Unit I: Theories, Evidences of Evolution and Extinction

UNIT-1 Theories, Evidence of Evolution & Extinction! Life’s Beginnings: Chemogeny, RNA world, Biogeny, Origin of photosynthesis, Evolution of eukaryotes. Historical review of evolutionary concept: Lamarckism, Darwinism, Neo-darwinism. Evidence of Evolution: Fossil record (types of fossils), transitional forms, geological time scale, evolution of horse, sources of variation: Heritable variation & their role in evolution. Background & mass extinction (causes & effects). Detailed example of K-T extinction. Life’s Beginnings Evolution is an orderly change in the form & behaviour of an organism with time or generation. It is a slow & time-taking process. Universe: 20 billion years ago Origin of earth: 4.5 billion years ago Origin of life: 3.5 billion years ago Theories of Origin of Life Life is a biochemical process contained in an organized system. The living system must have a genetic material (DNA or RNA) & must have the capacity of self-sustaining ability, metabolism, energy assimilation, development, irritability, growth & reproduction. Origin of life is called Protobiogenesis. Some of the ancient theories to explain the origin of life are given below: Theory of Special Creation According to this theory, life was created by supernatural power or God. God created life in 6 days & animals, including man, were created on the last day. According to Hindu mythology, Lord Brahma created different forms of life from various parts of his body. This theory is greatly supported by Father Saurez. Theory of Eternity of Life The pioneers of this theory believe that life is eternal & existed long before the creation of earth. This theory was put forward by Richter, Prethenius & Helmholtz. Theory of Panspermia (Cosmozoic Theory) Richter postulates that sports like living or seed existed on earth & on getting favourable conditions, they develop into existing life forms. The major criticism of this theory are: It does not explain how & from where spores reached the earth. It does not explain how life originated. Theory of Catastrophism This theory was supported by Georges Cuvier. According to him, after a gap of certain periods, the world undergoes a catastrophe (sudden calamity) that killed almost all living organisms. Then God created a new generation or new life forms from inorganic matter. Theory of Abiogenesis (Spontaneous Generation) According to this theory, evolution of life forms are developed from non-living matter spontaneously. Example: Meat left open generates flies & maggots. This shows spontaneous generation. Theory of Biogenesis This theory is based on the idea that life can be born only from preexisting life. This theory was proved by Redi, Spallanzani, & Louis Pasteur. Redi’s Experiment: He demonstrated that abiogenesis cannot exist. He placed two pieces of meat in 3 different jars. Jar A was uncovered, B was sealed & C was covered with a cloth. In a few days, he saw maggots appear on Jars A but not on B & C. Plenty of flies were seen sitting & laying their eggs on the cloth of Jar C. He concluded that flies arise from the egg laid by existing flies & maggots cannot appear spontaneously. Modern Scientific Hypothesis It was given by Oparin & J.B.S. Haldane. This theory was the now accepted one. This theory is also called the chemical theory or naturalistic theory. According to this theory, life originated in the primitive earth from abiotic chemical substances; therefore, this is called biochemical origin of life or molecular evolution of life. Oparin’s biochemical origin of life can broadly be discussed under 3 heads: i) Chemogeny: deals with synthesis of complex organic compounds from elements & molecules. ii) Biogeny: deals with the formation of protocells from molecules. iii) Cognogeny: deals with the evolution of different forms of living beings. (i) Chemogeny: It is described in the following headings: Atomic Phase At the time of origin of earth, its temperature was 5000°C to 6000°C & all the elements which form protoplasm like carbon, H, O, N, P, S were found as free atoms. As it cooled gradually, the heavy metals like iron, nickel, copper etc. formed the solid core of the future earth. Molecular Phase (Origin of molecules & simple inorganic compounds) Free atoms combined to form molecules & simple inorganic compounds, as the temperature of earth gradually reduced. Hydrogen atoms combined with all oxygen atoms to form water & forming no free atmosphere, i.e., oxidizing one. Hydrogen atoms also combined with nitrogen forming ammonia (NH3). So, water & ammonia were probably the first compounds (molecules) of primitive earth. Origin of Simple Organic Compounds (Monomers) The primitive atmosphere contained gases like CO2, CO, N2 etc. The nitrogen & carbon of the atmosphere combined with metallic atoms, forming nitrides & carbides. Water vapours & metallic carbides reacted to form the first organic compound, methane (CH4). Later on, hydrogen cyanide (HCN) was formed. Water vapours changed into rain due to cooling down of earth. Collection of water on earth formed primitive ocean which contained a large amount of methane, ammonia, HCN, carbides, nitrides, etc. Experimental Evidence for Abiogenic Molecular Evolution of Life (Spark Discharge Experiment) - Miller’s Experiment Stanley Miller & Harold Urey (1953) demonstrated it clearly; the ultra-violet radiation or electrical discharge or heat or a combination of these can produce complex organic compounds from a mixture of CH4, NH3, water vapour & hydrogen. Miller devised an apparatus simulating an early primitive atmosphere where synthesis of macromolecules must have started. In his experiment, he took the mixture of methane, ammonia & hydrogen in ratio 2:1:2 & water vapours in a tight apparatus & passed electrical discharge from electrodes. He passed the mixture through a condenser to cool down it. After about 18 days, this fluid was collected & analyzed. This dark red fluid was found to contain simple amino acids (glycine & alanine) & simple organic acids. Other substances such as urea, hydrogen cyanide, lactic acid & acetic acid were also present. Thus, he postulated that simple molecules could have given rise to complex life-bearing molecules as they are formed in the apparatus. Origin of Complex Organic Compounds (Polymers) Polymerization of amino acids, glycerol, fatty acids & sugars in the primitive ocean formed macromolecules of protein, fats & carbohydrates. These molecules formed main components of protoplasm, hence all the possibility of the origin of life in the primitive ocean could be established. RNA World The RNA world hypothesis: According to this hypothesis, RNA acts as genetic material to carry genetic information & catalyze chemical reactions in primitive cells. According to this view, RNA world existed on earth before modern cells arrived. Also, RNA molecules can catalyze its own synthesis. The unique potential of RNA to act as information carrier & catalyst forms the basis of RNA hypothesis. It has been suggested that the 1st molecules to possess both catalytic activity & information storage capacity may have been a polymer that resembles RNA but are chemically simpler. But, there is no evidence of this molecule in present-day cells. Using this pre-RNA polymers RNA was 1st synthesized. Once RNA molecules had been produced, they took over the function originally carried out by pre-RNA polymers. Pre-RNA → RNA → DNA The self-replicating RNA molecule evolved through natural selection & into a highly elaborated structure. Gradually RNA molecules were shielded within a closed membrane & they began to evolve as a carrier of genetic information. Later on, DNA took over as genetic material & proteins became the major catalyst & structural components of cells. Atoms ↓ Molecules ↓ Simple organic molecules ↓ Complex organic molecules (NA, AA, polysaccharide) ↓ Protobionts ↓ Coacervates ↓ RNA (genetic material) ↓ Pre-cell/Eobiont ↓ Prokaryotic-anaerobic-chemo-heterotrophs ↓ Chemo-autotrophs (S) ↓ Photo-autotroph (free oxygen) → Chlorophyll developed ↓ Cyanobacteria ↓ Eukaryotes (ii) Biogeny The complex organic compound synthesized abiogenetically on the primitive earth later tends to accumulate & formed large colloidal cell aggregates called protobionts. This cell aggregate can maintain an internal environment & now formed a stable & mobile aggregate called coacervates. They resemble living cells in basic properties like metabolism & growth but lack the complexity of living cells. They grew in size & multiplied to form the hypothetical primordial life form called eobiont. Earliest cells were anaerobic prokaryotic & chemo-heterotrophs (derived as a mode of nutrition from existing organic molecules born in oceans); with these organic materials decreased, synthesizing their own food. Facing the problem of increasing deficiency of inorganic compounds, some chemo-autotrophs & photo-autotrophs developed porphyrins, bacteria, chlorophyll & started the process of photosynthesis. This led to the evolution of anaerobic prokaryotic photo-autotrophs (3000-3500 mya). Later they developed chlorophyll & started using oxygen from the water to form aerobic photo-autotrophs.