Theories on the Origin of Life: Different Theories

Life on Earth began more than 3 billion years ago, developing from the simplest of microorganisms into a dizzying array of complexity over time. There are different Theories on the Origin of Life. With the advancements of science and technology, scientists cannot create life in the laboratory! Then the question arises, how did life come into existence on Earth? 

There are several theories on the origin of life on Earth. Some are based on pure belief, while others are based on certain scientific observations or imaginations. Let us understand the different theories of the origin of life on Earth in this article. 

Theories on the Origin of Life: Overview

The universe formed about \(20\) billion years ago. Earth formed about \(4.5\) billion years ago. Life originated approximately \(4\) billion years ago. Condition on earth at the time of origin of life includes:-

1. High Temperature.
2. Lighter elements were present in gases- Methane, ammonia, hydrogen, helium, water vapour.
3. No molecular oxygen was present, i.e., the atmosphere was reducing.
4. Heavy elements like FeNi form the core of the earth.
5. UV radiation was present in energy-giving radiation, and they favoured photochemical reactions.

Different Theories on the Origin of Life

1. Theory of Special Creation

This theory was based on religious beliefs. It was believed that God created life in heaven, which was placed on earth.
In the Bible, there is a chapter “Genesis” which says about how this life originated on earth. So, according to this, the first man was Adam, and the woman was Eve.
According to Hindu mythology, Brahma is considered as the creator in which the first man was Manu, and the first woman was Shraddha. 
But no proof is present to support these.

2. Abiogenesis

This theory was proposed by  Von Helmont according to which life originates from non-living things. Abiogenesis means abio \(=\) non-living, genesis \(=\) synthesis. He said the hair on the tail of a white horse gave rise to a worm called Gordius. The mud of the Nile river gave rise to various organisms.

3. Biogenesis

It means bio \(=\) life, genesis \(=\) synthesis. Life originated from pre-existing living forms. This theory was proved by three experiments proved by different scientists. This theory disproved the abiogenic origin of life.

Francesco Redi took pieces of meat or flesh and cooked then packed those pieces in various jars. The first jar was covered with a parchment membrane; the second jar was covered with a muslin cloth, while the third jar was kept open. He found larvae of the housefly on the flesh in the jar, which was kept open because houseflies came and laid their eggs on it. 

Fig: Francisco’s Experiment

Spallanzani did the same experiment, but he used hay infusion. Louis Pasteur used a swan neck flask in his experiment. In the flask, he kept a mixture of yeast and sugar. This mixture was heated and was not spoiled as long as that flask was with a swan neck tube. Because this tube did not allow the entry of microbes in the flask because of its structure. When the tube was removed, he observed some microbes in the mixture as spores must have entered the vessel, reproduced and divided. So life forms are coming from the life forms which are already existing.

Fig: Pasteur’s Experiment using Swan-neck Flask

4. Theory of Catastrophism

Cuvier, in \(1826\) postulated the theory of Catastrophism for the origin of life on the planet. This hypothesis postulates that since the earth might have experienced several ages, most life forms might have vanished completely and recreated subsequently with modifications and complexity. 

5. Panspermia theory or Extraterrestrial origin

Some Greek thinkers thought that life originated in space and it came on earth in the form of some structure called spores. Specifically, Arrhenius postulated that life on the planet might have come in the form of primitive life forms or panspermia or spores from extraterrestrial space.

Modern Theory of Origin of Life

6. Chemogenetic Theory

This modern hypothesis of the origin of life was first formulated by Haeckel. Later, independently, Russian scientist A. I. Oparin and English biologist J. B. S. Haldane proposed and developed the chemical origin of life. It includes chemogeny, cogenogeny and biogeny.

Chemogeny includes chemical evolution, which Oparin and Haldane explained in \(1924\). They believed in the chemical origin of life and supported abiogenesis initially, followed by biogenesis. They said that life came into existence from inorganic matter. Inorganic matter gave rise to organic matter. They said that when life originated, chemical changes must have taken place. According to them, at the time of origin of life conditions of earth were – 

1. High temperature above \({\rm{80}}{{\rm{0}}^{\rm{^\circ }}}{\rm{C}}\)
2. The energy was into two forms, i.e. UV rays and lightning.
3. Gases were ammonia, methane, hydrogen, helium and water vapour.
4. The atmosphere was reducing, and no oxygen was present.

Initially, the temperature was very high; all the water was in the form of vapour. When this temperature came down below \({\rm{10}}{{\rm{0}}^{\rm{^\circ }}}{\rm{C,}}\) it started raining. According to them, torrential rains resulted in the formation of huge water bodies where minerals and substances dissolved in them, and that mixture formed was called broth. 

Chemical evolution says in the presence of energy in the form of lightning, and UV rays, inorganic molecules (\({\rm{N}}{{\rm{H}}_{\rm{3}}}{\rm{,}}\,{\rm{H,}}\,{{\rm{N}}_{\rm{2}}}{\rm{,}}\,{\rm{C,}}\) and water vapours) and methane \(\left( {{\rm{C}}{{\rm{H}}_4}} \right)\) must have reacted to form simple organic molecules (amino acids, hydrocarbon chains). Then these organic molecules formed some bigger organic molecules like proteins, fatty acids, nitrogenous bases, sugars etc. It resulted in the formation of molecules that could replicate.

So, it can be said that chemical evolution or chemogeny started the biogeny or the evolution in terms of molecules that were able to replicate, like that of the formation of DNA and RNA.

Some of these organic molecules must have aggregated and reacted to form complex organic molecules. Like some DNA would have formed that could have given rise to RNA, and that RNA could have given rise to Proteins. This is assumed to be how biomolecules would have formed.

Cogenogeny: The large organic molecules, like protein, lipid, nucleic acids, synthesised abiotically, started forming molecular aggregates, called microspheres (named by Sydney Fox). These molecular aggregates were named as Coacervates by Haldane. These were the forerunners of first cells or protobionts. They were mixtures of proteins, nucleic acids, lipids, etc. and could divide. This led to the formation of protobiont or eobiont.

Biogeny is the biological evolution that began with the formation of coacervates. Nucleic acids and proteinoid aggregates came together to form giant nucleoprotein complexes, and the evolution of heterotrophic organisms began. It is thought that the first-formed cells were much similar to present-day mycoplasma and viruses.
Then chemoheterotrophs (using chemicals to obtain energy)  chemoautotrophs (using light and chemical for energy) came. Then with time, a pigment called bacteriochlorophyll developed in them, which they must have used to trap energy from Sun and water and by which they must have released oxygen in the atmosphere. This is how aerobic organisms came into existence. The first one, which is an aerobic organism, is our cyanobacteria.
This is how life originated, according to chemogenetic theory.

Fig: Main Steps of Oparin-Haldane Hypothesis

Chemogenetic Theory was proved by Urey and Miller. They conducted an experiment in \(1952\) and published an article in \(1953\). Miller-Urey experiment proved that inorganic molecules gave rise to organic molecules from which life originated. They were the first to produce amino acids from a mixture of inorganic chemicals by simulating the primitive Earth-like conditions.

Summary

Life originated approximately \(4\) billion years ago. Condition on earth at the time of origin of life includes- high temperature, reducing atmosphere, UV radiation, etc. Different scientists had different opinions about the origin of life. So, accordingly, they gave different theories. Like in ‘Theory of Special Creation’ was based on religious beliefs. According to it, life was created by God in heaven, which was placed on earth. ‘Von Helmont proposed abiogenesis’ theory according to which life originates from non-living things. ‘Biogenesis’ theory says life originated from pre-existing living forms. Their experiments proved this by F. Redi, Spallanzani and Louis Pasteur. Panspermia theory says that life originated in space, and it came on earth in the form of some structure called spores. Chemogenetic Theory was given by Oparin and Haldane, which says life came into existence from inorganic matter. Inorganic matter gave rise to organic matter.

FAQs

Q.1. What is the best theory that explains the origin of life?
Ans: Chemogenetic Theory given by Oparin and Haldane is the most accepted theory of the origin of life.

Q.2. What did the Miller-Urey experiment prove?
Ans: Miller-Urey experiment proved that inorganic molecules gave rise to organic molecules from which life originated.

Q.3. What are the four major theories of the origin of life?
Ans: Four major theories of the origin of life are:-
a. Panspermia theory
b. Abiogenesis
c. Biogenesis
d. Chemogenetic Theory

Q.4. What is the modern theory of the origin of life?
Ans: Chemogenetic Theory is the modern theory of the origin of life which explains how life originated from non-living matter. It says that organic molecules like amino acids, purines, pyrimidines, polysaccharides, fatty acids have evolved from inorganic molecules like methane, ammonia, hydrogen, helium and water vapour. This was experimentally proved by Urey and Miller in \(1953\).

Q.5. What is the theory of special creation?
Ans: Theory of special creation- This theory was based on religious beliefs. It was believed that life was created by God in heaven, which was placed on earth. 

Learn About Modern Theory of Origin of Life

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