Introduction Different theories have been proposed to explain the origin of life on earth. The most widely accepted theory is the biochemical theory of the origin of life . This theory explains the origin of life in two separate evolutions. The first phase is chemical origin in which earth, simple compound, complex compound are formed. The second phase is biological evolution in which life is originated from nucleic acid and nucleoprotein in the sea.
Biological evolution of life The biological evolution of life involves the following steps:
Formation of coacervates The giant molecules had a tendency to aggregate in various combination to form small colloidal masses as insoluble droplets, which finally precipitated and aggregated into the larger and denser colloidal system which were named, coacervates by A.I. Oparin. These contained more than one type of protein nucleoprotein and other organic and inorganic molecules in various combinations. These appeared as distant bodies suspended in the surrounding water. Coacervates were surrounded by a film of water.
Formation of free genes Gradually these coacervates droplets, by their absorption quality brought many other molecules together with the help of the liquid medium present surrounding them. Coacervate molecules split and reunited, again and again, coacervates developed the ability to grow and divide giving rise to microspheres (Sydney fox) which got associated with lipoprotein membrane and nucleic acids to produce eobionts. By this process, nucleic acids were produced, which were capable to replicate themselves but as they were not surrounded by any membrane, Oparin named them free genes.
Formation of primary organisms Among the various cell types that appeared during the early period two types are more important:
The origin of autotrophs With the gradual increase in the number of heterotrophs which consumed the organic nutrients, the decline in the natural food sources occurred in the ocean. Therefore, the early living organisms resorted to different methods for getting food. Some of the early Prokaryotes acquired enzymes, which would catalyze the synthesis of simple carbohydrates from inorganic substances of the sea. It is supposed to be the beginning of autotrophic nutrition. The energy utilized in this autotrophic nutrition for the synthesis of organic molecules was totally obtained from anaerobic breakdown because solar energy could not be trapped in the absence of chlorophyll. Therefore, early autotrophic nutrition is known as chemoautotrophic nutrition. This type of nutrition is seen in sulphur bacteria.
In due course of time, some autotrophic bacteria synthesized a new substance from metalloporphyrin, i.e. magnesium porphyrin of the seawater. This substance is known as bacteriochlorophyll which could absorb sunlight and help these prokaryotes in trapping solar energy and caused the fixation of carbon dioxide into carbohydrate molecules. This type of photosynthetic autotrophic nutrition is shown by certain marine planktonic sulphur bacteria.
Till then no free oxygen was available in the atmosphere. The bacteriochlorophyll underwent certain molecular changes which resulted in the formation of true chlorophyll. This change enabled the prokaryotes to synthesize carbohydrates by using water in place of hydrogen sulphide.
Thus, the prokaryotic cells, which were chemoautotrophic, became converted into true autotrophic like the blue-green algae of today. A fossil of blue-green algae namely Arehaeopheroides barbertonensis is also reported from Africa in 1968.
Oxygen Revolution The liberation of free oxygen into the atmosphere produced by the blue-green algae like prokaryotes due to the process of photosynthesis was in fact a revolution in the early history of the earth. This oxygen leads to a number of changes in the primitive atmosphere. In fact, the free oxygen oxidized methane and ammonia forming carbon dioxide, nitrogen and water. The free oxygen also formed a layer of ozone above the atmosphere, which started absorbing most of the ultraviolet radiations of sunlight which enable the organisms to migrate on the land from water. Due to free oxygen, the reducing atmosphere was changed into the oxidizing atmosphere. Methane, ammonia and cyanide ultimately vanished and do not exist in the surrounding.
The origin of eukaryotic With the presence of free oxygen in the atmosphere, conditions fit for aerobic respiration was established on the earth. The aerobic respiration liberated about twenty times more energy in the biological system than anaerobic respiration. Therefore, the prokaryotic gradually modified themselves and become adapted for the aerobic mode of respiration. They developed true nucleus, mitochondria and other organelles. The free eukaryotes originated in the sea probably about 1.5 billion years ago. These eukaryotes were like the unicellular organisms of today. However, there are different opinions about the origin of early eukaryotic cells. Some are given below:
Conclusion It can be concluded that life first originated in sea nearly about 1.5 billion years ago in the archaeozoic era. The life could also have originated on the other planets and stars having similar conditions to those of primitive earth but nothing is known with certainty about it.
Hakimuddin Kuwakhedawala is based in India with over 15 years experience as a teacher. Teaching and writing are his passion. Most of the articles of Hakimuddin are related to education, exam, environmental problems, etc.
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