Origin of Life Theories PDF

1 Unit V Origin of life - Theories of organic evolution Evolution Evolution mainly deals with the origin of life on earth. The conditions and the forms of life on earth were entirely different from what we see today. Everything evolved from one form to another for a better chance...

1 Unit V Origin of life - Theories of organic evolution Evolution Evolution mainly deals with the origin of life on earth. The conditions and the forms of life on earth were entirely different from what we see today. Everything evolved from one form to another for a better chance of survival. The Origin of the Universe Origin of life begins with the origin of the universe. The universe, an old vast and empty space comprising of galaxies, originated around 20 billion years ago. There was nothing but blackness filled with gas and dust. The Big Bang Theory is the most accepted theory regarding the origin of the planet earth and the existence of different life forms on it. According to this theory, the universe is a result of a massive explosion which occurred 20 billion years ago. Whether it is a hypothesis or a fact, a new universe was formed. The atmospheric condition after the explosion became more stable. The temperature reduced and gasses like hydrogen and helium formed which led to the formation of galaxies of today. Later, after 10 billion years, the earth was formed which was covered by water vapour, methane, carbon dioxide, and ammonia. There was no atmosphere but only gasses and moisture. The powerful rays of the sun stimulated and hastened the evolution. By making and breaking of bonds between gas molecules, earth came out with a new face. After millions of years i.e., once earth atmosphere was stabled, the first life on earth came into existence (around 4 million years ago). There began the story of the origin of life on earth. The Origin of Life There were lots of hypothesis regarding the origin of life on earth. Certain experts suggested that life came from outer space as spores while another group explained that life came from a non-cellular component such as decaying matters like mud. The latter theory was known as the theory of spontaneous generation, which was discarded later. In the year 1953, Oparin and Haldane suggested that life originated from non-living organic molecules like proteins and DNA. This was followed by the theory of chemical evolution which suggested that atmospheric conditions of earth led to the formation of organic molecules from inorganic molecules. Few scientists conducted experiments regarding the  2 same. However, once the first life came into existence, they started to evolve in different ways and forms. This laid a stepping stone to the theory of evolution. Life on earth appeared 500 million years after its formation. Different theories were given by different thinkers and scientists. (i)Theory of Special Creation It states that God created life out of nothing by his divine act off creation, i.e., the earth; light; plants and animals are all being created by the supernatural power. This theory has following connotations: (a) All living organisms (species or their types) that we see today were created as such. (b) Diversity of life form will not change in future. (ii) Theory of Panspermia or Cosmozoic Theory It was given by early Greek thinkers, which states that the spores or panspermia came from outer space and developed into living forms.  3 This theory was put forward by Richter and strongly supported by Arrhenius. The theory assumes that life was present in the form of resistant spores and appeared on earth from other planet. Since the condition of earth was supporting the life, these spores grew and evolved into different organisms. This theory was also known as “theory of panspermia or spore theory”. The theory initially got the support from the fact that fossils of microorganism were found in meteorites in 1961. But no mechanism is known about the transfer of spores from other planet or whether these spores could survive the journey in space. The absence of life forms on any planet except earth and no details about the spores, its origin and mechanism of crossing interplanetary space and reaching earth. In addition, this theory doesn’t add much into the fundamental details about origin of life. No scientific experiments were given to support the theory. As a result, the hypothesis didn’t receive much attention. (iii) Theory of Spontaneous Generation It states that life originated from decaying and rotting matter like straw, mud, etc. Louis Pasteur rejected the theory of spontaneous generation and demonstrated that life came from pre-existing life. In his experiment, he kept killed yeast cells in pre-sterilised flask and another flask open into air. The life did not evolve in the former but new living thing evolved in the later flask. (iv) Theory of Chemical Evolution/MODERN THEORY The modern theory is also known as “chemical theory” or theory of primary abiogenesis. In the modern theory, the hypothesis of abiogenesis was proposed with a condition that the non- living materials can give rise to life in the condition of primitive earth. The condition of the primitive earth is different from the present conditions which do not permit abiogenesis.  4 The idea of chemical theory was put forward by two scientists, A.I. Oparin and J.B.S Haldane. It has made following assumptions 1. Spontaneous generation of life under the present environment is not possible. 2. Earth’s atmosphere ~1 billion years ago is very different from the current conditions. 3. Primitive earth’s atmosphere was reducing in nature. 4. Under these conditions, the chemical molecules (inorganic molecules) react with each other through a series of reactions to form organic substances and other complex biomolecules. 5. The solar energy and UV radiation provided the energy for the chemical reactions. The first form of life came from pre-existing, non-living organic molecules (e.g., DNA, protein, etc.) and chemical evolution was followed by the formation of life, i.e., formation of diverse organic molecules were formed from inorganic constituents. The conditions on earth favouring chemical evolution were high temperature, volcanic storms, reducing atmosphere containing CH4, NH3, etc. (V) THEORY OF ETERNITY OF LIFE This theory assumes that life had no beginning or end. It believes that life has ever been in existence and it will continue to be so ever. It further believes that there is no question of origin of life as it has no beginning or end. The theory is also known as steady state theory.  5 The main objection against the proposed theory that it could not be explained; evidences support that initially earth forms and then life appeared on it. Where life exist before the formation of earth? Organic evolution is the theory that more recent types of plants and animals have their origins in other pre-existing forms and that the distinguishable differences between ancestors and descendants are due to modifications in successive generations. Charles Darwin (1809–1882) did not invent the idea of organic evolution; generations preceding him entertained the notion such as the French zoologist Jean Baptist de Lamarck (1744–1829), a pioneer in invertebrate paleontology, and Erasmus Darwin (1744–1802), grandfather of Charles. Until Charles Darwin’s time, however, the idea had never had wide currency because earlier workers lacked important data and the Huttonian concept of geologic time, which are both vital for the evolutionary argument.  6 Charles Darwin’s contribution was to propose a mechanism—natural selection—to explain how this change could occur. Evolution corresponds to change in the form and behaviour of organisms between generations. It refers to change within a lineage of populations between generations. Mechanism of Organic Evolution The phenomenon of genetic variation is basic for organic evolution. It is upon this that selective forces act for evolution to take place. The mechanism of evolution emphasizes on:  Descent and genetic differences which can be inherited to the next generation. Genetic drift, migration, mutation, natural selection as mechanisms of change.  Significance of genetic variation.  Consequences of a reduction in genetic variation and random nature of genetic drift  Impact of various species on each other’s evolution process via co-evolution.  Role of differential reproduction, variation, heredity in evolution by natural selection. Evolution is observed in a population as it contains genes in the gene pool, changes in this pool cause evolution. 1. Mutation It is a driving force for evolution. It can be a random change in the genetic composition having an impact on the gene pool of a population. This alteration in the nature of DNA in one or more chromosomes produces new alleles, hence a cause for genetic variation. Natural selection is decisive in mutation to eliminate the less-fit, allowing survival of fittest.  7 2. Natural selection It occurs when entities are left up to an environment where survival of fittest is observed. These characteristics when inherited in offspring reproduce a population better suited to the changing environment. Traits from less-fit entities are less likely to be passed to the upcoming generation. In natural selection, the significant selective force is the role of the environment. For instance, the ability to attract and mate can also be seen as a measure of fitness. Those who are better adapted, produce comparatively more offspring, passing on their genes at a higher success rate compared to the entities that are less adapted. 3. Gene flow Migration of entities may cause a cluster of them to move to a new geographical location. When the migratory entities interbreed with the newly introduced population, they result in the addition of new genes to the already existing gene pool as a result of the local population, thereby contributing to the gene flow. For instance, when pollens are blown by wind beyond the reach of the parent plant population, gene flow takes place. Some animals can be shifted away from a herd compelling them to move to a new location, interbreed and introduce new genes to the pool.  8 4. Genetic drift It can take place when a small population moves to a new geographical location, establishing in a completely isolated area. For instance, when a few fishes are introduced in a lake, the population evolves over time into a different one from the parent ones. This process does not take place in densely populated areas. 5. Development of Species A particular group of entities sharing a number of traits with the ability to interbreed with each other, leading to the production of fertile offspring are species. These usually share the same gene pool. Speciation is the evolution of species taking place when a specific population is isolated as a result of barriers, geographically. This phenomenon can also take place with the development of reproductive barriers. For instance, when a population tends to develop anatomical hurdles leading to difficulties in mating with members of the same population, a new species takes form. Species can also develop as a result of spatial differences and timing of sexual activity. A number of theories were put forward to substantiate and describe the evolutionary process. Some of these theories are listed below.