In this world, it is taken human as the most talented species who has brain and have the capacity to utilize it. The earth is supporting life by providing a suitable environment for its growth and survival. Thinking about life on earth, the question always arises that how life started on earth, how the first cell that came to origin. The curiosity to know the possible answers many researchers did experiments on it and made models based on the circumstances. Scientists are trying to prove the actual reason of origin since the 17^th century but till now there is no accurate and proved model about the origin.

The model of spontaneous generation proposed by Aristotle was accepted as the base of the study till the 19^th century. According to this model, one life form is derived from different forms (Oparin., 2003). Many ideas came to existence from the model and many artists drew many conceptual things based on the origin of life. Many questions arose on the concept of the model and many experiments done on it and in 1680s this model was proven wrong.  Next came the Etymology model proposed by Thomas Henry Huxley according to which a life can be originated from a previous living organism and proposed the word biogenesis and also from a non-living matter and proposed it as Abiogenesis (Biogenesis – Hmolpedia., n.a).

After him Louis Pasteur and Charles Darwin tried to prove that the life came from different part of the universe which was again proved wrong by a letter written by Darwin in which he had written that instead of thinking how life came to existence, the focus should be on how matter formed. He wrote that from many results of the experiments he can assume that life may have formed here from many inorganic elements inside a small warm pool (Koonin, E.V. and Martin, W., 2005: 647-654).

Next came the most important one, i.e. Primordial Soup hypothesis, which was proposed by Alexander Oparin and supported by the Miller-Urey Experiment (Vaneechoutte, and Fani., 2009: 437-440). According to this hypothesis, the following points can be concluded:

• The previous earth’s atmosphere is a type of chemically reducing.
• When this reactive atmosphere got exposed to various forms of energy, it formed simple organic compounds.
• These simple organic compounds get collected in a “soup” concentrated in various places of earth.
• Further by reacting and collaborating. Complexity of the matters increased and organic polymers were formed and then life came to existence ultimately.

Even after so many experiments and evidences provided, there is none which can be taken as the proven one. In the following study we are going to see what the most relevant theories are and how the actual existence of life occurred.

Even if there are no proved model for the existence of life, the most excepted one is the model proposed by Alexander Oparin and J.B.S. Haldane which is related to the molecular and chemical synthesis of life. According to this theory, the first cell came from many slow processes of the synthesis of the molecules naturally.

The researchers proposed that, at the early ages, the Earth may consists of a chemically reducing atmosphere which consists largely of methane, water, carbon dioxide or monoxide, ammonia, hydrogen sulphide and phosphate and scarcely or in negligible amount of molecular oxygen and ozone. This model was supported by the Miller-Urey experiment which showed the formation of the basic molecules like amino acids by electrifying the atmosphere composition proposed by Oparin and Haldane (Wickramasinghe., 2014).

But later, after death of Miller, it was found that from the original experiment, 20 more amino acids were formed apart from the 20 existing in life. Taking this into base many advanced experiments were done to find out that the composition of earth atmosphere at that time was different and it was not as reactive as stated by the above mentioned model.

Figure 1: Schematic presentation of Urey-Miller experiment set up

In 1967, Bernal advocated that the origin of life (biopoiesis) occurred in three stages:

• Beginning of the bio monomers.
• Formation of bio polymers
• Transformation from molecules to cells.

Modern scientists are also working on it to find out what are the possibility of this model to be correct.

Many study says that the complex molecules both organic and inorganic are formed in planets and space naturally. If this is considered then there are two possible ways, by which earth got the molecules.

• Terrestrial Origins: molecules are synthesized on the surface of the earth due to the impact of the shocks or any other source of energy.
• Extra-terrestrial Origins: interstellar dust clouds holding the formation of the organic molecules and then it rain on earth (Davis, and McKay., 1996:61-73).

If the first molecule is formed from one of these process then it must be sure because of the Late Heavy Bombardment which made way for the processes to be carried out on the earth surface. The estimations of scientists shows that if it is a case of the Late Heavy Bombardment then it must have pasteurized the earth to tens of meter deep (Kaufman., 2011). Due to this the heated and sterilized oceanic crust produces a lot more amount of organics than the products from the Miller-Urey experiments. Everett Shock found that a deep far hypothermal vent consists of vast amount of thermodynamic force to produce organic compounds. According to Shock, the energy available in the vent can be measured to a maximum of about 100-150 degree Celsius which is a favourable condition for the survival and reproduction of the hyper thermophiles bacteria and hyper acidophilic archaea. These two are estimated to be the second phase after the Last Universal Common Ancestor.

Self-replication (reproduction) and self-organisations are the attributes of living beings. Such features of self-replication and self-organisation has been observed in simple organic molecule i.e. RNA. Considering the fact that virus which are non-living until and unless they infect the host cell, once they get into the host cell they utilizes the host cell machinery to replicate themselves. These two facts supports the hypothesis of origin of life from self-replicating molecules.  At places, even now there are several sources providing the necessary physical conditions to support the reactions of inorganic molecules to form organic molecules. Such as iron-sulphur surfaces nearer the hydrothermal vents can form biomolecules (Sivaram., 2004).

Nitrogenous bases of nucleic acid can be formed from formamide, which can be formed by reaction between HCN and water. Also the simplest amino acid derived from it. Even heating ammonium cyanide in water can create adenine, which is a purine base. Similar many other possible reactions of inorganic molecules to form organic molecules have been found also it is important to note that, each reaction pathway is contrasting to other, such as formation of purines from HCN both at low and high temperature. Miller group reported that seven different amino acids and 11 types of nucleon-bases were formed in ice when ammonia and cyanide were stored in a freezer for 25 years. Others researches also have reported the formation of unusual bases upon co-incubation of ammonia and cyanide at low temperature conditions. Eutectic freezing is a possible explanation for such reactions (Bada., 2004:1-15). At freezing temperature water starts to form ice and under such conditions the water molecules comes closer to each other and packs itself into a lattice packing. During this process the soluble and insoluble impurities in water comes closer in the pockets inside the ice, where they can collide and stay under tremendous pressure. This conditions would also have favoured for formation of biomolecules.

At present researchers supports the hypothesis that, at early stages the earth atmosphere was neutral or slightly reducing, which is contrasting to the earlier consensus of a complete reducing atmosphere. So under such conditions the products of Urey-Miller experiment will further reduce, however other researchers have reported the formation of several amino acids by reaction of carbonate and water in presence of iron. Few other groups of researchers have focused their studies on highly reducing environment.

Recently it was found that cyanosulfidic array of reaction in presence of hydrogen cyanide, hydrogen sulphide, in presence of water and UV light can produce several amino acids, lipids components and ribonucleic acids.

From the above discussions, it can be inferred that the origin of biomolecules on earth were from inorganic sources. Now the question arise, how the components of complex bio macromolecules polymerized to form proteins, RNA and DNA. Perhaps organic polymers such as polypeptides, were synthesized and accumulated on rock or clay surfaces, provided the environment was hot and dry enough, the water molecule can be lost and amino acids can join (Irwin, and Schulze-Makuch., 2010).

According to the research of Chen and Szostak, essential cellular behaviours can rise from physicochemical properties of the basic protocells. Such important and cooperative behaviours of the membranes and the contents encapsulated can led to the transformation to true cells from replicating molecules (Chen., 2006:1558-1559). Further there will be formation of stabilized membranes which will put forward the crosslinking advantages of fatty acids and phospholipids. The micro encapsulation will prevent the passing of large molecules through it but will help in the exchange of smaller molecules. It will also help in the solubility and retention of energy as electrochemical gradient.

Based on the above suggestions many discussion and experiments were done. In 2012, a study proposed by Armen Y Mulkidjanian, states that condensed and cooled inland pools and geothermal vapour procures an ideal ability for life origin (Switek., 2012:1038).

According to the RNA world, the early Earth consists of RNA proteins having the characteristics of being catalytic and self-replicating but no DNA molecules. According to this, the present world is fully descendent from the concept of RNA world (Zimmer., 2014). After many experiments, the researchers were bound to think if RNA molecules had the ability to catalyse their replication. But to perform such replication, the presence of boron, oxygen and molybdenum is required which are present in Mars rather than in Earth at that time. And if the RNA world is true than the life originating molecules must have been transferred to Earth during the meteor ejections.

RNA played an important role in the formation of the life and the factors supporting this are the special feature of RNA to act as a self-catalyse and self-storage of information. Many scientists were able to form small RNA molecules which are capable of replication in labs. Szostak was successful to show that some RNA molecules having the catalytic characteristics join the smaller RNA sequences to replicate (Szostak., 2008:11-29).

From the above discussion it can be concluded that scientists are unsuccessful to find a proven theory about the origin of life in the earth. There are many theories prevailing related to the origin of life out of which some are just imaginary and cannot be possible in the earth’s environment and some are almost true but not totally true. Apart from all the theories discussed above some other theories like Big Bang theory, Iron sulphur world, Zn world hypothesis, clay hypothesis etc. are present which different scientists have proposed according to their study. As earth has changed a lot from its early self, so it was quite difficult to get the exact scenario of the origin of life. Still scientists are trying their best to get the accurate things that happened and they had been successful to a great extend but their destination is quite far away from them.

From all above readings it can be concluded that, at first there were very reactive gases prevailing in Earth and then started reducing reactively due to the energy sources came from the Late Heavy Bombardment. From the non-organic molecules, organic molecules were formed such as amino acids and from them the RNA molecules were formed. These RNA molecules were self-replicating and self-catalysing which led to the evolution of the organic cells. This is the most estimated theory of life origin. Since it is not proven as the ultimate one, scientists are trying their best to find out the correct one.

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