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Chapter 19 Evolution by TEACHING CARE online tuition and coaching classes

Chapter 19 Evolution by TEACHING CARE online tuition and coaching classes


 Origin of Universe and Earth.

Philosophers and scientists have been busy to solve the riddle as to how the universe and our earth were formed and how and when ‘life’ originated on earth. The branch of life science for the study of ‘Origin of life’ and evolution of different forms of life on earth was called Bioevolution or Evolutionary Biology by Mayer, (1970).

The study of universe or cosmos is called Cosmology. Our earth belongs to the Solar system having nine stars called planets constantly rotating around a common Sun. On the basis of the order of the distance from the sun these planets include Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto while moon is a satellite of earth. The universe is made up of matter and energy and it was formed about 10 to 13 billion years ago as a red hot, dense, rotating gaseous cloud of cosmic dust called Ylem or primaeval matter. The Ylem consisted of particles of matter (like neutron, proton and electron) and antiparticles of antimatter. Scientists like Lemaitre (1931), Gamow (1948), Dicke (1964) etc. supported the Big-Bang Hypothesis which explains that collision between these particles and antiparticles caused a tremendous explosion to form atoms of hydrogen. Cosmic evolution began with the fusion of hydrogen atoms with progressively heavy atoms of different elements. Stellar systems and stars were formed by spreading of original gaseous cloud into the space and divided into smaller and larger masses. Most of the stars are masses of red hot gases even today.

Kant (1755) and Laplace (1796) supported Nebular Hypothesis which explains our solar system to have evolved about 4.5 to 5 billion years ago from a rotating red-hot gaseous cloud containing millions of free atoms of different varieties. First of all sun was formed when this cloud condensed, next the planets were thrown off from the sun and later on, in turn satellites were thrown off by planets. However, Nebular hypothesis was rejected by astronomers of present century like Weizsacker (1944), Alfen (1950) and Hoyle (1955).

According to them the sun was originally surrounded by a disk of rotating gas. Later many rotating concentric whorls were formed by break up of the disc and by gravitation and rotation particles of each whorl collected and condensed to form a planet. Our earth was formed about 4.6 billion years ago as a red hot gaseous cloud of free atoms with temperature of 5000o to 6000oC.

Structure of present earth. The earth is orange like in shape. Its pole to pole diameter is 12640 km and equatorial diameter is 12783 km. It is about 15

crore km away from sun and about 484000 km away from moon. Due to tremendous temperature gases existed in atomic form but gradually they cooled down in hundreds and millions of years into molten core. According to density other elements got stratified. Earth contains the central solid core, the middle mantle and shell and outer crust. The earth rotates at its polar axis in one day and it rotates


 Origin of Life.

Life is the part and parcel of the universe and both are very intimately associated with each other. We know that “Life is the most unique, complex organisation of molecules, expressing itself through chemical reactions which lead to growth, development, responsiveness, adaptation and reproduction” that matter has achieved in our universe. Origin of life is a unique event in the history of universe.




  • Ancient theories of origin of life : Various theories have been put forward to explain the phenomenon of origin of life. A few of them were only speculations while others were based on scientific grounds. These theories are –
  • Theory of special
  • Theory of spontaneous generation or
  • Biogenesis
  • Cosmozoic
  • Theory of sudden creation from inorganic
  • Naturalistic
  • Theory of special creation : According to a Spanish Priest Father Suarez (1548 – 1617 B.C.), the whole universe was created in six days by the First day Earth and heaven, second day sky, third day dry land and vegetation, fourth day Sun, Moon and other planets, fifth day fishes and birds, and sixth day human beings other animals were created by God. This theory was based on some supernatural power.
  • Theory of spontaneous generation or Abiogenesis : This theory postulates that life originated from non-living matter spontaneously from time to This theory was supported by Plato, Aristotle, Anaximander, John Ray, Needham, Von Helmont, etc., upto the end of seventeen century. Huxley (1870) criticised this theory and propounded the theory “life originated from preexisting life only.”
  • Abiogenesis means origin of life from non-living
Ridi’s Experiments
  • Biogenesis : Scientists like Redi (1668) Spallanzani (1767), Louis Pasteur (1866–1862) provided experimental support for the Biogenesis concept of

Francesco Redi (1668) showed that maggots could not be created from meat. Actually, the smell of meat attracts flies which lay eggs on the flesh. These eggs hatched into flies.

Spallanzani (1767) showed that even primitive, unicellular organisms cannot arise from non-living matter.

Pasteur’s experiment with swan-necked flasks

Louis Pasteur (1860-62) obtained air samples in the flasks of broth (yeast and sugar solution) whose drawn-out necks were sealed cooling these contained a partial vaccum. Where a sample was required, the flask was opened. Air was drawn in and the flask was resealed. Flasks were incubated. These flasks which were opened in the streets became turbid while those exposed to dust-free air rarely contained bacteria.


Louis Pasteur also, used swan-necked flasks whose long, curved necks permitted exchange of air between outside and inside of the flask, but dust and bacteria were trapped along the wall of the neck. On tilting the flask, the bacteria got washed down into the broth, so that the latter became cloudy due to bacterial growth.

  • Cosmozoic Theory : Richeter (1865), Preyer (1880), Arrhenius (1908), Hoyle (1950) and Bondi (1952) believed in eternity of According to Arrhenius life was transferred from “cosmozoa” (life of outer space) to different





planets small units called ‘spores’. The spores were covered by a thick protective covering. When the spores got favourable conditions and temperature, the spore coat was dissolved and gave birth to initial living organisms. This theory does not explain as to how the life originated in space and how the life originated in spores remain impenetrable by ultraviolet and gama rays.

  • Theory of sudden creation from inorganic material : Cuvier (1769-1832) believed in According to him, the catastrophy destroys the whole life on earth, and after that, new life originates called it as Mechanistic theory.
  • Naturalistic Theory : Haldane, a British scientist, stated that in the early atmosphere of gas mixture probably carbon dioxide, ammonia and water vapours were predominantly When ultraviolet rays reacted on them, organic molecules were formed. Gradually, quantity of these oceans which later gave rise to amino acids, proteins, carbohydrates, nucleic acids, etc.
  • Oparin’s Modern Theory : Oparin (1924) proposed that “life could have originated from non-living organic molecules.” He believed in Biochemical origin of life. Haldane (1929) also stated similar views. Oparin greatly expended his ideas and presented them as a book “The origin of life” in

According to this theory, the Earth originated about 4,500 million years ago. When the earth was cooling down, it had a reduced atmosphere. In this primitive atmosphere nitrogen, hydrogen, ammonia, methane, carbon mono-oxide and water were present. Energy was available in the form of electric discharges by lightening and ultraviolet rays. As soon as the earth crust was formed, it was very much folded. Torrential rains poured over the earth for centuries and were deposited in deep places.

The atmospheric compounds, inorganic salts and minerals also came in deep places oceans, these molecules gave rise to a variety of compounds and finally to the self-duplicating molecules. Ultimately these molecules were enclosed in membranes derived from lipids and proteins, along with water and chemical compounds, giving rise to cell like units. Again random combinations may have led to the formation of chlorophyll– containing organisms which could produce their own food (autotrophs) by a process called photosynthesis. These organisms had a better chance to live because

they synthesise starch from carbon dioxide and water in presence of


sunlight. Starch could be used as further source of energy. During photosynthesis, oxygen was produced. The oxygen was used by

According to Oparin’s hypothesis–Origin of cells from coacervates


other organisms for respiration. Also oxygen, when acted upon by ultraviolet rays, formed ozone layer through which ultraviolet rays cannot pass. This layer is formed about 25 km. from earth’s surface. After the formation of ozone layer, organisms could come to the surface of the ocean and could survive even on land, if thrown out of oceans. The Oparins’s and Haldane’s theory of origin of life is most accepted these days as it is supported by Miller’s experiment duly supported by David Buhal, Melvin Kelvin’s experiment etc.

  • O2 is absent in the primordial atmospheres at the time of origin of life.

Miller’s Experiment : An American scientist (Biologist) Stanley Miller (1953) performed an experiment under support Oparin’s theory of origin of life. He believed that basic compounds which are essential for life can be




synthesised in the laboratory by creation in the laboratory, on a small scale, the conditions which must have existed at the time of origin of life on earth.

Miller took a flask and filled it with methane, ammonia and hydrogen in proportion of 2:1:2 respectively at 0°C. This proportion of gases probably existed in the environment at time of origin of life. This flask was connected with a smaller flask, that was filled with water, with the help of glass tubes. In the bigger flask, two electrodes of tungsten were fitted. Then a current of 60,000 volts was passes, through gases containing bigger flask for seven days. At the end of seven days, when the vapours condensed, a red substance was found in the U-tube. When this red substance was analyzed, it was found to contain amino acids, Glycine and nitrogenous bases which are found in the nucleus of a cell.

  • An experiment to prove that organic compounds were the basis of life, was performed by

From the above theory we conclude that life first originated in water. Therefore, water still continues to be an essential constituent of life.

The entire process of the origin of life, as proposed by Oparin, can be summarised as under –

(a)   The Chemical Evolution :

  • Step 1 : Formation of simple molecules : The reactions between simple atoms like carbon, hydrogen oxygen and nitrogen in the primitive atmosphere led to the formation of simple compounds like water, ammonia and methane. But since the earth was very hot, all these substances remained in the form of vapours in the Gradually, as the earth started cooling down, the dense clouds began raining on the earth. But the liquid was still very hot. Therefore, as the liquid water touched the earth’s surface, it again got vaporized to be returned to the atmosphere. This process continued for millions of years. As a result of these heavy downpours, the earth’s surface got filled with water to form rivers and oceans. Ammonia and methane got dissolved in the oceanic water. The mineral elements, which were dissolved in rivers, were also carried into the oceans when rivers joined it.

The scientists have found that simple unicellular organisms (resembling modern cyanobacteria) were present on this earth about 3600 million years ago. It is believed, therefore, that life must have originated on this earth about 4600 to 3600 million years ago.

It must be clear that the earth’s atmosphere at that time was quite different from as it exists today. The earth’s atmosphere at that time was reducing, not oxidizing (as it is today). The primitive atmosphere of earth had hydrogen, nitrogen, water vapours, carbon dioxide, methane and ammonia abundance. Oxygen was not available in free state in sufficient quantities.




  • Step 2 : Formation of Simple organic compounds : Continuous rains provided opportunities for different types of molecules to collide with each other and Methane is an active compound, and it reacted with other compounds to form organic compounds like ethane, butane, propane, ethyl alcohol. From such organic compounds which were formed in the ocean and which played a role in the origin of life include –
    • Sugars, glycerol and fatty acids : These were formed by the combination of carbon, hydrogen and
    • Amino acids : These were formed by the combination of carbon, hydrogen, oxygen and
    • Pyrimidines and Purines : These were formed by the combination of carbon, hydrogen and

These compounds were formed at the time when sunlight could not reach earth because of dense clouds in the sky. Under such circumstances, the energy required for the synthesis of above–mentioned chemical substances must have been obtained from the cosmic rays and lightening in the sky. Even today, NH3 and other compounds are formed in the atmosphere during electric discharges. Thus, it is clear that all the amino acids required by living organisms could be formed in the condition of primitive atmosphere. However, this type of synthesis requires reducing atmosphere because the substances produced through these processes get destroyed by oxidation on coming in contact with oxygen. Since free oxygen is available in the atmosphere today, present day atmosphere is no longer reducing–it has turned oxidizing. Therefore, in the present circumstances, such a synthesis of chemical substances is not possible.

  • Step 3 : Formation of complex organic compounds : The simple organic compounds combined in different ratios to form complex organic compounds like polysaccharides, fats and
    • Simple sugars combined in different ratios to form polysaccharides like starch, cellulose, glycogen The formation of such compounds had been very important for the origin of life because cell walls are made up of cellulose and energy–giving molecules are stored in the form of starch and glycogen.
    • The reaction between glycerin and fatty acids yielded
    • Different types of combinations between a variety of amino acids yielded different types of The formation of protein was a very important step in the origin of life because proteins are not only structural components of cell organelles, but many proteins, called enzymes, work as catalysts for biochemical reactions.

In fact, the present day organisms synthesise their complex molecules from simple organic substances with the help of enzymes only. However, enzymes themselves are proteins. Therefore, first of all protein must have been synthesized without the help of any enzyme.

  • Step 4 : Formation of nucleic acids and nucleoproteins : You have already learnt that the reaction between methane, ammonia and water resulted in the formation of purines and Some of the purines and pyrimidines combined with sugar and phosphorus to form nucleotides. Many molecules of nucleotides combined to form nucleic acids–DNA and RNA. The formation of nucleic acid in the oceanic water was a big steps in the direction of origin of life. Nucleic acids combined with the proteins to form nucleoproteins. Some of the nucleoproteins developed the capability to synthesise molecules similar to themselves, from organic and inorganic substance present in the ocean. In other words, the capability to reproduce had evolved. As a result of continuos reproduction, the number of nucleoproteins went on increasing. Since, organic substance were required for this, the organic substances started being depleted resulting in competition between the nucleoproteins. Physical and chemical changes sometimes led to the changes in the competition of nucleoproteins, and new types of nucleoproteins came into existence by mutations. Those new nucleoproteins which were successful in the competition, increased in number.












Lava   Fe


Other Metals






Amino Acids


e. g., Alanine; Leucine; Glycine

Energy of ultraviolet rays

and electric discharge

(b)  Organic Evolution :

  • Step 5 : Formation of Coacervates

Oparin believed that the formation of protein was a very important step in towards the origin of life. Due to their Zwitterionic nature, the protein molecules. This enabled these colloidal structures to maintain their identify inspite of being surrounded by water molecules–forming a type of emulsion. The coalescence of these colloidal structures led to the formation of structures called coacervates. These coacervates had the ability to exchange substances with the surrounding water and accumulating required substances within them.

  • Under certain conditions scientists have obtained cell like These are known as coacervates.

Sydney F. Fox’s experiment : Sydney F. Fox of Florida University, obtained some complex molecules by heating upto 90°C a dry mixture of many amino acids found in living organisms. The molecules so obtained very much resembled the proteins. He heated these molecules in water and allowed the mixture to cool down. In the fluid so obtained, he could observe minute structures resembling the cells. He called them microspheres. The microspheres are surrounded by membranes, and these also reproduce vegetatively just like years. Surprisingly the biochemical processes like breakdown of glucose also occur their. However, electron microscopic examination of these does not reveal any cellular structure. On keeping in distilled water, these become turgid, but these get shrunk if immersed in salt solution. Oparin’s coacervates and Fox’s microspheres are infact, similar structures.

  • Step 6 : Formation of Primitive cell : A primitive cell membrane was formed by the arrangement of lipid molecules between the surface of coacervates and external watery medium. This provided stability to the It is believed that a primitive cell was formed when–
    • Nucleic acids having the property of self-duplication entered the
    • Rearrangement of molecules occurred inside the coacervate surrounded by lipid

Earliest organisms : Evidences available so far indicate that the cells of the earliest organisms did not contain either nucleus or cell organelles. The molecules of nucleic acid were surrounded by a colloidal mixture (may be called protoplasm) of proteins and organic compounds. This, in turn, was surrounded by a thin membrane.





Water and soluble substances would pass through this membrane. Such cells which lacked nuclei were called prokaryotic cells. In 1966, some fossils have been discovered from 300 million years old rocks. These fossils are of prokaryotic organisms. Earliest organism is chemoheterotrops.

  • Step 7 : Origin of autotrophism : In the primitive organisms, the process of metabolism began because all the substances required for reduction were available in water. Of course, oxygen was not available which was required for oxidation of substances to yield energy. Therefore, the primitive cells respired anaerobically e., these used to obtain energy by fermenting the organic compounds obtained form the water, with the help of enzymes due to fast nutrition, growth and multiplication, their number in the ocean increased greatly. As a result, scarcity of organic substances developed in the ocean. A struggle started between the cells for obtaining nutrition.

At such a time, some of these organisms developed the capability of synthesizing organic substances. Such organisms began synthesizing energy– giving substances (carbohydrates) from simple inorganic substances abundantly available in the environment. In this way, evolution of autotrophs from heterotrophs took place.

This was the beginning of autotrophic nutrition. However, it was quite different from the photosynthesis which is carried out by green plants, because it utilized energy obtained by anaerobic respiration (not solar energy). Therefore, such type of nutrition is also called chemoautotrophic nutrition. Such type of nutrition is observed even


today in the sulphur bacteria.

6 CO2 + 12 H2S ¾¾Ferm¾ent¾at¾ion® C6 H12O 11 + 6 H2 O + 12 S

Chemical energy


At the same time, from different chemicals present in the oceanic water evolved porphyrins which where like modern chlorophyll led to the evolution of present chlorophyll, so that these cells started utilizing H2 O instead of


H2S for photosynthesis.

6 CO2 + 12 H2S ¾¾Sol¾ar ®  CH12O6 + 6 HO + 12 S



Till then, oxygen was not freely available in the atmosphere. However, gradually molecular changes in the bacteriocholorophyll led to the evolution of present chlorophyll, so that these cells started utilizing H2O instead of


H2S for photosynthesis.

6 CO2  + 12 HO ¾¾Sol¾ar ® CH12 O6  + 6 HO + 6O2



In this way, the prokaryotic cells which were chemoautotrophs, became photo autotrophic. These cells resembled modern cyanobacteria. In 1968, the forms of such types of cells have been recovered from 320 million years old rocks. These have been given the name Archaeospheroides barbertonensis. Due to the absence of well- defined nuclei in them, these have been included under the kingdom ‘Monera’. Thus, release of O2 in the atmosphere and its free avilability was the result of photosynthesis. This was a revolutionary change which greatly affected the course of organic evolution.

  • Step 8 : Origin of Eukaryotic cells : As a result of photosynthesis, oxygen was released in the atmosphere which started reacting with methane and ammonia in the atmosphere. Its reaction with methane yielded CO2 and H2 On the other hand, reaction between oxygen and ammonia resulted in the formation of CO2 and nitrogen. In the course of these changes, Ozone (O3) gas was formed from oxygen; the ozone spread in the form of an envelope surrounding the earth, the distance between the ozone layer and the earth’s surface being approximately 15 miles. As free oxygen became available on the earth, gradual changes took place in cell structure also. Membrane – bound organelles i.e., mitochondria, chloroplasts, golgi bodies, lysosomes evolved. Thus, prokaryotic cells. Most of the organisms on the earth today are eukaryotic.Gradual changes in the earth’s atmosphere led to gradual changes in the eukaryotic cells also. Instead of living separately, the cells started living together in the form of colonies. Simultaneously, multinucleation of multicellular structures forming tissues. Different




types of tissue combined to form special organs. From the organs, organ systems and ultimately complex bodies of organisms were formed.

  • Organic evolution would have not been taken place if individuals in a population did not show genetic
  • Synthetic theory is the most accepted theory of organic
  • The greatest evolutionary change enabling the land vertebrates to be completely free from water, was the development of shelled eggs and internal
  • The material for organic evolution is

 Evidences of Organic Evolution.

The following are the evidences in favour of Organic Evolution :

(i) Evidences from Classification                       (ii) Evidences from Comparative Anatomy

  • Analogy and Homology
  • Vestigeal organs

(iii) Evidences from Physiology                        (iv) Evidences from Serology

(v) Evidences from Embryology                  (vi) Evidences from Palaeontology

(vii) Evidences from geographic distribution          (viii) Evidences from Genetics

  • Evidences from Classification : All the known living animals and plants have been classified into various species, genera, families, order, classes, phyla and 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.
  • Evidences from Comparative Anatomy : In all the living animals, the basic substance of life is 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 –
  • 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 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





  • Homologous organs explains divergent
  • Similarity developed in distantly related groups as an adaptation to the same function is called convergent
  • The wings of an insect and a bat exhibit











  • Homology in the forelimbs of vertebrates


  • Vestigeal organs : There are present in the body of animals certain structures which have no function and are very much 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.

  • Vermiform appendix is the Vestigeal organ in
  • Hind limbs is a vestigeal organ of
  • Muscles of ear pinna is vestigial organ of
  • Wisdom teeth is vestigeal organ of
  • Evidences from Physiology : Various types of chemical tests exhibit many basic similarities in physiological and chemical properties that show a physiological relationship among

Take the example of thyroxin hormones, which are similar in all


metabolism. The thyroid of human beings can be very easily replaced by the thyroid of cattle without any ill-effects and so on.

Different vestigial organs in man


  • Evidences from Serology : This is a method by which the reactions of blood serum is 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.
  • Evidences from Embryology : With the exception of a few, every multi-cellular animal originates from a zygote. The development from zygote to adult shows many similarities in various organisms. The development is termed as

Haeckel, a German biologist, gave a theory “Ontogeny repeats Phylogeny”. An individual organism during its development (ontogeny) tries to repeat the history of its race by different stages (phylogeny). The vertebrate development confirms the above statement. For example, the embryos of all vertebrates pass through a gilled stage. In fishes, the gills are present in the adult condition. In amphibians, gills are present in some forms of frog and in




tadpole. While in reptiles, birds and mammals these gills are never functional but nevertheless they are always present in the embryo.

Haeckel, believed that all those stages, which have occurred during the course of evolution of a particular animal, also pass through its development.

There are objections to this theory, but is throws enough light to show the interrelationship of animals.

  • 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

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
  • In Cenozoic era mammals and birds were
  • There was no life in Azoic
  • Fossils are remains of organisms present in the
  • Ruling reptiles were dominant during Mesozoic


Era Age million years Period Epoch Dominated Fauna

(Age of Modern life)







Modern man, mammals, birds, fishes,




  Pleistocene Extinction of grate mammals. Primitive man common. Evolution of human

society & culture.



Tertiary Pliocene Evolution of primitive man–like forms

from man like apes. Formation of modern mammals.

2.5   Miocene Mammals at peak. Evolution of man–like


3.8   Oligocene Decline of egg laying and marsupial mammals.

Rise of first monkeys and apes.

5.4   Eocene Diversification of placental mammals.
6.5   Paleocene Rise of first primates, placental mammals

and modern birds.


Mes ozoi c


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




  195 Jurassic (Age of Giant Reptiles) Origin of advanced lizards, crocodiles, alligators, marsupials toothed birds. Dinosaurs became large. Reptiles dominant.
240 Triassic Origin of dinosaurs & primitive mammals (egg–laying mammals). Extinction of primitive amphibians.
Paleozoic (Era of Ancient life) 285 Permian Extinction of many marine invertebrates like trilobites. Rise of modern insects.

Evolution of mammals like reptiles.

375 Carboniferous

(Age of Amphibians)

Origin of reptiles and winged insects.
420 Devonian

(Age of fishes)

Origin of first land vertebrates (amphibians) Fishes abundant.
450 Silurian Origin of jawed fishes and wingless insects.
520 Ordovician

(Age of invertebrates)

Origin of chordates with first jaw– less fishes (origin of vertebrates). Invertebrates abundant.
570 Cambrian All invertebrate phyla established. Trilobites (swimming crustaceans which do not exist today) dominant
Proterozoic (Era of early life) 2300







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

Scanty fossils.

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

Archeaozoic (Era of invisible life)     No life?

(a)  Fossils :

  • Direct evidences of organic evolution are provided by fossils (L. fossil, dug up).
  • The science of discovering and studying fossil record is called Palaeontology (Gr. palaeo, ancient; logy, study of).
  • Fossils are remnants, models and impressions of extinct
  • Fossils are found preserved in earth’s sedimentary
  • Fossil may be an entire organism buried in sediment or snow, small part of ancient organism or impression of ancient leaf or
  • Fossilization occurs where organisms are buried and preserved by natural
  • Petrifaction is the most usual type of fossil in which hard parts like bones, shells, teeth or trunk of trees are




  • Other types of fossils are moulds, casts and A compression is a common type of plant fossil formation.
  • A compression is formed when the internal structure of plant is lost leaving a thin carbon film which gives the outline of the original
  • When the contents of intestine of ancient animals are preserved, it is known as coprolite.
  • Palynofossils are tiny microscopic spores, pollen and other vegetal remains of the
  • Pseudofossils are formed by minerals which crystalize and develop into patterns resembling outlines of
  • Twenty million years old fossil forests have been discovered and studied by the Birbal Sahni Institute of Palaeobotany,
  • About 25,000 years old frozen elephant-like mammoths were found buried in ice in Siberia in the early part of this
  • Leonardo da Vinci (1452-1519) is called the father of
  • The modern palaeontology was established by Georges Cuvier (1769-1832) and referred to as ‘Founder of modern Palaeontology’.
  • Palaeontological study helps in understanding and locating coal and hydrocarbon

(b) Age of fossils :

  • The age of the fossils or rocks can be determined by ‘Clock of the rock method’ or ‘Radioactive clock’.
  • ‘Clock of the rock’ method is based on conversion of unstable radioactive nuclei into stable nuclei over a fixed
  • Radioactive clock method was introduced by Boltwood in
  • The common radioactive elements which lose their radioactivity and change into their non-radioactive isotopes at a fixed rate are :

Important living fossils


1. Peripatus (Arthropoda) 2. Limulus (Arthropoda)
2. Nautilus (Mollusca) 4. Neopilina (Mollusca)
5. Lingula (Brachiopoda) 6. Latimeria (Coelacanth fish)
7. Sphenodon (Reptilia) 8. Didelphis (Opossum)
  • 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 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. 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’.

  • 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 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.

  • Recapitulation theory is given by Haeckel.

The biogeographical realms of the world


  • Atavism : Atavism or reversion is the sudden reappearance of some ancestral features. Such as Large canines, thick body hairs, short temporary

(b)  Protective resemblance :

  • A close resemblance with environment for defence and offence is found in many
  • Some organisms resemble other organisms and thus escape from This phenomenon is known as ‘mimicry’.




  • Phyllium is the leaf-insect because it resembles It is an example of mimicry or protective colouration.
  • Peppered moth, Biston betularia, is found abundantly in industrial cities of
  • The change of the lighter coloured variety of peppered moth, Biston betularia, to its darker variety (carbonaria) is due to mutation of a single Mendelian gene for survival in smoke-laden industrial
  • Phenomenon of “Industrial melanismdemonstrates natural
  • Biston betularia shows industrial melanism and it is the example of ‘evolution taking place before the eyes’.
  • Connecting links : Intermediate or intergrading forms between two groups of organisms :
Organism Connecting link between
1. Viruses Living and nonliving
2. Euglena (Protozoa) Plants and animals
3. Proterospongia (Protozoa) Protozoa and Porifera
4. Peripatus (Arthropoda) Annelida and Arthropoda
5. Neopilina (Mollusca) Annelida and Mollusca
6. Balanoglossus (Chordata) Nonchordata and Chordata
7. Dipnoi (Lungfish) Pisces and Amphibia
8. Archaeopteryx (Aves) Reptiles and Birds
9. Prototheria (Mammalia) Reptiles and Mammals

 Theories of organic evolution.

  • Lamarckism : Lamarck (1744 –1829) was one of the most brilliant stars on the horizon of the history of Though he was a man of great intellect yet he had to cut a sorry figure because of poverty, and secondly, as he could not get the approval of the famous and influential Cuvier. His work was recognized only few years after his death.

He was the first naturalist to put forward a general theory of evolution in his famous book. Philosophic Zoologique published in 1809. His evolutionary theory may be summarised in the form of following laws:

  • The internal forces of life tend to increase the size of an
  • The necessity in animals to produce new
  • The effect of use and
  • Inheritance of acquired
  • The internal forces of life tend to increase the size of an organism : Lamarck believed that there is some kind of internal force which is constantly working in living Its tries to make the animal bigger in size.
  • The necessity in animals to produce new structures : According to this law, each organ and structure is the product of some continuous necessity in the animals, to develop He believed that it was not due to the direct influence of the environment, but acted through the nervous system, the process being very slow. The plants, he said, were directly influenced by their environmental conditions.
  • The effect of use and disuse : Lamarck thought that the continuous use of a particular organ or structure tends to increase its size and its development. On the other hand, disuse results in disappearance of that structure, the process being slow and He supported this by a number of well-known examples. The long neck of giraffe was the result of continuous stretching to obtain food from the trees (long neck was acquired due to excessive use). The wings of certain birds Ratitae were not used for long, as there were no formidable enemies and food was available in abundance. So they did not fly and consequently lost the power of flight. Their wings became rudimentary (flightless condition acquired due to the disuse of wings).




  • Inheritance of acquired characters : Lamarcks stated that all the characters, which are acquired in one’s own lifetime are inherited by

Cirticism : The greatest draw-back in the Lamarck’s work was that it was too theoretical and there were no proofs to support it. His ideas were theoretically sound but practically they had no standing. He met a severe criticism from various workers –

  • The most serious blow came from Weisman who put his theory of continuity of germplasm which states that the inheritance is the sole concern of germ Characters introduced in the germ cells will only be inherited and not those which are present in somatic cells.
  • If the acquired characters were to be inherited, as Lamarck said, the world would have been full of cripples, blinds and deformed persons, as most of these characters are
  • Some workers have practically proved that mutilations are not inherited by offspring’s even if practiced for

Lamarck’s second and third laws show much truth in them; but they are not the sole cause of evolution. The laws of heredity of Weisman are practically opposed to the fourth law of Lamarck. Nevertheless, there came a number of workers who supported Lamarck and modified his laws. They were known as Neo-Lamarckian and the names of a few of them are : Cope, Haeckel, German savant, Gadow and Spencer.

  • Germplasm theory was given by weisman : Later on, Weisman also admitted that the germplasm may become modified to a certain extent by some environmental

There is always going on a struggle for existence among various agencies. The struggle for existence may be–

  • Intra-specific : When two different groups of animal species are opposed to each other, i.e., lion and deer or birds and
  • Inter- specific : When there is struggle between the members of the same species, as all of them have same wants and
  • Environmental : It is the struggle between the animals and their environment (e., climate, vegetation, open spaces, jungles and water, etc.,) A change in climate may affect adversely, resulting in the extinction of some and the survival of others.
  • Key point to lamarck’s view about organic evolution is that every offspring inherits characters acquired by the parental
  • Law of “Inheritance of acquired characters” is presented by
  • Lamarck’s theory was most severely criticized by curvier who greeted it by calling it “Nouvelle ”

Difference between Lamarckism and Neo–Lamarckism


Lamarckism Neo–Lamarckism
It is original theory by Lamarck. It is a modification of the original theory of Lamarck in order to make it more suitable to modern knowledge.
The theory lays stress on internal force, appetency and use and disuse of organs.

It believes that changes in environment brings about a conscious reaction in animals.

According to Lamarckism the acquired characters passes on to the next generation.

Neo–Lamarckism does not give any importance to these factors.

The theory stresses on the direct effect of changed environment on the organisms.

Normally only those modification are transferred to next generation which influence germ cells or where somatic cells give rise to germ cells.




  • Darwinism : Charles Robert Darwin was undoubtedly the first naturalist who put the idea of organic evolution on sound His statements and theories were based upon practical experiences and large number of proofs which he collected directly from the nature. He devoted his whole life for the purpose of finding out proofs in support of the theory of organic evolution.
  • Charles Darwin wrote Orgin of
  • Darwin was appointed up on a world survey ship of British government M.S. Beagle. His main ideas about the evolution are given below –
  • Over – production of offsprings (b) Limited supply of food and shelter (c) Struggle for existence :
    • Intra –specific (2) Inter –specific                            (3) Environment

(d) Survival of the fittest (e) Universal occurrence of variations  (f) Inheritance  (g) Natural selection

  • The theory natural selection is given by
  • Over-production of offsprings : The power of reproduction is enormous in the living The single Paramecium (Protozoa) divided about 600 times in a years. If all the progeny survive their total amount would exceed that of earth in a few months. Again if all the eggs of a lobster were to produce young ones, in about ten years time the sea would be full of lobsters and there will be hardly any space for other animals. Darwin gave one more interesting example of elephants. An elephant lives for about a hundred years and starts reproducing at the age of thirty. If each female produce six young ones, in 750 years, about 1,90,00,000 would be alive.
  • Limited supply of food and shelter : The amount of food and shelter is limited in a particular area. It is sufficient only for a definite number of
  • Struggle for existence : It is a common experience, that even with the enormous rate of reproduction, the number of species mostly remains stationary. The nature has provided a number of checks over their There is limited food, breeding places, shelter, presence of predators and parasites, etc.
  • Survival of the fittest : During the process of struggle for existence, some individuals perish; because they could not adapt themselves accordingly, and others survive. Those which survive always show some changes and departures from their original pattern. Darwin attributed this cause of the origin of species and said that in this way new species were formed which were most fit to
  • Universal occurrence of variations : The “departures form the original pattern” and changes in animals were termed as Darwin said that these variations occur universally in all the living beings, no matter how closely they are related with each other. Variation is perhaps the most fundamental evolutionary factor.
  • Inheritance : The useful variations are inherited by the progeny. For this, Darwin used the term ‘pangenesis’
  • “Theory of pangenesis” is given by
  • Natural selection : Darwin used the term ‘natural selection’ for all the forces (namely, over-production of offsprings, struggle for existence, survival of the fittest, variation and inheritance) when working

Most of the biologists agree with the Darwin’s theory as the best explanation of organic evolution. But there are a number of objections to this theory.

  • By performing the replica plating experiment, lederberg supported the “Natural Selection Theory”.





  • Darwin’s theory does not explain that the effects of ‘use and disuse’ of organs are
  • He considered the minute fluctuating variations as the cause of natural selection, but most of the variations are non-heritable.
  • He did not distinguish between germinal and somatic His theory of Pangenesis has no basic at


  • He believed that variations occur in all directions But now it is established that the variations

occur only on definite lines of change.

  • Darwin considered mutation too infrequent for the necessary basis of
  • Darwin’s theory explains survival of the fittest and arrival of the
  • It does not explain how natural selection could make use of certain adaptive characters in their initial stages, e., what would be the use of electric organs, electric fishes, until they have enough of power to produce a shock.
  • It does not explain the over-specialised and vestigeal
  • According to him, only useful characters are inheritable but on the contrary certain useless and non- adaptive characters are also passed
  • Geologists and astronomers think that the time required producing organic world is much more than the actual age of the earth

In the light of these criticism and objections various workers after him modified his theory. Most of the work was done after the rediscovery of Mendel’s work in 1900. This modified theory of Darwin is known as Neo- Darwinism.

(1)  Examples of Natural selection :

  • Industrial melanism : Industrial melanism is a phenomenon where the moths living in the industrial areas, develop black colour (melanin pigments) to match the body to soot-covered background, on the bark of

The industrial melanism is observed and worked out by a number of evolutionists like Fisher, Ford and Kettlewell.

Industrial melanism was observed in a peppered moth Biston betularia living, Manchester, in an industrial city of Great Britain.

The peppered moths exist in two forms, namely melanic forms and non-melanic forms. The melanic forms are black in colour because they contain melanin pigments. The melanic forms are also called carbonaria. The non- melanic forms are light coloured. The light colour is due to the absence of melanin pigments.

  • Resistance to DDT : In agriculture, insect pests are controlled by the spraying of insecticides like However, DDT cannot bring about 100% deaths in any species of insects. A certain number of individuals survive. These surviving individuals are resistant to DDT. The resistance is a character controlled by genes. The resistant flies arise as a result of the application of DDT. They reproduce more and more resistant flies. Thus the resistant populations are evolved as a selective advantage against insecticides.




  • Resistance of bacterium to drugs : L. Cavalli and G.A. Maccacro (1952) experimentally proved that the colon bacteria Escherichia coli develop resistance to the antibiotic chloramphenicol 250 times as great as that tolerated by normal bacteria by exposing the bacteria to increased concentration of the drug.

Difference between Darwinism and Neo–Darwinism


Darwinism (Natural Selection) Neo–Darwinism
1.             It is the original theory given by Charles Darwin (1859) to explain the origin of new species.

2.             According to this theory accumulation of continuous variations causes changes in individuals to form new species.

3.             It believes in the selection of individuals on the basis of accumulation of variation.

4.             Darwinism does not believe in isolation.


5.             It can explain the origin of new characters.


6.             Darwinism cannot explain the persistence of certain forms in the unchanged condition.

1.              Neo–Darwin is a modification of the original theory of Darwin to remove its short–comings.

2.              Instead of continuos variations, mutations are believed to help form new species.


3.              Variations accumulate in the gene pool and not in the individuals.

4.              Neo–Darwinism incorporates isolation as an essential component of evolution.

5.              The theory can explain the occurrence of unchanged forms over millions of years.

6.              Normally only those modification are transferred to next generation which influence germ cells or where somatic cells give rise to germ cells.

  • Mutation theory :
  • Hugo de Vries (1901), a Dutch Botanist, pioneered the theory of mutations to explain the mechanism of
  • The plant on which de Vries had experimented was Oenothera lamarckiana (Evening primrose).
  • The mutation observed by de Vries in Oenothera essentially was chromosomal number
  • Mutations are discontinuous variations, called ‘sports’ by Darwin and ‘saltatory variations’ by
  • Mutations are generally harmful and recessive.
  • Role of mutations in evolution is genetic
  • Mutations are due to changes in chromosomes, genes or
  • To be a successful event for evolution, a mutation must occur in germplasm
  • Mutations are the changes which may or may not be
  • Germinal mutation is a change that is
  • Hereditary variations in plants have been produced by use of X-rays.
  • Frequency of a mutated gene in a population is expected to increase if that gene is selected by
  • The possibilities of hereditary and evolutionary changes are greater in species that reproduce by sexual
  • Organic evolution would not have taken place if individuals in a population did not show gene




Difference Amongst Lamarckism, Darwinism and Mutation Theory


  Lamarckism Darwinism Mutation Theory
1.         Vital force The theory believes that every organism has an internal vital force that tends to increase its size upto a certain limit. Darwinism does not believe in internal vital force. No internal vital involved. force is
2.         Conscious Reaction Animals with well developed nervous system react consciously to any change in environments Darwinism does not involve any conscious reaction. No conscious reaction is believed to take part in the process of evolution.
3.         Appentency The theory considers appentency or desires on the part of animals an important force in the development of modifications. It is not a constituent of the theory. Appentency in not involved.
4.         Use and Disuse The organs put to more use are believed to develop more while organs not used begin to degenerate. The theory is silent about use and disuse of organs. The theory is silent about it.
5.    Inheritance Acquired Characters of The characters acquired by an organism during its life are believed to get transferred the next generation. According to Darwin, all the living cells produce minute particles or pangenesis, which pass into germ cells for transmission to the offspring. Only those variations are transferred to the offspring which originate in germ cells or in the cells which form germ cells.
6.         Struggle Existence for The theory does not clearly spell out struggle for existence in relation to high biotic potential. Organisms produce more offspring than the available food and space so that a struggle for existence ensues amongst them. The theory believes struggle for existence. in the
7.         Origin Variations of Variations appear in organisms in response to change in environment, conscious reaction, desire r use and disuse of organs. Variations appear automatically. Variations   appear   due   to change in genetic make up.
8.         Continuous Variations The theory is silent about them though it believes in a continuous modification of organs in a particular direction. It is based on the origin and selection of continuous variations. The theory is based on discontinuous variations or mutations.
9.         Natural Selection The theory does not take into account natural selection or survival of the fittest. Darwinism is based on natural selection or survival of the fittest. Mutations theory believes in natural selection or survival of the fittest.
10.      Progress


of Evolution is a continuous process which moves in a direction governed by environment and appentency. Evolution is a continuous process, the direction of which is governed by nature. Evolution is a jerky process, the direction of which is unpredictable            though ultimately it is governed by nature.
  • Variations : Dissimilarities between members of the same species are called variations. Tendency to differ helps organisms in their adaptations for different environmental conditions and these variations are inherited from one generation to another. Heritable variations are responsible for changes in a species to form new species. Variations are thus important in




Variations are progressive factors in evolution. Members of the same species exhibit variations by structural, physiological or psychological dissimilarities.

  • Types of variations : Variations are classified in three
    • Germinal and somatic variations : Germinal variations arise in germplasm of the organism. They occur in the gene pattern and are These variation reach the zygote through gametes and hence are inherited from generation to generation. For example colour of eyes or hair occur since birth whereas characters of height and body built develop later in life. Germinal variations are also called blastogenic.

Somatic variations are produced due to environmental factors. They develop in somatic cells or somatoplasm and are also called acquired variations. These are non-heritable. Darkness of skin due to working in the sun, development of intelligence by better education, achievements of a musician, muscular body of an athlete, bored nose and pinna in ladies, are some examples of acquired variations. Somatic or acquired variations are not important in evolution.

  • Continuous and discontinuous variations : Continuous variations are small and graded variations which are found in the members of same Darwin called them as fluctuating variations and realised their importance in evolution. Darwin considered all fluctuating variations inheritable from one generation to other to form new species. They were considered important factor for natural selection. For example children of the same class and age show continuous variations in their height and intelligence but they will be much different from children of other classes.

Discontinuous variations arise suddenly and they are distinctly visible in the group. These ungraded variations deviate so much from the average character that they are seen in new form among members of the same species. There are no grades or intermediate stages in such variations. Darwin called these variations as sports while Hugo de Vries called these as mutations. These discontinuous variations are stable and inheritable. Polydactyly or more than five digits in hands or feet in man, occurrence of four horns instead of two in goat etc. are mutations.

  • Determinate and Indeterminate variations : Determinate variations arise in a definite direction and time and are due to adaptations. These are found continuously and progressively in organic evolution and are affected by strong gene combination from generation to Leaf eating moth Diabrotica soror is a good example of this variation. In this moth many different colours are found other than main colour.

Indeterminate variations do not arise under any special condition and can arise suddenly in any direction.

These can develop to any extent.

(b)  Causes of variations :

  • Environmental conditions (2) Inherent tendency to vary

(3) Dual parentage                   (4) Nuclear reorganization

(5) Change in the gene pattern

  • Adaptations : All living species in the world have special character to adapt themselves according to their If species are unable to adapt, it will be difficult for them to live in new environment and gradually they will perish.
  • Definitions of adaptations : Somatic or structural and physiological changes in an organism, developed according to environmental conditions so that the organism is better fitted in its new environment, are called
    • Post-adaptation : Adaptation in a species to the environmental conditions in which it is already living is called post adaptation. Such adaptations cause perfection because a particular species already living in the environment gradually becomes more and more




  • Pre-adaptation : Some species living in a particular environment develop adaptations for new environmental conditions and later migrate to the new environment. Such adaptation is called pre-adaptation. Simpson (1953) called it prospective
  • Types of adaptations : Adaptation can be of following types according to the habit and habitat of the organisms :
    • Cursorial adaptation : Animals which run fast on ground develop such adaptations like in horse and
    • Fossorial adaptation : These animals live under ground and they dig burrows like rats and insectivores


  • Aquatic adaptation : Animals that live in the water as fishes, whale, siren
  • Scansorial adaptation : These animals are adapted for climbing on walls, trees, hills like reptiles

and some birds.

  • Volant or aerial adaptations : Animals that fly are mainly birds. Some animals like bats are also adapted for flying in
  • Cave adaptations : Some animals live in caves like salamander and some species of
  • Deep sea adaptation : Some species of fishes live in deep sea and are adapted
  • Desert adaptation : Some animals are adapted to living in dry climate of deserts like camel and kangaroo
  • Adaptive radiation : When different species evolve from one common ancestor and develop different modifications in their body according to their environmental conditions, it is called adaptive radiation. They exhibit adaptive divergence or radiant The concept of adaptive divergence was given by Osborn.





(vii)  Synthetic theory :

  • Dobzhansky (1937) in his book ‘Genetics and Origin of Species’ provided the initial basis of synthetic
  • Modern synthetic theory of evolution’ was designated by Huxley in
  • Some of the important workers who have contributed to the modern synthetic theory are : Dobzhansky, R.A. Fisher, J.B.S. Haldane, Sewall Wright, Ernst Mayr and G.L. Stebbins.
  • According to synthetic theory there are five basic factors involved in the process of organic These are :
    • Gene mutations,
    • Changes in chromosome structure and number,
    • Genetic recombinations,
    • Natural selection and
    • Reproductive
  • While the first three factors are responsible for providing genetic variability, the last two are responsible for giving direction to the evolutionary
  • Besides the five factors outlined above, there are two accessory processes, namely migration of individuals from one population to another and hybridization between races, species and even related genera, which contribute to the
  • The most accepted and recent theory of organic evolution is the synthetic

(viii)  Hardy-weinberg equilibrium :

  • Mutations introduce new genes into a species resulting a change in gene frequencies.
  • H. Hardy, an English mathematician, and Wilhelm Weinberg, a German physician, in 1908 established a simple mathematical relationship to the study of gene frequencies.
  • If certain conditions existed, gene frequencies would remain
  • The conditions necessary for gene frequencies to remain constant are :
    • Mating must be completely
    • Mutations must not
    • Migrations of individual organisms into and out of the population must not
    • The population must be very
    • All genes must have an equal chance of being passed to the next
  • According to Hardy-Weinberg concept, the gene frequencies will remain constant if all above five conditions are


  • The distribution of genotypes could be described by the relationship

A2 + 2Aa + a 2 = 1,

where A2


represents the frequency of the homozygous dominant genotype, 2Aa represents the frequency of the heterozygous recessive genotype and a2 represents the frequency of the homozygous recessive genotype.

  • Constant gene frequencies over several generations indicate that natural selection and evolution are not taking
  • Changing gene frequencies would indicate that evolution is in




 Human Evolution.

During the course of evolution different animal species evolved special organs for a successful life like wings in insects, birds and bats for flying, claws for holding, burrowing and climbing in rats and squirrels and fins or paddles in aquatic life for swimming. Similarly, most significant event in human evolution has been evolution of brain which enabled him to become most superior member of animal kingdom. The large and more complex brain evolved greater ability of thinking, logical power and capacity of taking decisions depending on the situation.

Present human species is named as Homo sapiens sapiens (Sapient = wise). T. H. Huxley (1863) in his book ‘Man’s Place in Nature’ made first attempt to explain scientific grounds of upbringing of man. Charles Darwin (1871) in his book ‘The Descent of Man’ gave his ideas about ancestry of man.

(i)  Systematic position of man in animal kingdom :

Phylum Chordata

Subphylum Vertebrata

Class Mammalia

Order Primates

Suborder Anthropoidea

Superfamily Hominoidea

Family Hominidae

Genus Homo

Species sapiens

  • Place of descent of man : Available fossils give the evidence that most of the fossils of prehuman ancestors including monkeys and apes have been recovered from Africa, Asia and However, descent of man is supposed to have occurred in Asia due to following reasons – (i) Civilization of Asia is oldest. (ii) Asia is the land of origin of all domesticated animals and crop plants. (iii) Migration of many animal species has taken place in Asia. (iv) Fossils of many ancestors of man have been found in Java and China which are part of Asia. (v) Climate of Asia was favourable at that time for the evolution of man.
  • Time of descent of man : Although time of descent of man is a controversial subject but fossil evidence indicates that time of evolution of man is from Miocene epoch to the beginning of Pliocene epoch of Tertiary period in Coenozoic It can be estimated that ancestral man evolved from man like apes about 1 crore 32 lakh years ago.
  • Evolutionary characteristics of man : The modern man possesses following special features, which have been acquired during the course of evolution–
  1. Bipedal locomotion. II. Large brain and cranial cavity.

III. Grasping hands and feet.                           IV. Erect posture.

  1. Stereoscopic (binocular) vision. VI. Sensitivity

VII. Social organisation.                               VIII. Expression by speech.

Morphological changes that have occurred in man during the process of evolution are :

  1. Increase in brain size and intelligence. II. Attainment of erect posture.

III. Flattening of face.                                   IV. Shortening of body hair and reduction in their number.

  1. Elevation and narrowing of nose. VI. Increase in height.

VII. Reduction of brow ridges.                         VIII. Rounding and enlargement of cranium.

  1. The bowl like form of pelvic girdle and broad ilia to support the viscera.
  2. Formation of




The vertebrates of class mammalia as well as other primates have common ancestory with monkeys and apes.

Mammals evolved from primitive reptiles in early Jurassic period (about 210 million years ago).

Humans belong to family hominidae in which Homo sapiens is the only living species. The evolutionary history of man has been built up on the basis of study of fossils and molecular homology.

The earliest hominid stock included fossils of Ramapithecus and Sivapithecus unearthed from Africa and Asia. Several species belonging to genus Homo can be recognised from fossil record. Human evolution took place in Africa and Asia.

A common ancestory for great apes and man has been deduced on the basis of similarities in DNA content, chromosome number and banding pattern of chromosomes.

  • Early human ancestors : The fossils Ramapithecus and Sivapithecus which lived in Africa and Asia (about 1015 million years ago) are believed to be the forerunners of Hominids. These were first man–like primates. The first fossil of Ramapithecus was a fragment of upper jaw recovered from the Shivalik Hills of India. Ramapithecus and Sivapithecus must have a sort face, small brain case, thickly enameled large teeth and they must have been used to walk on their
  • Australopithecus (the first man–ape)– Its fossils were described by Raymond Adart in 1925 from South These were intermediate between Ramapithecines and genus Homo.

Australopithecines are considered to be ancestral to all hominids of genus Homo.

Australopithecines must have been small statured averaging about four feet. They walked nearly or completely straight. The vertebral column had a distinct lumbar curve with pelvis broad and basin–like. The teeth were larger than those of modern man. though jaws and teeth were larger than those of modern man. Their face was prognathous and a chin was absent. The bulge of occipital region was small. Eyebrow ridges projected over the eyes.

Their brain capacity ranged from 450–600 ml i.e., slightly larger than that of modern adult Chimpanzee.

Thus, Australopithecine’s represented man with an ape–brain.

  • Ramapithacus has been known from shivalik hills in
  • Australopithecus stood
  • E. Lewis discovered fossils of Ramapithacus.
  • Australopithacus have 500 c. cranial capacity.
  • Homo erectus (The forerunner of Modern Humans) : In the Middle Pleistocene period, Australopithecines were succeeded by large brained form which were described under the name Pithecanthropus or Java man. Its first fossils were obtained by Dubois (1891). These were named Pithecanthropus erectus (erect ape– man). Similar fossils were found in a cave near Peking, China, and were named Sinanthropus

Mayer (1950) has replaced these names by Homo erectus–

  • Java Man (Homo erectus = Pithecanthropus erectus) : Its fossils occurred in the Pleistocene deposits about 500,000 years ago. Its cranial cavity was about 940 c; (intermediate between that of Australopithecus (600–700 c.c.) and modern man (1400–1600 c.c.) It was more than five feet tall with skeleton much like ours. Its forehead was low and slanting. The face was prognathous, and jaws were massive with huge teeth. The chin was absent and bony eye. He might have learnt the use and construction of tools and knew how to lit fire.
  • Fire was first used for protection and cooking by Java
  • Peking man (Homo erectus pekinensis = Pithecanthropus pekinensis–Sinanthropus pekinensis) : These perhaps lived 500,000–2,00,000 years ago. It was very similar to Java man with heavy bony eyebrow ridges, low slanting forehead and chinless face. However, their cranial cavity was much larger as compared to Java man ranging from 850–1200 and averaging 1075 c.c.




  • The skill of pithecanthropus was found in
  • The fossils of sinanthropus pekinensis have been discovered in pleistocene
  • Homo sapiens (Late Pleistocene Man) : Homo erectus were succeeded by early Homo sapiens, which were described under different names Homo neanderthalensis, Homo heildelbergensis, etc. But, since they are grouped under Homo

The fossils of primitive man were found in Europe, Asia and Africa. These are Heildelberg man, Neanderthal man, Solo man and Rhodesian man.

  • Heidelberg man : Their jaw is large and heavy and lacks a chin. Teeth are like those of modern man. Heidelberg is regarded as an ancestor to Neanderthal man and contemporary to Homo
  • Neanderthal man : These are considered to be on direct line of ancestry of modern man. Their fossils were found in the Neanderthal valley in Germany. Previously, it was named as sapiens neanderthalesis. These arose some 1,50,000 years ago and flourished in Europe, Asia and North Africa. These were similar to us below the neck, and were heavily built with outwardly curved thigh bones.

The skull bones were thick, forehead was low and slanting and the eyebrow ridges were heavy. The jaw was deep with no chin. The cranial capacity was about 1450 c.c. (almost equal to the modern man). But its lower and posterior portions were larger than the upper and anterior parts. It was quite intelligent to use and construct tools. It buried its dead and could perform ceremonies as well as

constructed hut–like dwelling structures.

  • Neanderthal man lives in
  • Rhodesian man (Homo Rhodensis) : Fossils of Rhodensian men were found in Rhodesia in the large limestone Their skull had a cranial cavity about 1300 c.c. with receding forehead and heavy eyebrow.
  • Cro–Magnon Man : These lived during last 30,000 years or more in Europe. These succeeded Neanderthals and became extinct about 10,000 years ago in the last glacial

These were about 180 cm. in height with a large skull, broad face, rounded forehead, narrow nose and a prominent chin. They lacked eyebrow ridges. The cranial cavity was about 1660 c.c. These were cave dwelling and hunters. They made tools from stones and ornaments from ivory.

  • Cro–Magnon is the most recent ancestor of “Homo sapiens”.
  • Cro–Magnon man was expert in making tools, weapons, paintings
  • Cranial capacity of modern man is 1350–500 cm3.
  • Modern Man (Homo sapiens–sapiens) : After last glacial period i,e., about 10,000 years ago, Homo sapiens– sapiens appeared and began to spread all over the globe. He learned to cultivate plants and domesticate animals of economic These were the first settlers who started living a settled life.







(v) Races of man : All modern living men belong to single subspecies Homo sapiens sapiens. Human species is much adaptable and due to fresh migration the members adapted to different geographical regions developing into different races. Each human race has characteristic genetic traits like height, proportion of body parts, shape of skull, colour of skin and eyes, colour and shape of hair, shape of lips and blood groups etc. Human races are studies under ethnology.

According to A.S. Romer different human races can be divided mainly into four groups.

  • Australoids : It is the most primitive and simple existing human Bushmen of Australia and Africa, Bhils of India and Veddans of Srilanka are included in this group. They have dark skin, elongated skull, flat nose, small forehead, eye brow ridges prominent, thick body hair and hair on head.
  • Negroids : Negroes of Africa and Pacific islands and Pygmies of Congo, Andman, Malaya, Newguinea and Philippines are included in this They live in tropical regions. Their colour of skin is black, head is long and broad, forehead is high, nose flattened, lips are thick and hair on head are wooly.
  • Caucasoids : These inhabit in temperate regions like countries around Mediterranian sea including North Africa, Europe, Russia, Arab countries and They have fair or brown coloured skin, long or broard head, thin nose and wavy hair on head. This race is often aggressive and dominant in nature.




  • Mongoloids : Members of this group often inhabit colder regions of the world and include Chinese, Japanese, Mangols, Eskimos and Red Indians. Colour of skin is red, light brown or yellow. Body hair are sparse but hair on head are very thick straight, hard and black. Head is broad, cheek bones are high, nose medium in size and eyes are small. However, some anthropologists divide the present man into 6 human
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