Title SP-3_Ch-15_Evolution.pdf ✅

Chapter Contents Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Origin of Life Evolution of Life Forms – A Theory What are the Evidences for Evolution? What is Adaptive Radiation? Biological Evolution Mechanism of Evolution Hardy-Weinberg Principle Speciation and Isolation A Brief Account of Evolution All the life forms like plants, animals or microorganisms that we see around comprise the biodiversity on earth. All these life forms have a long history of changes and the study of history of life forms on earth is called Evolutionary Biology. Evolution means the act of unfolding or unrolling. In simple terms, evolution is ‘change’ from one form to another. ORIGIN OF LIFE Origin of Universe Big Bang Theory : Origin of universe is the most important phenomenon in the origin of life. The Big Bang Theory attempts to explain the origin of the universe. This theory proposes that the universe is very old and its origin took place almost 20 billion (20 × 109 ) years ago. It states that a single huge explosion (i.e., Big Bang) took place in space and was so powerful that it is unimaginable in physical terms. Due to this explosion, the universe expanded in volume and the temperature of space came down. Slowly Hydrogen and Helium gases formed, which condensed due to the gravitational forces present in the surroundings. All this resulted in the formation of billions of galaxies about which we have already discussed in previous classes. In the solar system of milky way galaxy, Earth is supposed to have formed about 4.5 billion years back. The gaseous envelope or atmosphere was not present on early earth. Due to volcanic eruptions, the molten mass was released which slowly covered the surface of earth. From this molten mass, various gases released into the surroundings like methane (CH4 ), ammonia (NH3 ), carbon dioxide (CO2 ), water vapour etc. The ultraviolet (UV) Chapter 15 Evolution
72 Evolution NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 radiations present in the sunlight acted upon water and broke it into hydrogen and oxygen. The lighter H2 escaped. Oxygen readily combined with other gases like NH3 and CH4 to form again H2 O again long with CO2 and other gases. Therefore, free oxygen (like today’s environment) was not present on early earth. Therefore, the early atmosphere was reducing unlike today’s oxidising atmosphere. It was only present in the form of compounds like H2 O, CO2 , etc. Oxygen also participated in the formation of ozone (O3 ) molecules due to action of UV rays on it. These O3 molecules clustered together to form the ozone layer which acts as a shield to save our present atmosphere from harmful UV rays. The water vapour condensed to form water drops which fell as rain and slowly filled the depressions present on the surface of earth. This is supposed to be the origin of large oceans seen today. But during all these changes, life took approx. 500 million (500 × 106 ) years to appear or we can say life appeared 500 million years after the formation of earth or almost 4 billion (4 × 109 ) years ago (as earth formed 4.5 billion years ago). Theories on Origin of Life : 1. Theory of Special Creation states that life was created by supernatural power in a form which has not undergone any change. 2. Theory of Catastrophism was given by Cuvier, according to which after a gap of certain period (called age), the world undergoes a catastrophe (sudden calamity) which kills almost all the living organisms and then God creates a new generation or new life from inorganic matter. 3. Theory of Spontaneous Generation : The supporters of this theory stated that the life did not come from outerspace. Rather there was a natural ‘spontaneous’ generation of the living organisms from non- living matter present on the earth.  For example, they believed that the decayed and rotting material like mud, straw, etc. had the capacity to give rise to organisms like frogs, flies, etc.  However this concept was experimentally disproved by many scientists. One of them was Louis Pasteur. Louis Pasteur by careful experimentation demonstrated that life comes only from pre- existing life. He showed that in pre-sterilised flasks, life did not come from killed yeast while in another flask open to air, new living organisms arose from killed yeast. 4. Cosmozoic Theory (Did Life Come from Outer Space?) : Some scientists believe that life came on earth from other stars or planets. They were the early Greek thinkers (Richter Helmholtz) who said that the living organisms exist throughout the universe. Like we have our own earth, many other planets also have their own living organisms and life was transferred from one planet to other in the form of some small units of life called spores or seeds or sperms. The transfer of spores was termed Panspermia (Suggested by Arrhenius 1908). Panspermia states that life is present througout the universe (pan means complete). Transfer of life from one planet to another is called cosmozoic theory. 5. Theory of Biogenesis (i.e. life from life, omnis vivum ex. vivo) was proved by Redi, Spallanzani and Pasteur independently. They disproved (refuted) the theory of spontaneous generation (abiogenesis). Francesco Redi (1668) proved that flies could not arise from putrefying meat without their eggs. Spallanzani (1767) demonstrated that putrefaction of meat is due to microbes in the air and it can be prevented by boiling and sealing the meat in air tight containers. Pasteur gave a definite proof of life arising from pre-existing life using microbes and sterilization methods. He performed "swan neck flask" experiment. 6. Theory of Chemical Evolution or Oparin-Haldane Hypothesis : Oparin of Russia and Haldane of England proposed that the first form of life could have come from pre-existing non-living molecules (e.g. RNA, protein etc) and formation of life was preceded by chemical evolution i.e., formation of diverse organic molecules from inorganic constituents.
NEET Evolution 73 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Cloud Rain Lightning Sun Radiations High Temperature C, H, O, N, S, P Simple sugars, amino acids, glycerol, fatty acids, nitrogenous bases, etc. Complex sugars, Proteins, Fats, Nucleic acids (RNA, DNA) H , H O, NH , CO 2 2 3 2 CH , HCN etc. 4 Life forms Free atoms Simple inorganic molecules and compounds First simple organic compounds Simple organic compounds Complex organic compounds First Life Fig.: Process of chemical origin of life (as proposed by Oparin and Haldane) The formation of diverse organic molecules from inorganic constituents was termed chemical evolution. Oparin and Haldane proposed that energy and stimulus were provided to these reactions (i.e., chemical reactions) by the conditions present on the early earth like (1) High temperature (2) Volcanic eruptions (storms) (3) Reducing atmosphere containing CH4, NH3, etc. Early atmosphere was reducing unlike the present day oxidising atmosphere because it had no free oxygen. Experimental Evidence for Chemical Evolution : Based on the hypothesis proposed by Oparin and Haldane, S.L. Miller provided an experimental evidence of chemical evolution in 1953 in a laboratory set-up. S.L. Miller, an American Scientist, created the similar conditions as described by Oparin and Haldane on a laboratory scale. Miller sealed in a spark chamber a mixture of water (H2O), methane (CH4), ammonia (NH3) and hydrogen gas (H2). He made arrangements for insertion of two electrodes to provide electrical energy (simulation of lightning) to the spark chamber, which contained CH4, NH3, H2 were in 2 : 1 : 2 ratio and water vapour at 800oC. Electric sparks of 75,000 volt were provided to the mixture. The spark chamber was connected to another flask with an arrangement for boiling water (provision for evaporation). The other end of the spark chamber was connected to a trap by a tube that passed through a condenser (an arrangement for condensation and collection of aqueous solution, equivalent to rain and Haldane’s soup). The trap, in turn, was connected with the flask for boiling water (arrangement for circulation). The control experiment/ apparatus comprised similar arrangement except that it was devoid of energy source. After eighteen days, significant amount of the simple organic compounds (monomers), such as amino acids and peptide chains began to appear in the aqueous sample of the experimental set. Amino acids found were alanine, glycine and aspartic acid. Therefore, the obvious inference was that abiotic synthesis of organic monomers occurred in the simulated experimental conditions. By analogy, such synthesis could have occurred in the primitive atmospheric conditions on Earth. Several other scientists also conducted similar experiments by slightly changing the starting materials and sources of
74 Evolution NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 energy like UV radiations and became successful in generating sugars like ribose, hexose, nitrogen bases like adenine, guanine, pigments and fats. With hydrogen cyanide (HCN), even adenine and other nitrogen bases were produced in experiments run by other scientists.. Electrodes Spark discharge Water out Water in Condenser Water droplets Water containing organic compounds Liquid water in trap Boiling water To vacuum pump Gases CH NH H O H 4 3 2 2 Fig.: Diagrammatic representation of Miller’s experiment The meteorites strike the earth and their components fall on it. When the composition of meteorites was analysed, they were found to contain the similar compounds as produced in the above experiments. It indicates that similar chemical reactions were also taking place somewhere else in space. With experimental demonstration and evidence of meteorites, the concept of chemical evolution, i.e., origin of complex organic molecules from simple inorganic molecules was more or less accepted but the 2nd part of the conjectured story, i.e., origin of life forms from these organic compounds was still not proved. Abiotic synthesis of biomolecules is studied under following headings : 1. Chemogeny : Synthesis of organic molecules by chemical reactions. 2. Biogeny : Formation of self replicating biomolecules in broth (primordial hot soup or warm little pond). 3. Cognogeny : Evolution of various forms of life or diversification of existing groups. Enclosing the Prebiotic Systems The experiments of Miller and other scientists demonstrate that prebiotic molecules could have formed under the conditions which most likely existed on early Earth. Still, the formation of prebiotic soup of small molecules does not necessarily lead to the origin of life. For origin of life, atleast three conditions needed to have been fulfilled: 1. There must have been a supply of self-replicators i.e., self-producing molecules. 2. Copying of these replicators must have been subject to error via mutation. 3. The system of replicators must have required a perpetual supply of free energy and partial isolation from the general environment. The high temperature prevailing in early Earth would have easily fulfilled the second condition, that is, the requirement of mutation. The thermal motion would have continually altered the prebiotic molecules. The third condition, partial isolation, could have been attained by aggregates of artificially produced prebiotic molecules. These aggregates called protobionts could separate combinations of molecules from the surroundings; maintain an internal environment but are unable to reproduce. Two important protobionts were coacervates and microspheres. Oparin (1924) observed that if a mixture of a large protein and a polysaccharide was shaken, coacervates were formed. Their interiors, which are primarily protein and polysaccharide, with