Biochemical theory of the origin of life Part -II

The mystery of the origin of life was solved by Oparin and Haldane by explaining the biochemical theory of the origin of life. In this article, you will find the process of origin of life in the seawater. To know the biochemical evolution of life, please visit this page.


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:
  1. Formation of coacervates
  2. Formation of free genes
  3. Formation of primary organisms
  4. Origin of autotrophs
  5. Origin of eukaryotic

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:
  • Moneran type: In this types of primitive cells, the nucleoprotein aggregated together loosely, directly embedded in the cell substances. These cells regarded to be the ancestors of bacteria and the blue-green algae.
  • Protistan type: They had nucleoprotein clumps condensed into a central mass surrounded by an affine membrane which restricted the nucleoproteins from the cell substances. In other words, we can say that the central mass of nucleoproteins constituted nucleus. These cell types are supposed to be the ancestors of the majority of modern organisms

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:

  1. independent origin: it suggests that the eukaryotic cells originated independently side by side to the prokaryotic cells.
  2. evolutionary theory : According to this theory coacervates evolved into prokaryotes which later evolved into eukaryotes by gene mutations and differentiation. This theory is recently supported by Raff and Mahler.
  3. Symbiotic theory : This theory was given by Wallin and further supported by Margulis. It suggests that the evolution of eukaryotic cells have taken place from the symbiotic association of two or more prokaryotic cells because the mitochondria and chloroplasts of present-day eukaryotic cells are exactly like those of the mitochondria and chloroplasts of primitive prokaryotic cells. The discovery of DNA in chloroplasts by Ris and Plaut and in mitochondria by Nass has further supported this view.


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.


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