Chemogeny or Chemical Evolution of Life Cycle: Earth, the third planet from the Sun, is the only planet that holds life in the whole universe. Do you know how life originated on Earth? To get the answer, many scientists proposed many theories. Out of many, the modern theory of the origin of life given by Oparin and Haldane was accepted. The theory said ‘Abiogenesis first and Biogenesis ever since’. It means the origin of the chemical evolution of life happened first, followed by biological evolution.

According to the theory, about 4 billion years ago, there was no oxygen on Earth. The atmosphere had ammonia, methane and water vapours. These gases made all the simple and complex compounds. Read on more about all the steps which lead to Chemogeny or chemical evolution of the life cycle and also a detailed Chemogeny or Chemical Evolution of Life definition.

Chemogeny or Chemical Evolution of Life Sciences

Modern Theory of Origin of Life was proposed by a Russian biochemist, Alexander I. Oparin (1923 A.D.) and was supported by a British scientist, J.B.S. Haldane (1928 A.D.), also called Oparin-Haldane theory.

It stated that primitive life on Earth evolved from non-living organic molecules (e.g., RNA proteins, etc.) in water bodies on the early Earth through chemical evolution through a series of chemical reactions about 4 billion years ago (during the Precambrian era) (i.e. about 500 million years after the formation of the Earth). Because it has a scientific explanation and has been tested empirically, it is the most satisfactory theory. Primary abiogenesis is another name for Oparin’s theory.

Theories on the Origin of Life

Various steps of the modern theory are:

1. Chemogeny (Chemical Evolution)
2. Biogeny (Formation of Primitive Life)
3. Cognogeny (Nature of Primitive Life and Its Evolution)

Chemogeny or Chemical Evolution of Life Definition

Chemogeny or chemical evolution of life definition: Chemogeny or Chemical Evolution of the life cycle is the formation of complex organic molecules from simpler inorganic molecules through chemical reactions in the oceans during the early history of the Earth, first step in the development of life on this planet. The period of chemical evolution lasted less than a billion years. Students can view the chemogeny or chemical evolution of life chart below for a clear understanding.

Fig: Chemical Evolution of Living Organisms

(a) Origin of Earth’s Primitive Atmosphere

1. 5-6 billion years ago, the Earth must have originated from a broken part of the Sun or cosmic dust gradually condensed and formed the entire solar system.
2. Initially, the Earth had hot gases and vapours of various elements. Gradually the gases condense into a molten core, and different elements become stratified according to their densities. 
3. The heaviest metallic elements formed the core of Earth-like sodium, potassium, silicon, aluminium, magnesium, sulphur etc.
4. The lighter elements flowed to the surface like hydrogen, oxygen and argon, carbon, and nitrogen and formed the primitive atmosphere.

(b) Formation of Simple Inorganic Molecules (Water, Ammonia and Methane)

1. Chemical evolution was favoured on the primitive planet some 4 billion years ago. The original temperature was very high (5000°C- 6000°C), and elements like hydrogen, oxygen, carbon and nitrogen could not exist in a state. 

2. When the Earth’s surface temperature was less than 100°C, nitrogen was in the form of ammonia (-NH3), carbon was in the form of methane (-CH4), and oxygen was in the form of water vapours (H2O) in its atmosphere, but there was no oxygen.

3. As a result, the primordial atmosphere was “decreasing.” Protoplasmic compounds were the name given to these substances.

Fig: Water, Methane and Ammonia

4. As the Earth cooled, it became a solid crust, which eventually gave rise to depressions and elevations. Meanwhile, water vapours in the atmosphere condensed and eventually fell to the ground as rain. 

5. Water collected in the depressions dissolved minerals like chloride and phosphates, resulting in the formation of huge water bodies known as oceans. 

6. Large quantities of hydrogen, nitrogen, carbon dioxide, methane, ammonia and water vapours were present on the primitive Earth; the atmosphere was devoid of oxygen. 

(c) Formation of Simple Organic Compounds

1. The Earth’s surface cooled to 50-60°C, molecules and minerals in water bodies combined and recombined in a variety of ways, forming simple organic compounds such as alcohols, glycerol, aldehydes, fatty acids, purines, pyrimidines, simple sugars (e.g. ribose, deoxyribose, glucose, etc.) and amino acids.

2. In the absence of a consumer, enzyme catalysts, or oxygen, these organic molecules accumulate in water bodies. In the current oxidising environment, such transition is impossible because oxygen or micro-consumers will disintegrate or kill any living particles that may arise by chance.

a. Condensation Reactions

HC≡ CH + H₂O → CH₃CHO (acetaldehyde)

CH₃CHO + CH₃CHO → CH₃CHOHCH₂CHO (aldol)

b. Oxidoreduction

CH₃CHO + H₂O → CH₃COOH + C₂H₅OH

c. Polymerization

CH₃COOH + C₂H₅OH → CH₃COOCH₃CH₂ + H₂O

CH₂OHCOOH + NH₃ → CH₂NH₂COOH + H₂O

3. One of the following factors generated the energy for these photochemical reactions:

(i) Volcanic eruptions (Intense dry heat of Earth)
(ii) Solar radiations (UV-rays)
(iii) Electrical energy during lightning
(iv) Decay of radioactive elements.

4. The complex organic compounds formed in the ocean gradually got settled in the primitive oceans.

Fig: Chemical evolution

(d) Formation of Complex Organic Compounds (Carbohydrates, Proteins, and Fat)

1. Simple organic compounds underwent random chemical reactions and polymerization to eventually produce complex organic compounds such as polysaccharides, lipids, nucleotides, nucleic acids, polypeptides, and so on.

2. Electrical discharge, lightning, solar energy, ATP, and pyrophosphates were the primary energy sources for chemical reactions and polymer synthesis. The concentration of monomers caused by water evaporation favoured polymerization.

3. The water bodies were dominated by these polymers because they were more stable. The chemical equilibrium changed towards the synthesis of stable polymers from unstable monomers when the concentration of polymers was high.

4. The synthesis of these complex chemicals, which are crucial protoplasm elements of live cells, established the possibilities for the genesis of life in the primaeval ocean.

5. As a result of the foregoing chemical reaction, marine water becomes a rich blend of organic molecules named hot dilute soup of primordial soup or broths by Haldane.

Experimental Proof of Abiogenic Molecular Evolution of Life

Stanley L. Miller, a biochemist, and Harold C. Urey, an astronomer, demonstrated the production of simple organic compounds from simpler compounds in reducing conditions in 1953 A.D. (Fig.)

Fig: Miller and Urey Experiment Apparatus

1. It’s referred to as a simulation experiment. Its goal was to determine the validity of Oparin and Haldane’s claims about the formation of organic molecules in primitive Earth.
2. They exposed a methane-ammonia-hydrogen-water mixture (simulating the primordial atmosphere) to an electric spark (of about 75,000 volts, simulating primitive earth lightning and providing a temperature of about 800°C) between two tungsten electrodes in a gas chamber (called a spark discharge apparatus) placed in reducing conditions for about one week.
3. In a ratio of 2:2:1, methane, ammonia, and hydrogen were used. They passed the hot products through a condenser (for condensation and collection of the aqueous end product, Equivalent to Haldane’s soup).
4. They used a condenser (for condensation and collecting of the aqueous end product, similar to Haldane’s soup) to send the hot products through.
5. Except for the energy source, the control experiment met all of the conditions. They used chromatographic and colourimetric methods to investigate the chemical components after eighteen days.
6. They discovered peptides, purines, pyrimidines, and organic acids, as well as amino acids (lysine, alanine, aspartic acid, glutamic acid, and so on).
7. Purines and pyrimidines were nucleic acid precursors. Proteins require these amino acids to be formed.
8. Aldehydes and HCN were discovered to be intermediary products. A small number of organic compounds were detected in the control experiment.
9. Similar chemicals were discovered in meteorite material, indicating that similar processes are occurring elsewhere in space.

Observation

1. They noticed a condensed liquid with a dark colour.
2. It was gathered and chromatographically analysed, and the liquid was found to be a mixture of sugars, amino acids (glycine, aniline, etc.) and fatty acids.

Conclusion

The experimental results support the Oparin-Haldane theory of the origin of life that organic molecules are created from inorganic molecules during the course of the origin of life.

(e) Formation of Complex Aggregates

1. Oparin called these minute, spherical, stable, and motile aggregates coacervates (L. acervus = pile — Fig.) and Sidney Fox called these microspheres.
2. Coacervates are generated when a protein and a polysaccharide are shaken together, according to Oparin. Protein, polysaccharides, and water make up the majority of the core of these coacervates.
3.  It was extracted in part from the surrounding aqueous solution, which contained fewer proteins and polysaccharides. However, because these coacervates lack a lipid-based outer membrane, they were unable to replicate. 
4. Coacervates were later surrounded by limiting membrane and fatty acids like lecithin and cephalin. 
5. After the formation of the limiting membrane, various substances accumulated inside the coacervates.
6. Coacervates started absorbing organic substances from oceanic soup and became anaerobic heterotrophs, which grew bigger in size, variable in chemical composition and multiplied by breaking down into smaller droplets after attaining growth.

Fig: Coacervates

Experimental Proof of Formation of Complex Organic Compounds

Many amino acids polymerised and produced polypeptide chains, called proteinoids, when a mixture of 18-20 amino acids was heated to the boiling point (160 to 210°C for many hours) and then cooled in water, according to Sidney W. Fox (1957 A.D.).

However, according to Fox, these proteinoids combined with water to create colloidal aggregates known as coacervates or microspheres. These were roughly 1-2 m in diameter and shaped and sized like coccoid bacteria.

Fig: Coacervates under a microscope

These may be made to constrict on the surface,  resembling budding in bacteria and fungi (yeast). He also discovered that inorganic chemicals and HCN might be used to synthesize porphyrins, nucleotides, and ATP.

(f) Formation of Protobionts

Fig: Protobionts

1. For the origin of life, the following three conditions must be fulfilled:

(i) There must be a continuous supply of self-producing molecules, called replicators.
(ii) Copying of these replicators must have been subject to mutation (change).
(iii) The system of replicators must have required a continued supply of energy and their partial isolation from the general environment.

2. Thermal motions generated by high temperature were most likely the main cause of mutations in replicators (prebiotic molecules), while partial isolation was achieved within their aggregation. Complex organic chemicals synthesised abiogenetically on the early Earth tended to gather and form huge colloidal cell-like aggregates called protobionts, according to Oparin and Sidney Fox.
3. Such non-cellular forms of life are thought to have evolved 3 billion years ago. These would be massive molecules, including RNA, protein, polysaccharides, and other biomolecules. These aggregates can separate molecules from their surroundings and maintain a controlled interior environment.
4. These were probably the first and most primitive of primordial organisms.
5. Life thus presumably originated in the ocean about 3.7 billion years ago, and the first organisms were heterotrophic, anaerobic, and virus-like nucleoprotein structures.

Summary

Modern Theory of Origin of Life was proposed by a Russian biochemist, Alexander I. Oparin (1923 A.D.) and was supported by a British scientist, J.B.S. Haldane (1928 A.D.), also called Oparin-Haldane theory. This led to the evolution of the first terrestrial photoautotrophs, the end of biosynthesis. Chemical evolution was followed by biological evolution. In 1953, Stanley Miller and Harold Urey experimented with testing the biochemical origin of the life hypothesis offered by Oparin and Haldane. Therefore, life existed from slow changes in the Earth’s atmosphere, giving rise to the first cell up to today’s extent.

Learn All Concepts on Origin of Life

Frequently Asked Questions (FAQs)

Q.1. Who proved the chemical evolution of life?
Ans: “The Origin of Life on Earth” is the Chemogeny or chemical evolution of life book. It was first proposed in 1936 by Russian scientist Aleksandr Ivanovich Oparin.

Q.2. What were conditions like on the early Earth?
Ans: There was no ozone layer on the early Earth, and it was presumably very hot. There was no oxygen on the early Earth, either. Few things could live on the early Earth if it didn’t have an oxygen atmosphere. Anaerobic bacteria were most likely the first living organisms on the planet.

Q.3. What is the first step in chemical evolution?
Ans: Molecules in the early environment formed simple organic substances like amino acids during the initial stage of chemical evolution. Aleksandr Ivanovich Oparin (1894– 1980), a Russian scientist, first proposed this concept in 1936 in a book titled “The Origin of Life on Earth.”

Q.4. Which of these are the three requirements for chemical evolution?
Ans: The formation of replicating molecules and the accumulation of organic molecules on ancient Earth are two crucial aspects of chemical evolution. This process can be divided into three stages. Inorganic, organic, and biological.

Q.5. What is the result of chemical evolution?
Ans: The initial step in the evolution of life on this planet was the formation of complex organic molecules from simpler inorganic molecules through chemical reactions in the oceans throughout the early history of the Earth.

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