What is Abiogenesis?

In the field of evolutionary biology, abiogenesis or the origin of life from non-living things is a scientific theory that explains that life started on earth through spontaneous, natural means because of the conditions found at that time. In better words, life came from abiotic matter or simple chemical constituents. English biologist Thomas Henry Huxley proposed this term. Certain environmental conditions are recognized as important for the creation of life in this way. The details will be discussed further.

Difference between Biogenesis and Abiogenesis

Biogenesis defines the production of life from living things only. In other words, it explains when a new life or cell, arises from a pre-existing life or cell it is referred to as biogenesis. Antoni van Leeuwenhoek established microscope lenses to see the living things in a drop of water to understand how life began. He observed various minute organisms and studied their morphology and structure. He later studied the life histories of different organisms, such as fleas, and indicated that they arose from pre-existing organisms. Louis Pasteur conducted an experiment that showed that living things come from living things. 

The concept of abiogenesis is dependent on various complex mechanisms and not just on a single event. It defines evolution based on different processes that include molecular self-replication, self-assembly, the emergence of the cell membrane, and the auto-catalysis process. 

The emergence of the Abiogenesis concept

In the year 1920, scientists J.B.S. Haldane and Aleksandr Oparin artificially set almost similar ideas remembering the conditions required for the origin of life on earth. They claimed the organic fragments to be made from nonliving things in the presence of an energy source like ultraviolet radiation. Both of them also believed the primitive atmosphere of the earth was reducing in nature. It contained methane, ammonia, water vapor and lacked oxygen. They assumed that the life forms first developed in the warm environment like the primitive oceans and were heterotrophic. Oparin observed that the life developed from the coacervates, which are microscopic lipid molecules tethered by electrostatic forces. Later, these molecules started to act like the living cell at some point. Oparin worked with coacervates and confirmed that microscopic molecules work in better efficiency in membrane-bound spheres than in a free aquatic medium. Haldane was not familiar with this idea, and he delivered his idea of the genesis of organic compounds in the stratosphere that supplied enough ultraviolet light. Eventually, those molecules became upgraded and generated cells.

The Miller-Urey experiment to build up the abiogenesis theory

In the year 1953, two chemists Harold C. Urey and S. Miller, thoroughly studied the “Oparin-Haldane theory” and produced organic forms from the inorganic components through some experiments. These components were assumed to be found in the prebiotic atmosphere on earth. During their experimental setup, they assembled warm water of a temperature of 4500K (Kelvin) and a mixture of four gases together, which are ammonia (NH₃), methane (CH₄), water vapor (H₂O), and molecular hydrogen(H₂). They stimulated it with electrical discharge to create the earth's primitive environment. The chemical ingredients were directed to create a warm environment and prebiotic atmosphere in addition to the primitive ocean. After a week, Miller and Urey found basic organic molecules, including amino acids (the building block of proteins). Those findings created an impact on the environmental condition of the early earth.

It did not exactly prove how life got started on earth, but it supported the hypothesis by showing how it could have happened.

Modern conceptions of Abiogenesis

These hypotheses are built on the similar principles of the Oparin-Haldane theory, and the Miller-Urey experiment. There are several models available to explain the succession from an abiogenic to a living organism. The complex molecules first converted into self-replicating molecules that lack metabolic functions or first became the metabolizing protocells then gained the capability of self-replication.

There is a debate involving the habitat of the organism. Some evidence mentions that life is supposed to have originated from the ‘hydrothermal vents’ on the ocean floor. It concludes that abiogenesis occurred deep below the earth's surface. 

Different life forms originated with particular biochemical constituents expressing the originality of abiogenic materials from which they evolved. After the experiment of Miller-Urey, several concepts about the origin of life on earth have been built. The researchers discovered that the synthesized amino acid not only could generate small protein molecules (peptides) but also initiated the mechanism of evolution by forming the RNA (Ribonucleic acid), where the nitrogen-containing compounds (nitrogenous bases) were bound to sugar and phosphate molecules could generate from the prebiotic starting materials.

According to a new search, the possible life form had no phosphate on early earth as there was very little phosphate around. Instead, sulfur must have been used to break down food and keep cells alive. Phosphate is an essential part of the structure of the DNA double-helix.

RNA world hypothesis

RNA world explains that self-replicating RNA molecules existed and were sustained long before the evolution of Deoxyribonucleic acid (DNA) and proteins. The term RNA world was coined by Walter Gilbert and the concept was first introduced by Alexander Rich. This theory gets wide acceptance all over the world.

Analogous to DNA, RNA can store and copy genetic information through particular processes. RNA and protein-based enzymes can generate chemical reactions important for life. A crucial cellular component, the ribosome is made up of ribosomal RNA (rRNA) and proteins. Due the presence of the 2' hydroxyl group, RNA is susceptible to degradation. Also, it is mainly single-stranded, which makes it susceptible to degradation by nucleases. It is presumed that some of the ancient RNAs may have developed the capability of methylating other RNAs for their benefit as methylation protect the nucleic acid from degradation.

Significance of RNA world

Proceedings of the National Academy of Sciences have explained two types of primitive habitats, which are significant for establishing RNA as the life source. Those are called 'hydrothermal vents' and 'warm little ponds. Give below are the features of RNA that help to establish the importance of RNA being the first genetic material. The features are:

• Self-reproducing or replicating: RNA can self-replicate. That means RNA can be synthesized using RNA as a template instead of DNA. 
• Catalysis: It catalyzes a few simple chemical reactions, which enhance the creation of building block molecules. 
• Amino acid-RNA ligation: A type of RNA, which is tRNA (transfer RNA) can ligate an amino acid molecule to the 3' end of the RNA strand to use its chemical structure for further processes.
• Peptide bond formation: In modern cells, cellular organelle ribosomes can generate longer proteins by linking amino acids by peptide bonds.
• Acting as information storage: It is very similar to DNA with two chemical differences. The RNA contains the sugar ribose, whereas the DNA contains the deoxyribose sugar molecule. The second difference is, the RNA contains uracil as the nitrogenous base, but the DNA contains thymine instead of it. The structural change is thought to be initiated by the process of evolution. The origin of DNA is thought to be related to RNA. For example, some viruses and lower organisms use RNA as their genetic material in present times.

Limitation of RNA as a permanent genetic material

The chemical features of RNA are significantly fragile. RNA is generally single-stranded and thus easily accessible to nucleases. Due to the presence of the 2'-OH group, it is susceptible to hydrolysis. They can be destroyed easily or dissolved through hydrolyzing. These characteristics make it unsuitable for future information storage. In the process of evolution, the ribose sugar is converted into deoxyribose to make it a stable structure. 

Problems regarding abiogenesis

• It has no detailed theoretical explanation of life's origin from complex organic components.
• There is no example of a successful experiment that can help to prove the generation of more complex molecules other than amino acids.
• There is no such process that clarifies the development of purine-pyrimidine bases of RNA from the short RNA building blocks.

Some alternative theories of the origin of life

Some other theories of life formation have been discussed in Biology. These include

1. Community clay: Two organic chemists explained the first molecules of life might have evolved on clay. They suggested that the minerals in clay could have arranged organic molecules into organized patterns. This event follows the modern arrangement of DNA molecules.
2. Chilly start: The Earth is known to be a billion-year-old planet. Ice covering the earth's surface 3 billion years ago might have protected the organic compounds in the water below. Cold might have helped these molecules to survive longer.
3. Simple beginnings: Instead of a complex one (RNA), life has been established from the smaller molecules interacting with each other. The evolution of life forms might have started in cycles of reactions.
4. Panspermia: It was believed that life existed in outer space and it was brought on earth by meteoroids, asteroids, etc. This theory potentially consideres all the living things outsiders of the earth.

Significance of abiogenesis

1. This theory of abiogenesis is very important as it provides detail of life’s origin and evolution
2. It gives rise to the most primitive forms of life and explains about pre-biotic atmosphere. 
3. Higher or developed living organisms have evolved from these primitive life forms.
4. Biological evolution and abiogenesis are known to be distinct branches of Biology. They are closely related to the evolutionary conceptual framework.

Context and Application

It is mainly studied for understanding the evolutionary link of modern earth. A part of biochemistry is also involved in this explanation. Several fields of biology explain it as an important part. Those fields are:

• Bachelor of Science in Zoology
• Master of Science in Zoology
• Masters in Evolutionary Biology

Common Mistakes

It is a quite common problem with students that they confuse abiogenesis with spontaneous evolution. In biology, abiogenesis is a theory that explains the life started from inorganic molecules. Another theory is based on complexity. It has been created through spontaneous and continuous reactions of non-living matter. In biology books, there is a common example found of generating flies from a lump of rotten meat. It is sometimes considered an example of abiogenesis, which is not true. It is an example of spontaneous life generation.

Practice Problems

1. What does the abiogenesis theory suggest about molecules quality?
   1. Self-replication
   2. Self-maintenance
   3. Both a & b
   4. None of them

Correct Answer: a. Self-replication. Self-replication defines the situation where an organism copies its information.

2. The Oparin-Haldane concept was introduced in which year?
   1. 1920
   2. 1950
   3. 1953
   4. 1900

Correct Answer: a. 1920. In the year 1920, Oparin and Haldane gave some ideas about the origin of life on earth.

3. In which of the following processes does RNA convert into DNA?
   1. Replication
   2. Transcription
   3. Translation
   4. None of the above

Correct Answer: d. None of the above. RNA is converted to DNA through the “Reverse Transcription” process. The enzyme reverse transcriptase catalyzes this conversion process.

4. Who first coined the term 'RNA world'?
   1. Miller
   2. Alexander Rich
   3. Gilbert
   4. Darwin

Correct Answer: c. Gilbert. The researcher Walter Gilbert first coined the term RNA world.

5. According to the Spontaneous theory, what can be generated from rotten meat?
   1. Life forms
   2. Amino acid
   3. Water
   4. Nitrogenous base

Correct Answer: a. Life forms. The spontaneous theory explains the generation of flies from rotten meat in an enclosed place.