient leaf or stem.

(f) Fossilization occurs where organisms are buried and preserved by natural processes.

Age of fossils

(a) The age of the fossils or rocks can be determined by ‘Clock of the rock method' or 'Radioactive clock’.

(b) 'Clock of the rock' method is based on conversion of unstable radioactive nuclei into stable nuclei over a fixed period.

(c) Radioactive clock method was introduced by Boltwood in 1907.

(d) The common radioactive elements which lose their radioactivity and change into their non-radioactive isotopes at a fixed rate are : 



                                    Potassium 40       \[\to \]       Argon 40

                                    Carbon 14              \[\to \]      Nitrogen 14

                                    Uranium 238        \[\to \]       Lead 207

                                    Rubidium 87        \[\to \]       Strontium 87

                                    Thorium 232         \[\to \]       Lead 206


Living fossils : Living fossils are the organisms which underwent little  change during long geological periods e.g.,

Peripatus                                 -     Primitive arthropod

Limulus (king crab)              -     Arthropod

Neopilina                                 -     Mollusca

Lingula                                      -     Mollusca

Latimaria                                 -     Coelocanth fish

Sphenodon (fuatra)            -     Reptelia

Didephis                                   -     Opossum

Plastasterias                           -     Echinodermata

Echidna and platypus         -     Mammalia


(vi) Evidences from geographic distribution : If the study of horizontal distribution of animals on the face of this earth is made, it would be seen that animals are not evenly distributed. Two identical places with the same climate and vegetation may not have same sort of animal fauna Alfred Russel Wallace (1823-1913) divided the whole world into six major biogeographical regions or realms. Nearctic – Northern America, Palaearctic –N. Africa, N. Asia, Europe, Neotropical – Central and S. America, Oriental –Asia (South of himalayas i.e., India, Ceylon, Malayasia, Indonesia, Philippines), Ethiopian – S. Africa, Australian –Australia, New Zealand.   For example, Elephants and Rhinoceros occur in India and Africa only and not in Brazil, although the climatic conditions are alike. Again, members widely separated areas. For example, lung fishes are found in South America, Australia, Africa and nowhere else.

The explanation of this uneven distribution is quite obvious, and can be explained on the basis of organic evolution. A particular species, after arising from one place, migrates to other far off places. While doing so, it has to come across various climatic changes for which it tries to modify it self. In this way, the new species are added. After its dispersal, if some barriers arise, the species becomes confined to that particular region and is isolated from the parent species. This provides a very interesting theory on the mechanism of organic evolution.



Charles Darwin during his voyage around the world studied the fauna and flora of Galapagos islands (off the west coast of south America). Galapagos islands are called 'a living laboratory of evolution'. Darwin studies 20 related varieties of the bird belonging to family Geospizidae different in shape and size of beak. These birds are now called 'Darwin's finches'.

(vii) Evidences from Genetics : Johan Gregor Mendel in 1866 published his work on experimental breeding. He bred two individuals differing in certain well-defined characters, and observed the ratio in which various contrasting parental characters appeared in successive generations. Since then, selective breeding has started in the case of domestic animals and plants to obtain the evidences of organic evolution. These furnish the direct evidences of evolution.

Conclusion : According to the above description, we have seen certain evidences in favour of organic evolution. Although the evidences are indirect, merely interpretations based on certain phenomenon, certain organs, systems and other facts but they throw enough light to show us that the present day forms of life have originated from simpler forms in millions and millions of years. The process was continuous gradual, and accompanied by trial and error.

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