Origin of Life: Don’t we feel nice looking back at our old pictures or old childhood memories? It would surely be nice to look back at the story of our growth from childhood till today, isn’t it? How every single event that occurred in life has shaped us and made us who we are today? Here in this article, we will take you on a journey to our grand past.

The origin means beginning, and life is “the ability of an organism to reproduce, grow, produce energy through chemical reactions to utilize the outside materials”. Origin of life refers to the beginning of life. Once upon a time, the Earth was a lifeless planet, and then gradually, things started to change, and the chain of events led to the “Origin of Life”. It is a complex and controversial subject. Let’s start from the very beginning. 

Origin of the Earth

1. Before we begin to understand the origin of life, it is important to understand the origin of the universe, solar system, and Earth. 
2. It all started with “The Big Bang” approximately 20 billion years ago. 
3. The Big Bang led to an unimaginable expansion of the universe and ultimately the formation of the solar system and Earth nearly 4.5 billion years ago. 
4. In the beginning, the Earth was in hot, gaseous form. 
5. Slowly with time, it cooled down, leading to the formation of the solid Earth’s crust. 
6. At the same time, the condensation of vapour led to great torrential rains, which continued for thousands of years, leading to the formation of oceans and seas. 
7. The primitive ocean was different from the present ocean as it had so many organic and inorganic compounds dissolved in it; hence it is also referred to as primordial soup. 
8. The atmosphere of primitive Earth was not like the present-day atmosphere. The gaseous content was very different. There was no gaseous oxygen in the atmosphere at all, and the atmosphere was reducing type. 
9. Since the present-day atmosphere is oxidising now, it is believed that gaseous oxygen is not supporting the origin of the new life forms. However, the evolution of existing life continues.

Different Theories on the Origin of Life

Fig: Primitive Earth

Theory of the Origin of Life

After studying various pieces of evidence and performing various experiments, It was theorised that life appeared around 4 billion years ago. As new information came to light, different new theories were proposed to attempt to explain the origin of life. Some of the most important theories are discussed below:

Theory of Special Creation: This theory was proposed by  Father Saurez. Every religion has its own theories on the origin of life. This cumulative theory suggests that life originated on this Earth from supernatural powers like God.  Every religious community has its own theory on the origin of life. Despite having no scientific proof whatsoever, this theory still prevails among religious groups.

Cosmozoic Theory: Proposed by Richter, this hypothesis states that the “seeds” or “panspermia” of life exist all over the Universe and can be transferred through space, and life on Earth was brought by a comet in the form of spores or seeds. This theory was discarded as there were no scientific explanations to support this theory.

Theory of Spontaneous Creation: This theory is also known as the theory of abiogenesis.  It was proposed by Aristotle and backed by well-known philosophers like Van Helmont and Needham. This theory suggests that life originated from non-living things, decaying and rotting matter like straw, mud, etc. For example, it was believed that fishes and frogs originated from mud, and maggots originated from decaying meat. 

Theory of Biogenesis: The word biogenesis means that the genesis of life is biological, i.e., it emphasises that life can only come from preexisting life forms—the famous statement “Omnis cellula e cellula” Virchow, and established by Pasteur. Many scientists worked on various experiments to discard the theory of abiogenesis and supported the theory of biogenesis. Some famous experiments are discussed below.

Francesco Redi: He placed fresh meat in 3 different jars. He left the first jar uncovered, sealed the second jar with cork, and covered the third jar with gauze. He maintained this setup undisturbed for some time. Maggots were observed in the open jar but not in the sealed one. The covered jar with gauze had maggots growing on top of gauze; however, the maggots were found inside the third jar proving that life did not originate spontaneously from non-living substances.

Fig: Redi’s experiment

Lazzaro Spallanzani: He took beef broth in two separate flasks and boiled them for about an hour. He left one open and sealed another flask to restrict any kind of entry of air. After a while, He found out that there was no life in the sealed flask while the open flask had microbes. This experiment discredited spontaneous generation theory.

Fig: Spallanzani’s experiment

Louis Pasteur: Pasteur is famous for his sterilization technique called Pasteurization. In his attempt to understand the origin of life, he took a broth in two swan-necked flasks and heat-killed all the yeast present in these. Neck of the first flask was left as such, but the recognised neck was removed from the second flask and then the flasks were left to sit for a few days. When Pasteur checked the sample from both the flasks, he found that there was no life in the first flask, but microbes were found in the second flask. He ended up proving once and for all that life does not originate from non-living rotting and established biogenesis theory.

Fig: Pasteur’s experiment

Theory of Biochemical Evolution

This is the first modern theory worked out independently by two different scientists, A. I. Oparin and J. B. S. Haldane; hence this theory is also referred to as the Oparin-Haldane model. According to this model, the origin of life occurred in two steps:

Fig: Steps to The Origin of Life According to Oparin-Haldane Model

Chemical Evolution of Life

Chemical evolution presents the idea of the formation of biomolecules before the origin of life. It is believed that simple inorganic compounds like water vapour, ammonia, hydrogen gas, etc., combine to form organic monomers. These simple organic monomers formed organic compounds like simple sugars, purines, pyrimidines, amino acids, etc. In primordial oceanic water, these organic molecules polymerised and formed complex organic molecules like proteins, nucleic acids, amino acids, etc. 

Fig: Steps of Origin of Life According to Oparin-Haldane Model

Biological Evolution of Life

1. The basic concept of biological evolution revolves around the creation of a self-replicating body from non-living substances. Very little is known about the transition from non-living matter to living entities.
2. It is believed that around 2000 million years ago, the first self-reproducing cellular organisms were chemoheterotrophs that appeared in the primordial soup of the primitive ocean. They metabolized the chemical present in their surroundings.
3. Complex biomolecules formed in the primitive environment somehow led to the formation of protocell or pre-cell structures like Coacervates and Microsphere. 
4. Coacervates and microspheres are both microscopic, lipid membrane-bound droplets. Coacervates are seen to have a single lipid membrane, whereas microspheres are seen to have a double lipid membrane. 
5. These prelife structures resembled living in some ways like, they metabolised some compounds employing biochemical reactions.  Though they had cell-like features, they lacked genetic material.
6. It is now broadly believed that the first-ever genetic material was RNA. The discovery of ribozymes presented us with the evidence that led us to believe in the concept of the RNA world. RNA can self-replicate, and they possess catalytic functions, which are essential for a living cell to have.
7. Presumably, the first-ever cell was a chemoheterotroph and received energy from inorganic molecules present in their environment. 
8. Slowly organic molecules were exhausted from the surrounding, leading to the evolution of chemosynthetic autotrophs. Gradually with time, these chemoautotrophs evolved into photoautotrophs. 
9. These autotrophs were DNA-based anaerobes that used sunlight to produce energy. 
10. In other words, these autotrophs were cyanobacteria that performed photosynthesis and produced oxygen. This led to the Great Oxidation Event, in which the reducing atmosphere started to turn oxidising.
11. As the environmental condition started to stabilise slowly, multicellular beings began to appear. 

Experimental Proof of Oparin and Haldane’s Model

In 1953, Stanley Miller and Harold Urey performed an experiment to test Oparin-Haldane’s model. They tried to build an apparatus to create an artificial reducing environment resembling the early Earth.

The apparatus: They designed a glass apparatus. They took methane, ammonia, water vapour, and hydrogen gases in a flask. The ratio of methane, ammonia, and hydrogen was 2:1:2. All these gases were circulated in a flask equipped with electrodes. In another flask, they took boiling water at a fixed temperature of 800^OC. The electricity was used to generate a spark in the medium to mimic lightning. The whole setup was left to the provided environment for about a week or more. After the specific time period, they collected and analysed the remaining contents of the apparatus.

Fig: Diagrammatic representation of Miller’s experiment

Observation: To their surprise, they were able to recognize five amino acids aspartic acid, glycine, alpha-amino-butyric acid, and two versions of alanine and other simple organic molecules. Different kinds of amino acids and simple organic acids such as formic acid, acetic acid, propionic acid, lactic acid and succinic acid etc were produced in the laboratory in an experimental setup similar to that of Miller’s experiment while changing different environmental conditions. 

Conclusion: Following facts were concluded after the experiment:

1. Their experiment proved the Oparin and Haldane theory to be true and the most accepted one to date. 
2. They established that the origin of life requires the presence of a reducing environment.

Summary

The present-day Earth is not what it used to be. The universe started with an incredible explosion called the Big Bang. Continuous expansion formed galaxies, the solar system and our Earth.  The primitive Earth was nothing like present-day life-supporting Earth. It was hot and gaseous. Slowly the Earth cooled down to form a solid earth crust. Torrential rain led to the formation of the ocean while the atmosphere remained reduced. There are various theories about the origin of life on Earth, like the theory of special creation, panspermia, the theory of spontaneous creation, etc. The theory of biochemical evolution given by Oparin and Haldane is the most accepted one.

It suggests that life started in the ancient ocean, called primordial soup. Simple inorganic molecules came in a continuous collision and formed simple organic molecules, forming complex organic molecules. These organic molecules led to the formation of prelife structures like coacervates and microspheres. Based on our scientific understanding and evidence, it is believed that the first life forms were chemoheterotrophs. The slow and gradual change led to the evolution of Cyanobacteria. Cyanobacteria caused the great oxidation event leading to oxidation of the atmosphere of the Earth. Urey and Miller’s experiment established their theory to be true.

Frequently asked questions (FAQs)

Q.1. Who proposed the theory of the origin of life?
Ans: It was Aristotle who proposed the first-ever scientific theory on the origin of life.

Q.2. What is the most accepted theory about the origin of life?
Ans: The theory of Biochemical evolution by Oparin and Haldane is the most accepted one.

Q.3. What is the study of origin and evolution?
Ans: Exobiology refers to the study of the origin and evolution of life.

Q.4. Where did life start or originate?
Ans: It is believed that the first form of life appeared in the primitive ocean, also known as primordial soup.

Q.5. What was the first life on Earth?
Ans: First-ever life form on Earth was microscopic chemoheterotrophs.

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