Origin of Life Origin of Life Theories of

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Origin of Life • Origin of Life • Theories of origin of life

Origin of Life • Origin of Life • Theories of origin of life

 • What is life? • The condition that distinguishes animals and plants from

• What is life? • The condition that distinguishes animals and plants from inorganic matter, including the capacity for growth, reproduction, functional activity, and continual change preceding death. • The period between birth and death

How the life does originate on earth? • Biopoiesis, is the natural process of

How the life does originate on earth? • Biopoiesis, is the natural process of life arising from non living matter, such as simple organic compounds. • Chemical evolution describes chemical changes on the primitive Earth that gave rise to the first forms of life.

 • The first living things on Earth were prokaryotes similar to present day

• The first living things on Earth were prokaryotes similar to present day bacteria. • Prokaryote fossils have been found in 3. 4 million year old rock in the southern part of Africa, and in even older rocks in Australia.

For the origin of life, there are certain pre -requisits • i. Formation of

For the origin of life, there are certain pre -requisits • i. Formation of Earth • ii. Availability of inorganic raw materials • iii. Atmospheric conditions suitable for interaction of inorganic raw materials that give rise to organic molecules • iv. Conditions that help to interact organic molecule to produce the life.

Theories on the Origin of Life • Origin of life on earth is also

Theories on the Origin of Life • Origin of life on earth is also called as protobiogenesis. • It is clear the that life came into existence from non living substances of primordial earth about 3 billion year ago.

 • Origin of life can be explained by following theories : • 1.

• Origin of life can be explained by following theories : • 1. Theory of Special Creation 2. Theory of Spontaneous Generation or Theory of Abiogenesis/ Autobiogenesis 3. Theory of Eternity of Life 4. Theory of Biogenesis 5. Modern self-assembly theories

 • 1. Theory of Special Creation : • It is also termed as

• 1. Theory of Special Creation : • It is also termed as Mythological Approach. • This concept was put forward by Spanish monk Father Suarez. • According to this concept, all the living beings on earth were created by supernatural power “God” either once or at successive intervals.

 • According to Christianity (Bible), the whole world was created within six days:

• According to Christianity (Bible), the whole world was created within six days: • On 01 st day “God” made the Heaven & Earth. • On day 02 nd He separated the sky from the water. • On 03 rd day, He made the dry land plants

 • On 04 th day: Sun, Moon and Star were created. • On

• On 04 th day: Sun, Moon and Star were created. • On day 05 th Fishes were created. • And finally He made the land animals and Human beinggs on day 6 th.

 • According to Muslim faith; the first man was Adam Alehsalam (peace be

• According to Muslim faith; the first man was Adam Alehsalam (peace be upon him) and the first woman was Eve. • According to Hindu Mythology; the whole world was created by “God Brahma”. • Gods, Men and Devils developed from his Head. • Birds developed from his Chest.

 • Goat developed from his Mouth • Plants from his hairs of the

• Goat developed from his Mouth • Plants from his hairs of the body. • The first man was Manu and first woman was Shradha. • As this concept does not have any scientific evidences, it was not accepted by scientific community.

2. Theory of Spontaneous Generation or Theory of Abiogenesis/ Autobiogenesis: • Untile 17 th

2. Theory of Spontaneous Generation or Theory of Abiogenesis/ Autobiogenesis: • Untile 17 th century people believed in abiogenesis (Gk. A=without; bios=life; genesis=origin) or in spontaneous generation. • This concept was given by early Greek philosophers like Thales, Anaximander, Xenophanes, Plato, Aristotle etc. in 600 BC.

 • According to Epicurus (342 271 B. C. ) worms and numerous other

• According to Epicurus (342 271 B. C. ) worms and numerous other animals were generated from the soil or manure by the action of moisture and heat of the sun an air. • According to Aristotle (384 322 B. C. ) living creatures were born from live species, no doubt, but they also arise spontaneously like common worms, bee larvae, wasps, ticks, glow worms and various other insects are born from dew, rotten slime, manure, dry wood, sweets and meats etc.

 • Even two thousand years later, Van Helmont (1577 1644) described that organism

• Even two thousand years later, Van Helmont (1577 1644) described that organism like mice could arise from wheat grains or if dirty sweaty shirt is place in a pot for 21 days, it give rise to mice. • Thus on the basis of above examples it is presumed that life originated from non living things spontaneously, without any interruption.

 • 3. Theory of Eternity of Life: • This theory is a collection

• 3. Theory of Eternity of Life: • This theory is a collection of the views of various scientist like Richter (1865), Preyer (1880), Helmholtz (1884), Arrhenius (1908) Nobel Prize Winner of 1903 in chemistry, Hoyle (1908), Bondi (1952). • According to them, “life is immortal”.

 • They believed that living substance has existed together with non living matter

• They believed that living substance has existed together with non living matter from the very beginning and has changes its forms. It is never created. • Under this concept, views of Ritcher (1865), Helmholtz (1884) and Arrehenius (1908) are important to discuss: • Ritcher in 1865 proposed a theory named as Cosmozoic theory i. e. “protoplasm” reached the earth in the form of spores or germs (termed as cosmozoa or panspermia) from some unknown part of the universe with cosmic dust of meteorites.

 • Helmholtz in 1884 also speculated that protoplasm in some form reached the

• Helmholtz in 1884 also speculated that protoplasm in some form reached the earth along with falling meteorites (mass of matter that had fallen from the sky upon the earth). • Arrehenius in 1908 gave Cosmic Panspermia Theory. • He believed that organisms existed throughout the universe and their spores or germs could freely travel through the space from one star to the other. • As theory of eternity of life does not explain the origin of life it is also discarded.

 • 4. Theory of Biogenesis : • This theory was given by Italian

• 4. Theory of Biogenesis : • This theory was given by Italian Physicians Francisco Redi (1621 1697), Abbe Lazzaro Spallanzani and Louis Pasteur. • This theory could not explain the origin of first life on earth but able to explain continuity of life on the basis of some simple experiments. • They demonstrated that “Abiogenesis” could not exist and living organisms are always produced from pre-existing living forms by reproduction and not from non-living or lifeless matter.

 • Experiments conducted by Franceso Redi: • In 1668, Francesco Redi, an Italian

• Experiments conducted by Franceso Redi: • In 1668, Francesco Redi, an Italian scientist, designed a scientific experiment to test the spontaneous creation. • • He put meat into three separate jars. Jar 1 was left open Jar 2 was covered with netting Jar 3 was sealed from the outside

Jar with meat only Jar 2 covered with netting Jar 1 was left open

Jar with meat only Jar 2 covered with netting Jar 1 was left open Jar 3 sealed from the outside

 • He observed that • in Jar-1 which was Left open, flies were

• He observed that • in Jar-1 which was Left open, flies were laying eggs on the meat and after few days maggots (fly larvae) developed. • • In Jar-2 & 3 was covered with netting, and parchment therefore flies were unable to lay eggs on the meat as a result no maggots developed. • He concluded that life originated from pre existing life and not spontaneously.

 • In England, John Needham challenged Redi's findings by conducting another experiment in

• In England, John Needham challenged Redi's findings by conducting another experiment in which he placed a broth, or “gravy, ” into a bottle. • He heated the bottle so as to kill anything that could produce life inside, then sealed it. • Days later, he reported the presence of life in the broth and announced that life had been created from nonlife. • However, in actuality, he did not heat the broth enough to kill all the microbes.

 • Spallanzani's Experiment • Abbe Lazzaro Spallanzani, also an Italian scientist, reviewed both

• Spallanzani's Experiment • Abbe Lazzaro Spallanzani, also an Italian scientist, reviewed both Redi's and Needham's data and experimental design and concluded that perhaps Needham's heating of the bottle did not kill everything inside. • He constructed his own experiment by placing broth in four separate bottles. He boiled the broth in two bottles, then sealing one bottle and leaving the other open.

 • • Flask 1 was left open Flask 2 was sealed Flask 3

• • Flask 1 was left open Flask 2 was sealed Flask 3 was boiled and then left open Flask 4 was boiled and then sealed • • • Spallanzani's Experiment Step 1 Flask-1 Left Open Turned cloudy Microbes were found

 • • • Spallanzani's Experiment Step 2 Flask-2 Sealed Turned cloudy Microbes were

• • • Spallanzani's Experiment Step 2 Flask-2 Sealed Turned cloudy Microbes were found • • • Spallanzani's Experiment Step 3 Flask-3 Boiled and left open Turned cloudy Microbes were found

 • • • Spallanzani's Experiment Step 4 Flask-4 Boiled and sealed Did not

• • • Spallanzani's Experiment Step 4 Flask-4 Boiled and sealed Did not turn cloudy Microbes not found

 • He concluded that air is the medium through which microorganisms or germ

• He concluded that air is the medium through which microorganisms or germ travel and germinate when got favourable conditions. • So although his experiment was successful, a strong rebuttal (disproof, denial, refusal) blunted (unclear, dull) his claims.

 • Pasteur's Experiment • Louis Pasteur, French scientist, accepted the challenge to re

• Pasteur's Experiment • Louis Pasteur, French scientist, accepted the challenge to re create the experiment and leave the system open to air. • He subsequently designed several long necked S-curved shaped flask (swan neck) that were oriented downward so gravity would prevent access by airborne foreign materials and act as filter.

 • He placed a nutrient enriched (sugar + yeast powder) broth in one

• He placed a nutrient enriched (sugar + yeast powder) broth in one of the swan neck bottles, boiled the broth inside the bottle to kill the microorganism. • He observed that if the boiled broth in swan neck is kept for years no life appeared because the germ laden dust particles in the air were trapped by the curved shaped neck of the flask that served as filter.

 • He then broke off the top of the bottle, exposing it more

• He then broke off the top of the bottle, exposing it more directly to the air, and noted life forms (colonies of moulds and bacteria) in the broth within days. • He noted that as long as dust and other airborne particles were trapped in the S shaped neck of the bottle, no life was created until this obstacle was removed. • He reasoned that the contamination came from life forms in the air. Pasteur finally convinced the learned world that even if exposed to air, life did not arise from nonlife.

 • Pasteurization originally was the process of heating foodstuffs to kill harmful microorganisms

• Pasteurization originally was the process of heating foodstuffs to kill harmful microorganisms before human consumption. • Now ultraviolet light, Steam, Pressure and other methods are available to purify foods —in the name of Pasteur.

 • 5. Modern Theory of Origin of Life or Modern Hypothesis of Origin

• 5. Modern Theory of Origin of Life or Modern Hypothesis of Origin of Life : • Modern self assembly theory holds that protobiogenesis occurred due to inherent property of chemical molecules of self organization or self assembly. • This theory include conversion of micro molecule into macro molecules to cell like microsystem.

 • According to this theory • life originated on early earth through physico

• According to this theory • life originated on early earth through physico chemical processes of atoms which combined to form molecules • These molecules further reacted to produce inorganic and organic compounds. • Organic compounds interacted to produce all types of macromolecules which organized to form the first living system or cells.

 • Thus according to this theory ‘life’ originated upon our earth spontaneously from

• Thus according to this theory ‘life’ originated upon our earth spontaneously from non living matter. • First inorganic compounds and then organic compounds were formed in accordance with ever changing environmental conditions. • This is called chemical evolution which cannot occur under present environmental conditions upon earth. • Conditions suitable for origin of life existed only upon primitive earth.

 • Modem views regarding the origin of life can broadly be divided into

• Modem views regarding the origin of life can broadly be divided into chemical evolution and biological evolution: • A. Chemical Evolution (Chemogeny): • Process of chemical evolution can be divided into six steps. • Step I: • Step III: • Step IV: • Step VI: The Atomic phase Formation of inorganic molecules Synthesis of simple organic molecules (Monomers) Formation of Hot dilute Soup Synthesis of complex organic molecules Origin of coacervates

 • Step I. The Atomic Phase: • Early earth had innumerable atoms of

• Step I. The Atomic Phase: • Early earth had innumerable atoms of all those elements (e. g. , hydrogen, oxygen, carbon, nitrogen, sulphur, phosphorus, etc. ) which are essential for the formation of protoplasm. • Atoms were segregated in three concentric masses according to their weights.

 • Step II. Formation of Inorganic Molecules: • Free atoms combined to form

• Step II. Formation of Inorganic Molecules: • Free atoms combined to form inorganic molecules such as H 2 (Hydrogen), N 2(Nitrogen), H 20 (Water vapour), CH 4 (Methane), NH 3 (Ammonia), C 02 (Carbon dioxide) etc. • Hydrogen atoms were most numerous and most reactive in primitive atmosphere.

 • First, hydrogen atoms combined with all oxygen atoms to form water and

• First, hydrogen atoms combined with all oxygen atoms to form water and leaving no free oxygen. • Thus primitive atmosphere was reducing atmosphere (without free oxygen) unlike the present oxidising atmosphere (with free oxygen). • Hydrogen atoms also combined with nitrogen, forming ammonia (NH 3). So water and ammonia were probably the first molecules of primitive earth.

 • Step III. Formation (Monomers): of Simple Organic Molecules • The early inorganic

• Step III. Formation (Monomers): of Simple Organic Molecules • The early inorganic molecules interacted and produced simple organic molecules such as: • Simple sugars (e. g. , ribose, deoxyribose, glucose, etc. ), • Nitrogenous bases (e. g. , purines, pyrimidines), • Amino acids, glycerol, fatty acids, etc. • CH 4 (methane)+ C 02 + H 20 —> Sugars + Glycerol + Fatty Acids • CH 4 + HCN + NH 3 (amonia)+ H 20 —> Purines + Pyrimidines • CH 4 + NH 3 + C 02 + H 20 —> Amino Acids

 • Torrential rains must have fallen. • As the water rushed down, it

• Torrential rains must have fallen. • As the water rushed down, it must have dissolved away and carried with it salts and minerals, and ultimately accumulated in the form of oceans. • Thus ancient oceanic water contained large amounts of dissolved NH 3, CH 4, HCN, nitrides, carbides, various gases and elements.

 • Some external sources must have been acting on the mixture for reactions.

• Some external sources must have been acting on the mixture for reactions. • These external sources might be: • (i) solar radiations such as ultra violet light, X rays, etc. , • (ii) energy from electrical discharges like lightning, • (iii) high energy radiations are other sources of energies (probably unstable isotopes on the primitive earth). • There was no ozone layer in the atmosphere.

 • Experimental Evidence Molecular Evolution of Life: for Abiogenic • Stanley Miller and

• Experimental Evidence Molecular Evolution of Life: for Abiogenic • Stanley Miller and his supervisor Harold Urey at the University of Chicago, demonstrated it clearly that ultra violet radiation or electrical discharges or heat or a combination of these is passed through a mixture of of methane, ammonia, water (stream of water), and hydrogen can produce complex organic compounds. • The ratio of methane, ammonia and hydrogen in Miller’s experiment was 2: 1: 2.

 • He found a large number of simple organic compounds including some amino

• He found a large number of simple organic compounds including some amino acids such as alanine, glycine and aspartic acid. • Miller conducted the experiment to test the idea that organic molecules could be synthesized in a reducing environment. • Other substances, such as urea, hydrogen cyanide, lactic acid and acetic acid were also present.

 • In another experiment Miller circulated the mixture of the gases in the

• In another experiment Miller circulated the mixture of the gases in the same way but he did not pass the electric discharge. • He could not get the significant yield of the organic compounds. • Thus on the basis of these experiments it is considered that the essential ‘building blocks’ such as nucleotides, amino ac ids, etc. of living organisms could thus have formed on the primitive earth. • Later on many investigators have syn thesizeda great variety of organic compounds including purines, pyrimidine’s and simple sugars, etc.

 • Step IV: Formation of Hot Dilute Soup • A soup like broth

• Step IV: Formation of Hot Dilute Soup • A soup like broth of chemicals formed in oceans of the early earth from which living cells are believed to have appeared, was termed by J. B. Haldane (1920) as ‘prebiotic soup’ (also called ‘hot dilute soup’). Thus the stage was set for combination of various chemical elements. Once formed, the organic molecules accumulated in water be cause their degradation was extremely slow in the absence of any life or enzyme catalysts.

 • Step V. Formation of Complex Organic Molecules (Macromolecules): • A variety of

• Step V. Formation of Complex Organic Molecules (Macromolecules): • A variety of amino acids, fatty acids, hydrocarbons, purines and pyrimidine bases, simple sugars and other organic compounds accumulated in the ancient seas. • In the primeval atmosphere electrical discharge, lightning, solar energy, ATP and polyphosphates might have provided the source of energy for polymerisation reactions of organic synthesis.

 • Thus simple sugars could form polysaccharides and fatty acids could combine to

• Thus simple sugars could form polysaccharides and fatty acids could combine to produce fats. • Amino acids could form proteins, when other factors were involved. • So the small simple organic molecules combined to form large complex organic molecules

 • Sugar + Sugar ——— > Polysaccharides • Fatty Acides + Glycerol ———

• Sugar + Sugar ——— > Polysaccharides • Fatty Acides + Glycerol ——— > Fats • Aminoacids+ Aminoacids ———–> Proteins • Nitro. Bases + Pent. Sugars + Phosphates ———> Nucleotides • Nucleotides + Nucleotides ———–> Nucleic Acids

 • Which came First RNA or Protein? • The RNA first Hypothesis: •

• Which came First RNA or Protein? • The RNA first Hypothesis: • In the early 1980 s three scientists (Leslia orgel, Francis Crick and Carl Woese) independently proposed the RNA World as the first stage in the evolution of life in which RNA catalyzed all molecules necessary for survival and replication. • Thomas Ceck and Sidney Altman shared Nobel Prize in chemistry in 1989 because they discovered that RNA can be both a substrate and an enzyme.

 • If the first cells used RNA as their hereditary molecule, DNA evolved

• If the first cells used RNA as their hereditary molecule, DNA evolved from an RNA template. • DNA probably did not evolve as a hereditary molecule until RNA based life became enclosed in membrane. • Once, cells evolved DNA probably replaced RNA as the genetic code for most organisms.

 • The Protein First Hypothesis: • A number of authors (for example Sidney

• The Protein First Hypothesis: • A number of authors (for example Sidney Fox, 1978) claimed that a protein catalytic system must have developed before a nucleic acid replicative system. • Sidney Fox had shown that amino acids polymerized abiotically when exposed to dry heat to form proteinoids.

 • Cairns-Smith’s Hypothesis: • It was proposed by Graham Caims Smith, according to

• Cairns-Smith’s Hypothesis: • It was proposed by Graham Caims Smith, according to which both proteins and RNA originated at the same time. • Formation of Nucleoproteins: • The giant nucleoprotein molecules were formed by the union of nucleic acid and protein molecules. • These nucleoprotein particles were described as free living genes. Nucleoproteins

 • Step VI: Origin of Coacervates • The large organic molecules such as

• Step VI: Origin of Coacervates • The large organic molecules such as proteins, carbohydrates, fats, nucleic acids etc which were synthesized abiotically come together along with water due to the intermolecular action formed a large colloidal aggregate. • This water aggregate is called as microsphere or coacervates by Oparin. • Each coacervate was a collection macromolecules surrounded by a membrane. of

 • Each coacervate grew by absorbing molecules from their surrounding environment and divided

• Each coacervate grew by absorbing molecules from their surrounding environment and divided by budding. • The sun provided the energy for many chemical reactions, thus many chemical reactions including the decomposition of glucose took place inside the coacervates. • That gave rise the origin of first cell.

 • B. Biological Evolution (Biogeny): • The biological evolution completed in five steps:

• B. Biological Evolution (Biogeny): • The biological evolution completed in five steps: • (biogeny) is also

 • Step I: Formationofprimaryorganism (Ebiont or Protobionts or Protocells) • The formation of

• Step I: Formationofprimaryorganism (Ebiont or Protobionts or Protocells) • The formation of Coacervates (term given by Oparin’s) or Microspheres (term given by Sydney Fox) which possess some of the basic prerequisites of proto cells led to the organization of phospholipid films resulted in the formation of first living cell called as Protobiont/Ebiont.

 • The polarized film of phospholipids (e. g. lecithin and caphalin) organized in

• The polarized film of phospholipids (e. g. lecithin and caphalin) organized in such a way that they form a surface membrane around these coacervates. • Thus the coacervates were appeared as soap like bubbles having: • phospholipid surface membrane just like the plasma membrane of present day cells and • self replicating RNA and protein molecules.

 • At some later stage in the evolutionary process RNA transformed into DNA

• At some later stage in the evolutionary process RNA transformed into DNA which is genetic material. Thus the genetic code was stored in the DNA and RNA become associated with protein synthesis. • It is estimated that the Protocell (Ebiont) appeared some 3700 million years ago in the ancient oceans. • Thus life originated some 3700 million years ago.

 • Step II. Origin of Prokaryotes: • Prokaryotes were originated from proto cells

• Step II. Origin of Prokaryotes: • Prokaryotes were originated from proto cells about 3. 5 billion years ago in the sea. • The changes (i. e. mutation) in replication process of DNA caused structural changes in the proto cell which were transferred from one generation to the next. • Thus the first true living beings were prokaryotes. • They were evolved in an environment where no free oxygen was present and abundant organic molecules were present, therefore, they were anaerobic in respiration and nutritionally they were chemoheterotrophs.

 • Step III. Evolution of Modes of Nutrition: • (i) Origin of Autotrophism:

• Step III. Evolution of Modes of Nutrition: • (i) Origin of Autotrophism: • Autotrophism means the ability to synthesis their own food. • These organisms were capable of producing their own organic molecules either by chemosynthesis (i. e. Chemoautotrophism) or by photosynthesis (i. e. photoautotrophism). • The photoautotrophism is again of two types: • A. Anaerobic photoautotrophism • B. Aerobic photoautotrophism

 • (a) Origin of Chemoautotrophism: • As the environment over the earth become

• (a) Origin of Chemoautotrophism: • As the environment over the earth become cooler, the drop in temperature stopped synthesis of organic molecules in the sea water. • However, the chemoheterotrophs continued to utilized the available organic molecules present in their surrounding that lead to shortage of nutrients in the sea. • This shortage of food molecules leads to a sort of competition.

 • Therefore, some of the early chemoheterotrophs developed a new mode of nutrition

• Therefore, some of the early chemoheterotrophs developed a new mode of nutrition i. e chemoautotrophism before the organic material disappeared. • They were called as chemoautotrophs. • They developed the ability to synthesis the organic molecules from inorganic raw materials or less complex nutrients because they developed the enzymes of metabolic pathways.

 • They were the first autotrophs and were also anaerobic. • These anaerobic

• They were the first autotrophs and were also anaerobic. • These anaerobic chemoautotrophs were like present day anaerobic bacteria (e. g. sulphur bacteria, iron bacteria, nitrifying bacteria). • They released large amount of CO 2 in the atmosphere which provide way for the utilization of CO 2 in synthesizing organic compounds by trapping solar energy.

 • (b) Origin of Photoautotrophism: • During the course of evolution, pigment molecules

• (b) Origin of Photoautotrophism: • During the course of evolution, pigment molecules (Chlorophyll) were developed in certain proto cells. • The development of these chlorophyll molecule enabled certain proto cells to utilize light energy and synthesize carbohydrates. • This phenomenun is known as photoautotrophism and the organism were called as photoautotrophs.

 • These photoautotrophs were anaerobic in respriation. • The photoautotrophs are of two

• These photoautotrophs were anaerobic in respriation. • The photoautotrophs are of two types: • I. Anaerobic Photoautotrophs • II. Aerobic Photoautotrophs

 • I. Anaerobic Photoautotrophs • They did not use water as a hydrogen

• I. Anaerobic Photoautotrophs • They did not use water as a hydrogen source and did not release oxygen but simply cleaved hydrogen sulfide into ‘H’ and ‘S’. • Hydrogen was used in manufacturing of food and sulphur was released as a waste product. No oxygen was evolved and atmosphere remained reducing. • They were similar to present day sulphur bacteria in which hydrogen sulfide split into hydrogen and sulphur.

 • II. Aerobic Photoautotrophs • The Aerobic photoautotrophs used water as a source

• II. Aerobic Photoautotrophs • The Aerobic photoautotrophs used water as a source of hydrogen and carbon dioxide as source of carbon to synthesize carbohydrate in the presence of solar energy and release oxygen in the atmosphere. • The first aerobic photoautotrophs were similar to present day cyanobacteria (blue green algae) which had chlorophyll. They released oxygen in the atmosphere as the by product of photosynthesis.

 • Oxygen Revolution: • As the number of photoautotrophs increased, oxygen was released

• Oxygen Revolution: • As the number of photoautotrophs increased, oxygen was released in the sea and atmosphere. • This free oxygen than reacted with methane and ammonia present in the primitive atmosphere transforming them into carbon dioxide and free nitrogen. • CH 4 + 202 ———— > CO 2 + 2 H 2 O • 4 NH 3 + 3 O 2 ———– > 2 N 2 + 6 H 2 O

 • The oldest fossil belonging to blue green algae, named Archaeospheroides barbertonensis which

• The oldest fossil belonging to blue green algae, named Archaeospheroides barbertonensis which is 3. 2 billion years old. • Oxygen releasing prokaryotes first appeared at least 2. 5 billion years ago.

 • Step IV. Formation of Ozone Layer: • As oxygen accumulated in the

• Step IV. Formation of Ozone Layer: • As oxygen accumulated in the atmosphere, the ultra violetlight changed some of oxygen into Ozone. • 2 O 2 + O 2 ——— > 2 O 3 • The ozone formed a layer in the atmosphere, blocking the ultraviolet light and leaving the visible light as the main source of energy.

 • Step V. Origin of Eukaryotes: • The eukaryotes developed from primitive prokaryotic

• Step V. Origin of Eukaryotes: • The eukaryotes developed from primitive prokaryotic cells about 1. 5 billion years ago. There are two views regarding the origin of eukaryotes. • (i) Symbiotic Origin: • According to Margulis (1970 1981) of Boston University, some anaerobic predator host cells engulfed primitive aerobic bacteria but did not digest them.

 • These aerobic bacteria established themselves inside the host cells as symbionts. •

• These aerobic bacteria established themselves inside the host cells as symbionts. • Such predator host cells became the first eukaryotic cells. • The predator host cells that engulfed aerobic bacteria evolved into animal cells while those that captured both aerobic bacteria and blue green algae became eukaryotic plant cells. • The aerobic bacteria established them selves as mitochondria and blue green algae as chloroplasts.

 • (ii) Origin by Invagination: • According to this view cell organelles of

• (ii) Origin by Invagination: • According to this view cell organelles of eukaryotic cells might have originated by invagination of surface membrane of primitive prokaryotic cells.