Origin and Evolution of Life & Classification : 12th Class Biology Notes – NIOS

Origin and Evolution of Life & Classification : 12th Class Biology Notes – NIOS (Origin and evolution of life biology (314) notes for senior secondary – NIOS)

Origin of life  – simplest primordial life from non-living matter.

Evolution of life – gradual formation of complex organisms from simpler ones.

Origin and Evolution of Life & Classification : 12th Class Biology Notes - NIOS

Chemosynthetic Theory of Origin of Life

  • This theory proposed by A.I. Oparin.
  • The earth originated about 5 billion years ago.
  • Initially made up of hot gases and vapours of various chemicals.
  • Gradually it cooled down and a solid crust was formed.
  • The early atmosphere contained ammonia (NH3), water vapour (H2O), hydrogen(H2), methane (CH4).
  • At that time there was no free oxygen.
  • Heavy rains fell on the hot surface of earth, and over a very very long period the water bodies appeared that still contained hot water.
  • Methane and ammonia from the atmosphere dissolved in the water of the seas.
  • In this water, chemical reactions occurred and gave rise to amino acids, nitrogenous bases, sugars and fatty acids which further reacted and combined to give rise to biomolecules of life such as proteins and nucleic acids.

Probable stages in the origin of life

First stage

  • The sources of energy were the ultraviolet rays or electric discharge (lightening) or heat.
  • Either alone or a combination of these energy sources caused reactions that produced complex organic compounds (including amino acids) from a mixture of ammonia (NH3), methane (CH4), water (H2O) and hydrogen (H2).
  • The amino acids are the building blocks of proteins which are the main components of protoplasm

Stanley Miller and Harold C. Urey in 1953 set up an experiment with an air-tight apparatus in which four gases (NH4, CH4, H2 and H2O)
were subjected to an electric discharge for one week. On analyzing the liquid, they found a variety of organic substances in it, such as amino acids, urea, acetic acid, and lactic acid

Second Stage

Simple organic molecules combined to form large molecules which included peptides (leading to the formation of proteins), sugars, starch and fat molecules.

Third stage

  • The large molecules of different kinds combined together to form multi-molecular heaps or complexes.
  • Some simple fat molecules arranged themselves around this molecular complex in a sort of membrane.
  • It was observed in the laboratory experiments that when such complexes reached a certain size they separated from the surrounding solution in the form of what were termed “coacervate drops” of microscopic size, moving in the liquid with a definite boundary.
  • Now, some sort of “metabolism” could occur within these coacervates with synthesis of certain substances and breakdown of others.

Fourth stage

  • Some sort of nucleo proteins or nucleic acids may have evolved by random combinations which have provided two more properties to coacervate–like bodies.
  • Thus, cells were produced that could be called the simplest primordial life.
  • The primitive “drop”–like forms of life were all heterotrophs, unable to manufacture their own food but derived it from environment.

One of the innumerable changes in genetic make up of the primitive heterotrophs led to the formation of chlorophyll (green colouring matter of the leaves) molecules.

The chlorophyll–bearing units of life for the first time started using solar energyfor production of food as well as for the first time started liberating free oxygen into the atmoshphere.

Origin and Evolution of Life Question Answer

  1. Approximately how many years ago was the earth formed? – 5 billion years
  2. Who gave the Chemosynthetic Theory for origin of life? – A.I. Oparin
  3. Name the four gases present in the primitive atmosphere of the earth. – NH3, CH4, CO2, water vapour
  4. Name one source of energy which was used for chemical combination inprimitive atmosphere. – Lightening/geothermal energy/UV rays (any one)
  5. Where did life originate in water or on land? – Water
  6. What are ‘coacervates’? – aggregates of (life-like) molecules
  7. In the origin of life, first large molecules were formed from inorganiccompounds. Name any two such large molecules. – amino acids, fatty acids, sugars (any two)
  8. Name the two scientists who experimentally tried to verify Oparin’s hypothesis. – Miller and Urey


What is Evolution ?

The formation of complex organisms through ‘gradual change’ from simple ancestral types over the course of geological time is termed Evolution or Organic Evolution.

According to the Theory of Organic Evolution

  • The various present day organisms were not created in the same form in which they exist today, but have gradually evolved from much simple ancestral formsfrom a common ancestor.
  • The characteristics of organisms had been changing in the past; they are changing even today, and will continue to do so in the future as well.
  • This is due to the fact that the environment in which organisms live also changes and organisms need to adapt to the changed environment in order to survive.

This process of slow and gradual change is called Organic Evolution.

Evidences of organic evolution

  1. Morphological evidences
  2. Embryological evidences
  3. Palaeontological evidences
  4. Molecular evidences
  5. Evidences from Morphology

1. Morphological evidences for evolution are derived from –
(i) Homologous and analogous organs
(ii) Vestigial organs
(iii) Connecting links

(i) Homologous Organs

  • Homologous organs are the organs which are similar in structure and origin but may look very different and perform different functions.
  • Forelimbs of vertebrates are a good example of homologous organs. They are built on the same fundamental plan yet they appear different and perform different functions

Analogous organs :

  • The structures which are functionally similar but structurally different are called analogous organs.
  • The wing of an insect, and that of a bird or bat or pterodactyl are examples of analogous organs.
  • The function of the wing is the same (for flying) but the insect wing has no structural resemblance with that of the vertebrates.

(iii) Vestigial Organs-

  • Vestigial organ is any small degenerate or imperfectly developed (non-functional) organ or part which may have been complete and functional in some ancestor.
  • Example : Body hair in male, Appendix, Tail vertebrae, Wisdom tooth etc.

(iii) Connecting Links –

  • The animals or plants which possess characters of two different groups of organisms are known as connecting links.
  • The connecting links establish continuity in the series of organisms by proving that one group has evolved from the other.
  • A good example is that of a fossil bird Archaeopteryx, which was a connecting link between reptiles and birds.
  • This bird had a beak with teeth and a long tail (with bones) like the lizards.
  • It had feathers onthe wings and on the body like the birds

2. Evidences from Embryology :

Embryology is the study of development of an organism

3. Evidences from Paleontology

Paleontology is the study of fossils. Fossils are the remains or traces of animal and plant life of the past, found embedded in rock either as petrifiedhard parts or as moulds, casts or tracks.

4. Molecular Evidence of Evolution

  • All organisms have cell as the basic unit of life.
  • The cell is made of biomolecules common to all organisms.
  • Ribosomes, the cellular organelles are of universal occurrence in organisms.
  • DNA is the hereditary material of all organisms, except for some viruses.
  • ATP is the molecule which stores and releases energy for biological processes.
  • The same 22 amino acids form the constituents of proteins of almost allorganisms.
  • The genetic code is universal (exceptions are very few).

Origin and Evolution of Life Question Answer

  1. Define organic evolution.
    Ans. The process of slow and gradual change as a result of descent withmodification, from a common ancestor.
  2. Name one fossil animal which forms a connecting link between reptiles and aves.
    Ans. Archaeopteryx
  3. Which organ of man is homologous to the wings of birds?.
    Ans. Fore-limb/arm
  4. Define vestigial organ.
    Ans. Functionless organs of the body
  5. Give one example of a connecting link among the living beings.
    Ans. (i) Lungfish between fish and amphibia (ii) Egg laying mammals between reptiles and mammals.
  6. Give two examples from molecular biology which support organic evolution.
    Ans. (i) All organisms have cell as the basic unit of life. The cell is made of biomolecules common to all organisms.(ii) Ribosomes, the cellular organelles are of universal occurrence in organisms.

Darwin’s Theory of Natural Selection

An English Scientist, Charles Darwin (1809-1882) explained the mechanism ofevolution through his theory of natural selection. He is still regarded as ‘the father of evolution’ because of two very significant contributions.

He suggested (i) tha tall kind of organisms are related through ancestry and (ii) he suggested a mechanism for evolution and named it natural selection.

Origin and Evolution of Life Question Answer

  1. Who gave the theory of natural selection?
    Ans. Charles Darwin
  2. What is the modern interpretation of Darwin’s theory of evolution called?
    Ans. Neo-Darwinism/synthetic theory
  3. What are the two major contributions of Charles Darwin regarding evolution?
    Ans. All organisms are related through ancestry he suggested natural selection as the probable mechanism for evolution.
  4. Give two main features of Neo-Darwinism.
    Ans. (i) Variation in population forms the basis of evolution (ii) Differential reproduction

Elemental Forces of Organic Evolution

Evolution is caused by action of forces on Natural Selection of Variation. Reproductive Isolation keeps the species distinct therefore the elemental forces of Organic Evolution are:  (i) Variation (ii) Natural Selection (iii) Isolation.

(i) Sources of organic variation

Variations may occur by

  1. Mutation, which is a sudden genetic change. It may be a change in a single gene (genic mutation or point mutation) or may affect many genes (chromosomal mutation).
  2. Genetic recombination, which occurs in sexually reproducing organisms at every
  3. Gene flow is when there is chance mixing of genes of closely related species through sexual reproduction.
  4. Genetic drift occurs in small populations when a part breaks off from a large population.

Speciation– The evolution of new species is termed speciation.


Meaning of Classification- Classification means identifying similarities and differences between different kinds of organisms and then placing similar organisms in one group and different kindsof organisms in different groups.

Taxonomy, may thus be defined as the science of classification of organisms into categories, maintaining certain rules.

Early taxonomists classified organisms according to morphological features only.

Taxonomic categories

  • While classifying an organism, it is assigned to categories which show its evolutionary relationship with other groups of organisms.
  • Each level or categoryis termed taxon (plural-taxa).
  • The lowermost category of classification or taxon is species.

The various taxonomic categories are given below :

  • Species : Group of individuals of one kind which can interbreed to produce fertile offsprings.
  • Genus : Group of species resembling each other in several features indicating common ancestry.
  • Family : Group of genera (singular-genus) resembling each other. e.g. Felis domestica (the cat) and Panthera tigris (the tiger), both belong tothe family Felidae.
  • Order : Includes families showing similar characteristics.
  • Class : Includes related orders.
  • Phylum : Includes related classes.

The various phyla belong to their respective kingdoms. There are five kingdoms about which you will learn later. Humans belong to the kingdom Animalae.

  • Kingdom : Animalae (Animals)
  • Phylum : Chordata (Animals with notochord / backbone)
  • Class : Mammalia (Animals that suckle their young ones.)
  • Order : Primates (Mammals with larger brains and binocular vision)
  • Family : Hominidae (Humans and human like ancestors)
  • Genus : Homo (Fossil men and modern man)
  • Species : H.sapiens (Modern man)

Scientific naming of organisms

  • A simplified system of naming organisms called binomial nomenclature has been the standard for more than two centuries now.
  • It was proposed by the Swedish biologist, Carolus Linnaeus (1707-1778).
  • Binomial nomenclature simply means two-name system of naming. The name of every category of organism has two parts, that of the genus followed by that of species.
  • The generic name is written with a capital letter and the specific name with a small letter.
  • e.g. Homo sapiens is the scientific name of modern man, Mangifera indica is the botnical name of mango.

Prokaryotes and Eukaryotes

  • The organisms that are most primitive or the first to evolve on earth are the bacteria.
  • They do not possess a nuclear membrane around their single chromosome.
  • Absence of a well-defined nucleus or in other words a primitive nucleus terms them prokaryotes (pro = primitive, karyon = nucleus).
  • All bacteria including blue-green algae (Cyanobacteria) are prokaryotes.
  • As a constrast, organisms other than bacteria possessing a well-defined nucleus are eukaryotes (eu = true;
  • karyon = nucleus).

Differences between Prokaryotes and Eukaryotes


  • Size – 0.1-10 μm
  • Genetic Materials – Circular DNA, no linear DNA, no histones associated with DNA, nucleoid form, no nuclear membrane
  • Site of Nuclear material – DNA in cytoplasm
  • Organelles – No membrane bound organelles
  • Cell wall – Always present, Contains peptidoglycan
  • Respiration – By mesosomes
  • Reproduction – Mostly asexual e.g. bacteria and cyanobacteria (blue-green algae)


  • Size – 10-100 μm (larger volume)
  • Genetic Materials – Histones present on which DNA molecule wrapped, well defined linear chromosomes, with free terminal end nuclear membrane present
  • Site of Nuclear material – DNA inside distinct nucleus
  • Organelles – Mitochondria, golgi body, lysosomes present in the cell
  • Cell wall – None in (animals) and made of cellulose/chitin in plants and fungi
  • Respiration – By mitochondria
  • Reproduction – Asexual and sexual e.g. Protoctista, fungi, plants Animals
difference between prokaryotes and eukaryotes

The Five Kingdoms of Organisms

R.H. Whittaker in 1969 suggested the five kingdom classification which is based on the following three criteria.

  1. The presence or absence of a well-defined nucleus.
  2. Unicellular or multicellular
  3. Mode of nutrition

The five kingdoms are Monera, Protista or Protoctista and Fungi, Plantae and Animalae.

The five kingdoms are Monera, Protista or Protoctista and Fungi, Plantae and Animalae.

The five kingdom classification of organisms

The five kingdoms are Monera, Protista or Protoctista and Fungi, Plantae and Animalae

Origin and Evolution of Life and Classification Question Answer

1. Name the scientists who proposed :
(a) Binomial nomenclature – Carolus Linnaeus
(b) Five Kingdom Classification – R.H. Whittaker

2. Which were the first organisms to appear on earth?
Ans. Bacteria

3. Name the taxonomic categories which come before and after family.
Ans. Genus

4. Name the categories above order level in a correct sequence.
Ans. Kingdom, phylum, class, order

5. Rewrite the following in correct form –
(a) Mangifera Indica – Mangifera indica
(b) Homo Sapiens – Homo sapiens
(c) Felis leo – Felis leo

6. Place the following in their respective kingdoms
(a) Bacteria which curdle the milk- Monera
(b) Cow – Animalae
(c) Grass – Plantae
(d) Amoeba – Protoctista
(e) Bread mould – Fungi


  • You have heard about diseases such as influenza, polio, mumps, rabies, small-pox, AIDS and dengue are caused by viruses.
  • They are non-living and made up of DNA or RNA surrounded by a protein coat.
  • They can replicate.
  • However, they cannot reproduce on their own.
  • They reproduce when inside a living cell.
  • Logically, therefore, they cannot be placed in any of the five kingdoms because they can multiply in their host cells, and can mutate – like living organisms but, can be crystallised exhibiting a non-living feature.

Discovery of Viruses

In 1892, the Russian botanist Iwanowsky prepared an extract from tobacco plants suffering from tobacco mosaic disease.
Dutchman Beijerinck gave the term virus in 1898 (Virus – poison in Latin) to these infective particles.

Size of Virus

  • Viruses are extremely small and can be seen only under the electron microscope.
  • They are smaller than the smallest bacteria.
  • Can pass through fiters which retain bacteria.
  • Their size is indicated in nanometres (nm).
  • Their size ranges from 10 nm to 300nm in diameter.

Structure of virus

  • Virus has a simple structure consisting of a core and a cover.
  • The core particle is the genetic material, either DNA or RNA.
  • The cover is a protein coat called capsid.
structure of virus image photo origin and evolution of life
  • Virus can reproduce only when inside the living cells.
  • A virus cannot reproduce by itself. For its reproduction it needs to enter the cell of some organism.
  • From the host cell, it uses the raw material and enzymes and energy generating machinery of the host cell to produce its own DNA.
  • A numberof virus particles are thus formed inside the host cell.
  • The host cell bursts to release the new virus particles.

Virus — living or non-living?

  • Though viruses possess nucleic acids as genetic material like the living organisms, they cannot make copies of DNA for reproduction on their own.
  • They can make copies of themselves to reproduce only inside a living cell.
  • And because their genetic material is DNA or RNA, they exhibit mutations followed by variations in their infective properties.
  • Further, they are considered non-living because they are non-cellular, they have no enzymes of their own and they can be crystallised

Infective properties of virus

  • Viruses are known to attack bacteria, plants or animals.
  • Viruses which invade bacteria are called bacteriophages.
  • Viruses are highly specific in their relationship with the host and tissue.
  • For example– Polio virus attacks particular nerves; mumps virus attacks the particular pair of salivary glands (parotid glands) of humans.

Viruses keep on ‘mutating’!

  • Mutation means change in genetic material.
  • For example – Influenza virus which has RNA as its genetic material, mutates and so every year flu is caused by a different virus and scientists find it difficult to find a cure for influenza or flu.
  • Certain cancers are also known to be caused by viruses. These viruses have RNA as genetic material and are called retroviruses.

Certain viruses, their hosts, diseases caused by them and mode of transmission.

Certain viruses, their hosts, diseases caused by them and mode of transmission.


  • Viroids are circular RNA molecules, consisting of several hundred nucleotides.
  • They infect plants and even kill them.
  • In plants, they use enzymes of the plant cells to replicate like the viruses do.
  • When they infect plants, these RNA molecules cause defects in the regulatory systems controlling plant growth.
  • Hence viroid infected plants show stunted growth and abnormal development.

Virus related question answer

1. Give one feature because of which viruses are considered non-livng.
Ans. They cannot reproduce on their own / they can be crystallised (any one)

2. Name one chemical common to viruses and all other organisms.
Ans. Nucleic acid / protein (any one)

3. Complete the following :
(a) Core particle of virus contains – DNA or RNA
(b) Coat of virus is made of – Protein

4. In what way is viroid structurally different from a vrius?
Ans. A virus has a DNA or RNA molecule surrounded by a protein coat, whereas a viroid is only an RNA molecule.

5. Why are viroids considered a menace for plants that they attack?
Ans. They infect plants and when inside the plant cells, use the host plants’enzymes to replicate & increase in number resulting in stunted and abnormal growth of plant.

Origin and evolution of life 2nd part (coming soon..)

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  1. Bhaiya phy Chem class 12 ka bhi notes banao na ise tarah ye notes bahut acha likha gya hai book ka har ek point wise hai padhna easy ho rha hai chem phy ka bhi aisehi chahiye or kaheen nhi mil ra

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