→Theories,Evidences of Evolution & Extinction.
Life Beginnings:
Introduction:-
Origin of life on earth is a great mystery of all the time with respect to the scientific enquiry.Though the spiritual ,religious point of view it is quite interesting;Greek mythology,Indian Vedas and Roman philosophies describes a lot about the origin of life on the history of earth.
Different thinker and philosopher put forwards their theory to describe the beginnings of life,some of which are mentioned below.
1.Theory of special creation:
Spanish monk,Father Suarez(1548-1617B.C) propounded that the world with all its complex organisms has been created by God in six natural day.From the day of creation these organism are existing unchanged.
2.Greek Theories :
Several theories of historical importance were described by Greek philosophers before the birth of ‘Jesus Christ’ .Some popular once are given below:-
i) Thales:
Thales propounded the theory of aquatic or marine origin of life.He consider oceanic water to be the mother from which all living forms originated.
ii) Anaximander :
In sixth century he explained the origin of universe on a rational basis rather than by myths.
He proposed that all living beings have arisen from a primordial fluid,or slime to which they ultimately return.
The plants and animals were formed as this mud dried.It was presumed that man himself was first shaped like fish and lived in water.
iii) Anaximenes:
He described that air to be the source of life .He proposed theory of abiogenesis.
iv) Epedocles:
He believed in spontaneous generation and proposed that evolution of animals was by a series of attempts by nature to produce more perfect forms.
a) Higher forms of life evolved gradually .
b) Imperfect forms ( i.e less adapted forms )
where gradually replaced by perfect forms.(i.e the better adapted form).
c) Plant life came first and animal life developed later.
d) Perfect forms were produced by the extinction of imperfect forms.
where gradually replaced by perfect forms.(i.e the better adapted form).
c) Plant life came first and animal life developed later.
d) Perfect forms were produced by the extinction of imperfect forms.
3.Pre-modern Theories
Evolutionist of mediaeval age were :-
i) Francis Bacon:
Francis Bacon reviewed Aristotalian ideas and presumed that new species could arise from the old species by degenerative process caused due to mutability in the species.
ii) Jan Swammerdum:
The Dutch scientist Jan Swammerdum proposed the preformation theory which says that the ova contain miniature of the adult in preformed state.The act of fertilization (i.e union with sperm) provides initiation for growth and the miniature grows into adult.
iii) Maupertius:
Maupertius was the first to propose a general theory on evolution.He proposed that hereditary material was particulate and transmitted through both material and paternal sides of the family.He also thought that hereditary character may changed by environment.
iv) Carolus Linnaeus:
Linnaeus is known as the ‘father of taxonomy’.He believed in special creation.He presumed that species are created by God and are immutable and fixed entities.
v) Erasmus Darwin:
Erasmus Darwin is the grand father of Charles Darwin, who gave the first clear statement of the inheritance of the acquired characteristics, according to which the effects produced by the environment on the organisms are transmitted to the offsprings.
vi) Modern Theories:
The contribution made by Lamarck,Darwin,Cuvier,Weisman and Huxley etc are great importance,since this provoked the real scientific thinking of evolutionary process and their theories are still being helpful though in a some what modified form.
Chemogeny :
As the name suggest chemogeny means the chemical genesis or chemical evolution (synthesis) of molecules which act as a precursor for life on earth.Though it is a early transition and further it leads to the biogeny and ultimately the biochemogeny.RNA world concept has a significant role in describing the origin of life on earth but we will discuss it at the end of this discussion.
Before the origin of life on earth we all known that the origin of earth itself is a big controversy and different hypothesis has been proposed to explain the origin.The early atmosphere of the earth is some how different from present one; and it is strongly reducing in nature .Primitive earth atmosphere contain hydrogen, methane,ammonia and water vapours.
A.I Oparin suggested that from these precursor molecules more complex organic molecules were formed gradually under the influence of electric charges( lightening) ultra violet rays or corpuscular radiation.
Let us discuss the chemical origin of essential organic compounds phase wise,along with the formation of essential biomolecules responsible for origin of life.
Phase-1 Origin of organic compounds
In this phase when temperature of the earth surface cooled down varities of free radicals are combined first to form the hydrocarbon in the following manner;
CH + CH →HC ≡ CH Acetylene
ċH2 + ċH2→ H2C = CH2 Ethylene
ċH2 + ċH2→CH4 + C Methane and carbon
2CH4→HC ≡ CH + 3H2 Acetylene
Where metal carbide reacts with steam to form unsaturated as well as saturated hydrocarbon (having large open chain hydrocarbon).
CaC2 + H20→HC ≡ CH + CaO
Acetylene
3Fe4C5 + 16H2O→C15H32 + 4Fe3O4
Due to the reaction of super heated steam with unsaturated as well as saturated hydrocarbaon it produces oxy- and hydroxyl derivatives like ketone,aldehyde and acid respectively.
O
ǁ
HC≡CH +H2O→CH3 ̶ C – H
CO combine with water to produce formic acid.
CO + H2O→HCOOH
Though this was a reducing environment the above hydroxy derivatives produce the large chain complex compound called glucose and fructose whose condensation led to the formation of sugar and starch.
Like this other macromolecules like fatty acid and glycerol has formed from the polymerization of aldehyde and ketone.Fatty acid and glycerol again combined to form fats, as we aware that the fats are the constituent of cell membrane so it can also be consider that these complex molecules make the origin of life more reliable.
Then the main parts i.e the protein constituent should be formed to make the life some what existable.Therefore combination of hydroxy compounds ,ammonia and water form the building block of protein i.e amino acid which on linked by peptide bonds to give proteins molecules.These process of formation of molecules was happened in large amount,because today’s atmospheric condition is not suitable for this phenomena and its required a lot of energy to progress.This process of formation of molecules was happened in earlier because today’s atmospheric condition is not suitable for this phenomena and it required a lot of energy to progress.This process is generally occurred in ocean only, though it is the main source of all organic compounds in solution form earlier and scientist J.B.S Haldane named it as ‘Hot Dilute Soup’ from which life arose,later this hypothesis were tested by Stanley Miller and Harold Urey in laboratory condition.Finally from this above description we can say that life first arose from water i.e sea water.
Phase – 2 Origin of complex organic macromolecules
By the formation of amino acids,proteins starch,glucose the complexity arised largely along with the simpler substance of hot dilute soup condense to form nucleioside,nucleiotide,purine and pyrimidine too.Where some proteins act as enzyme which trigger the reaction and we know that the proteins results the life more existable.The preassuming model of life existence was protocell model which will be discuss later.
Phase – 3 The formation of master piece i.e nucleic acid
Nucleic acid formation is the crucialu steps in origin of life; generally it is done by the polymerization of nucleotide. Earlier it is necessary that to produce a self replicating molecules having the large number of nucleotides precursor. In addition to this reading of genetic code in self replicating molecules and translation of molecule into amino acid is very much essential. It was suggested that the RNA was the first genetic material available for carry the genetic information.
Phase – 4 Formation of nucleoproteins
Though RNA plays a main role in proteins synthesis ; some of them act as a enzyme like catalyst and called as ribozymes. This leads to more production of the RNA molecules; where proteinoids ( a mixture of several amino acids under anhydrous condition and at lower temperature 130 ̊ – 180 ̊c leads to polymerization of amino acid polypeptide chain and produce protein like molecules : Organic Evolution by V.B Rastogi ) molecules combined with nucleic acid produce nucleoproteins having the ribozymes activities as most of the RNA (ribozymes), and that nucleoproteins called protoribosomes, some of them seems like present day virus called as protovirus though they have the free genomic material (RNA) and protein coats.
Biogeny :
Biogeny refers to the biological genesis or biological origin (synthesis) of life. This hypothesis focused on the biological origin of life on earth i.e the life forms began with precursor life form, and its subsequent concept of chemogeny.
The formation of first living cell is triggered by the aggregation of many more organic molecules including more than one nucleoprotein,proteins and other macromolecules and finally formed the large colloidal particles called ̔coacervates̓ we will study it later on at ̔Protocell Model ̓
The polarized phospholipid ( having +ve charge on phosphate head and –ve charge on fatty acid tail ) surrounded the inner nucleoprotein constituent. As a result of which it seems like present day cell.This assemblage of nucleoproteins and its protection phospholipid film separate the inner particles from outside environment and favour the inner catalyst to carried out the catalytic reaction requiring the minimum energy which was earlier not possible because of freeness of colloidal particles. This coacervates having the nucleoprotein and phospholipid membrane considered as the first living cell. As due course of evolutionary process and changing environment along with stability of the primitive cell it replace the RNA to DNA as the genetic material and RNA for other catalytic as well as protein synthesis process.
Origin of Photosynthesis :
As the early atmosphere is reducing one , there was no free oxygen in the atmosphere . Probably the organism which was present is simply the anaerobes ones.Therfore we can classified the evolutionary origin of photosynthetic microbes according to their modes of nutrition.
1. Chemoheterotrophs- These types of organism is purely anaerobes in nature and they were derive their nutrient from the macromolecules present in the sea therefore called as chemoheterotrophs
2. Chemoautotrophs-These types of organism utilized the less complex nutrients and derive their energy using the enzymatic metabolic pathway by the process of fermentation.There were also anaerobes and using the evolved co2 from fermentation process by trapping the solar energy synthesizes the orgaqnic compounds.
3. Anaerobic photoautotrophs- These type of protocell develop thye chlorophyll molecule; utilizes the light energy and produce carbohydrates molecules. These are the first photoautotrophs an anaerobics in nature.
4. Aerobic photoautotrophs- These aerobic photoautotrophs is like cyanobacteria as they utilize the oxygen (not free one )and carbon which in the form of water and carbondioxide in photosynthesis and produce oxygen, therfore they were called as ̔ oxygen producing photosynthesizers ̓ . (Organic Evolution; V.B Rastogi, 2003,Medtech publication).
This increase in photoautotrophs increase the quantity of oxygen in atmosphere which reacts with methane and ammonia to form CO2and N2.
CH4 + 2O2→ CO2 + 2H2O
NH4 + 3O2 → 2N2 + 6H2
Finally the reducing atmosphere changes to oxidizing ones with huze amount of free oxygen and this leads to the evolution of first eukaryotic cell.
Evolution of Eukaryotes :
Probably the protocell which were formed is like prokaryotic and archaebacteria type because this archia is the only one who can survive at extreme condition . From this archae protocell it is being assumed that the eukaryotes has developed. The evolution of eukaryotes can be interprete by two theories i.e
1. Endosymbiotic Theory
1. Endosymbiotic Theory
2.Invagination Theory
1. Endosymbiotic Theory : This theory explain that a symbiotic relationship between large and small result in evolution of eukaryotes. Here when large cells engulfed small cells large cells couldn’t digest the small and subsequently the inner small cell remain lived within the large cell and evolved into organelles.The organelles which evolved have the capacity to break the inner metabolites of large cell to produce energy for themselves and provides energy to large cell as well.Where this type of organelles can be considered as mitochondria of eukaryotes and they multiply within the large cell and evolved to other organelles to utilize sunlight and make food and considered to be chloroplast of eukaryotic cells.In this manner precursor job of mitochondria and chloroplast were done by these organelle and which pave a path to evolution of eukaryotes.
2. Invagination Theory : This theory explain that the eukaryotes might have developed by the invagination of prokaryotic cell membrane. It is little bit same as that of endosymbiotic theory where the large cells engulf the small ones by invagination of cell membrane,here multiple invagination occurs and the small bacteria converted to mitochondria and invaginating portions act as the endoplasmic reticulum and some other bacteria evolved into chloroplast too.
RNA World Concept :
RNA world concept was first proposed by Alexander Rich in 1962 and the term coined by Walter Gilbert in 1986 ( en.m.wikipedia.org ).This hypothesis suggests that the life on earth originated from a self replicating molecules i.e RNA .Here the thought provoked about the RNA molecule is that the RNA molecule act as a versatile molecule i.e it can form a copy by itself ,store the instruction,act as catalyst like ribozymes though ribosomes are primary composed of RNA,along with carry information for protein synthesis.
RNA world concept has a worth noting supportive role in chemogeny hypothesis though it relates to the chemical origin of life on earth.As an enzyme RNA has a great role in protein synthesis though it act as ribosomes which whose subunits combined to proceed for for translation and other catalytic action like peptide bond forming by rRNA ( this translation process you will study in later on ),RNA polymerase to synthesize new short RNA strand.The evidence of fossil records about DNA has some logical criteria to have that the RNA was the primary and precursor molecule to have life on earth.
In comparision to DNA ,RNA molecules is very much similar to DNA,but having two striking differences one is RNA have ribose sugar instead of deoxyribose and second is it has uracil nitrogenous base instead of thymine,but pertaining to stability of DNA is more stable than RNA because the 2 ̔ positioned hydroxyl group in ribose sugar is more sensitive to hydrolysis ,which cause mutation and the information can not be transmit properly so it is necessary to have stable molecule.Due course of time the DNA molecules replaces RNA and act as a genetic material,but the role and job of RNA molecule can not be ignored though it was earlier the versatile one presently also the versatile one and remain as more important forever as long as life going on earth.
Protocell Model :
In earlier discussion we came across the formation of proteins in the hot sea water.By conveying a situation where several aminoacid mix-up and heats under anhydrous condition and at lower temperature (130 ̊ – 180 ̊c) leads to polymerization and form protein like molecules called ̔Thermal Proteinoids ̓ ( V.B Rastogi,Organic Evolution …).
After that several proteinoids aggregate to form microsphere and resemble as spherical bacteria.It is formed by aggregation and with addition of water to form proteinoids,it is more stable form and having cell membrane like structure and experiments shows that it carried out the enzymatic activities inside it and divided by either binary fission or by budding.The aggregation is some times attract each other and some times repel each other.Therfore when solution of oppositely charged colloids are mixed coacervates droplet formed and the organic polymer produced inside it accumulate other materials from environment and it seems poor protocell model which define the early cell in origin of life on earth.These coacervates droplets may breakdown due to unstability in it and contain some bioproteins ( V.B Rastogi, Organic Evolution,Medtech publication)
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