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Steps in the Origin of Life

The earth when it was formed about 4.8 billion years ago, was a hot revolving ball of gas consisting of atoms of various elements. Heavy elements such as iron and nickel were found in the center while comparatively lighter ones like those of aluminium and silicon formed the middle layer. The lightest elements like hydrogen, oxygen and carbon were found in the outermost layer. Due to the extremely high temperature, the atoms of these elements could not combine to form molecules.

As the earth started cooling gradually, the atoms started combining with one another to form molecules.

 

Formation of Inorganic Molecules and Compounds

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With a considerable decrease in the earth's temperature over thousands of years, the atoms of different elements came together at random and formed inorganic molecules. Since the lighter elements (hydrogen, oxygen, carbon and nitrogen) were the most abundant in the outermost layer, their atoms reacted with each other to form the first inorganic molecules. Thus, the earliest molecules formed were those of hydrogen (H2), nitrogen (N2), ammonia (NH3), methane (CH4), carbon dioxide (CO2) and water vapour (H2O). All the atoms of oxygen probably combined with those of hydrogen and carbon to form water vapour and carbon dioxide. Hence, the lack of free molecular oxygen was responsible for the reducing type of atmosphere that existed on the primitive earth. The energy required for the configuration of these molecules must have come from the ultraviolet radiation in the sunlight.

Formation of Simple Organic Compounds

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As the earth cooled further, the primitive inorganic molecules interacted and combined with one another to form simple organic compounds. Simple sugars, fatty acids, glycerol, amino acids and nitrogen bases (purines and pyrimidines) were probably the simple organic compounds that resulted from the interactions of the inorganic molecules.

Water vapour present in the primitive atmosphere formed the clouds, which then resulted in rainfall continuously for several centuries. This rain water filled the hollows and basins of the earth's crust to form the oceans. Water in these oceans contained ammonia and methane. These compounds reacted among themselves to form the primitive organic compounds, which had carbon-carbon linkages. Thus, ocean water provided the basis for formation of organic compounds.

Formation of Complex Organic Compounds

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The smaller and simpler organic compounds that were formed initially in the earth, gradually started combining among themselves to form complex organic compounds. Simple sugars combined among themselves to form complex polysaccharides such as starch and cellulose. Fatty acids and glycerol molecules combined to form lipids. Amino acids combined among themselves to form polypeptides and proteins. Purines and pyrimidines combined with simple sugars and phosphates to form nucleotides, which in turn combined to form nucleic acids. Heat of the sun probably provided the energy required for the formation of complex organic compounds.

Haldane suggested that due to the accumulation of complex organic molecules, the sea ultimately became a sort of 'hot, dilute soup' where in, the molecules collided, reacted and aggregated to form more complex molecules.

Formation of Molecular Aggregates

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It is suggested that the large organic molecules formed abiotically in the primitive earth came together spontaneously and due to intermolecular attraction, formed large colloidal aggregates called Coacervates. An envelope of water molecules formed around each such aggregate due to the hydrophilic nature of some of these compounds. A membrane of fatty acids protected and enclosed these molecules, increasing the chances of chemical reactions. Gradually, breakdown and building up reactions started for which the energy required was provided by the breakdown reactions. The coacervates selectively absorbed proteins and other materials from the ocean resulting in their active growth. The coacervates not only started growing rapidly but also started multiplying.

Formation of First Cells (Protobionts)

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The coacervates were in a state of dynamic equilibrium, constantly taking in new materials from the oceans and releasing degraded materials. Thus, they had all the basic properties of life such as metabolism, growth and reproduction. However, they lacked the complexity of molecular organization, catalytic proteins (enzymes) and precise control of nucleic acids. Later, the nucleic acids are said to have taken control of coacervate and the process of replication became precise in the due course of time. With the nucleic acids being established as the genetic material, the coacervates got transformed into the primitive living systems which have been called as protobionts or eobionts.

Some of the proteins in protobionts are said to have developed the ability to catalyse chemical reactions, thereby functioning as the first enzymes. The formation of enzymes greatly enhanced the rate of synthesis of various molecules in the protobionts.

In the course of time, the protobionts became enclosed by a protein lipid membrane, allowing the accumulation of some molecules and the exclusion of others. This property improved the ability of protobionts to survive and compete with others. With the processes of metabolism, growth and reproduction becoming regular, precise and regulated, the first cells or organisms were formed. The term progenote has been suggested by Carl Woese to describe the first cell which served as the ancestor of all the forms of life existing today.

The first forms of life developed among the organic molecules, in the oxygen free atmosphere. Hence, they presumably obtained energy by the fermentation of organic compounds. They were heterotrophs, requiring ready-made organic compounds as food.

Chemoheterotrophs

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They were prokaryotic like bacteria. They were anaerobes. They must have been dependent on the organic molecules present in the broth for body building and obtaining energy.

Chemoautotrophs

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They were unable to synthesize organic molecules from inorganic raw materials, with the help of chemical energy obtained by the degradation of chemical compounds present in the sea.

Photoautotrophic

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The next step was to development of pigment molecules chlorophyll. It would absorb solar energy and convert it into chemical energy. This process is termed as photosynthesis. The earliest formed organisms were photoautotrophic bacteria. They were anaerobic and did not produce O2 as byproduct during photosynthesis, because they did not use water as a reagent.

Aerobic Photoautotrophs

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They evolved 3300 to 3500 million years ago. They were like present day cyanobacteria and could release O2 into the atmosphere because they used water as the reagent. Thus, the whole reducing atmosphere changed to an oxidising atmosphere.

Autotrophs are said to have arisen much later in the primitive earth due to a mutation in the primitive heterotrophs. The appearance of autotrophs, particularly photo autotrophs changed the situation. The appearance of photosynthetic organisms resulted in the release of free molecular oxygen into the atmosphere gradually transforming it into an oxidizing type from the existing reducing type.

The major steps in the origin of life can be summarized as follows

illustration of stages in origin of life

Steps Involved in Origin of Life

primitive atmosphere contents

simple organic molecules formation from atmosphere

microsphere formation by polymeristaion

prokaryotes and eukaryotes formation in detail

A possible scenario for the origin of life

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