Evolution - Origin Of Life| NCERT concept | Neet Concept | Biology

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ORIGIN OF LIFE

• Life is an inherent capacity possessed by an organism to maintain and reproduce itself . The origin of life is considered as an unique event in the history of universe . . . .
• Most accepted theory to explain the origin of universe is the Big - Bang Theory , proposed by Abbe Lemaitre ( 1931 ) which states that the universe had an explosive beginning . Universe originated about 20 billion years ago by a thermonuclear explosion or big bang .
• To explain the origin of life , many theories were put forward .
• Theory of spontaneous generation or autogenesis or abiogenesis believe that under certain conditions non - living substances gave rise to living beings spontaneously . This concept was supported by Thales , Anaximander , Aristotle , Plato , etc.
• Theory of Panspermia was proposed by Richter in 1865 , according to which life came from outside in the form of spores .
• Oparin - Haldane theory of origin of life was put forward by A.I. Oparin , a Russian biochemist and J.B.S. Haldane , an English biologist . According to this theory , life originated on earth spontaneously from non - living matter , first inorganic compounds and then organic compounds were formed in accordance with everchanging environmental conditions .
• Oparin - Haldane theory is also called chemical theory or naturalistic theory .
• Haldane gave the term hot dilute soup or prebiotic soup for broth of chemicals formed in oceans of the early earth from which living cells are believed to have appeared .
• The Oparin - Haldane theory was experimentally supported by Stanley Miller ( 1953 ) a student of Harold Urey . In Stanley Miller's experiment , electric discharge was created in a closed flask containing mixture of four gases - CH₁ , H₂ , NH3 and water vapour at 800 ° C temperature . The mixture was passed through a condenser and gases were circulated for about one week resulting in the formation of amino acids , such as alanine , glycine and aspartic acid .
• The synthesis of carbohydrates , fats , amino acids and other complex organic molecules from simple organic molecules probably occurred in sea in primaeval atmosphere . Electric discharge , ATP , solar energy , lightning might have provided the source of energy .
• Biological evolution is defined as the process through which the characteristics of organisms change over successive generation of genetic variation and natural selection .
• Oparin gave the term coacervates for non - living structures , from which first living cells were formed . Coacervates are surrounded by a thin film of water , lack a definite membrane but are able to grow and divide in favourable environment .
• Proteinoids are protein like structures formed by dehydration synthesis of amino acid . Sydney Fox demonstrated that microspheres could be formed from proteinoids placed in boiling water .
• The first living organism which originated from organic molecules in oxygen free atmosphere ( reducing atmosphere ) were anaerobes . They were capable of anaerobic respiration and lived in the absence of oxygen . They depended on the existing organic molecules for their nutrition and thus they were heterotrophs .
• Some of these prokaryotic heterotrophs might have evolved into autotrophs when the supply of existing organic molecules was exhausted .
• The eukaryotes developed from primitive prokaryotic cells about 1.5 billion years ago . There are two theories regarding origin of eukaryotes .
• According to symbiotic theory , some anaerobic predator host cells engulfed primitive aerobic bacteria but did not digest them . These aerobic bacteria became symbionts . Such predator host cells evolved into first eukaryotic cells .
• According to origin by invagination theory , cell organelles of eukaryotic cells might have originated by invagination of surface membrane of primitive eukaryotic cells .
• Primitive eukaryotes led to the evolution of protists , plants , fungi and animals . Life was present on earth about 3.9 billion years ago .

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MANISH MEVADA

M.Sc, M.Phil, B.Ed

GUJARAT BIOLOGY NEET

NEET MATERIAL IN GUJARATI 

KNOWLEDGE ON THE WAY.