Table of Contents
What are Viruses?
Viruses are small infectious agents that can divide only inside a host cell such as plants, animals or bacteria. Because they can be grown in-vitro and require a cell machinery to divide, they are also known as genetic parasites.
They are a diverse group of organisms that include viruses that have RNA genome (poliovirus), DNA genome (herpesvirus), double stranded genetic material (smallpox) or single stranded genetic material (influenza virus). Their replication models and structures are equally diverse. However, viruses do have some common features, such as very small size (<200 nm), their ability to replicate only within host cells and absence of ribosomes.
Are Viruses Alive?
It is a controversial and open-ended question that still has no answer. Living organisms possess certain properties such as, their ability to grow, reproduce, respond to stimuli, carry out metabolic processes and maintain homeostasis.
On checking if viruses fit in this criteria, it was observed that, viruses do reproduce within host cells and cause infectious diseases but they do not carry out metabolic processes. They also do not have ribosomes, hence cannot replicate on their own.
In conclusion, many scientists believe them to be living while many believe them to be non-living.
Hypotheses
Where did the virus come from? To answer this question, three hypotheses were put forward. Let us look at them one by one.
The Progressive Hypothesis
This hypothesis states that viruses originated in a progressive manner. The viruses were thought to be composed of mobile genetic elements. Mobile genetic elements are pieces of the genome that have the ability to move, i.e., they can exit from one cell and enter another.
This origin theory can be linked to retrotransposons which are a common occurrence in eukaryotes. Retrotransposons are also mobile genetic elements that copy-paste themselves into different locations by converting RNA back to DNA using reverse transcription. These mobile genetic elements may have been the basis of the origin of viruses.
The Regressive Hypothesis
This hypothesis states that the viruses may have originated in a regressive or reduced manner. It was believed that certain bacteria that are obligate parasites may have evolved from free-living ancestors. It was then deduced that viruses might have evolved from complex free-living organisms that lost their genetic information over time and adopted a parasitic mode of life.
The descendants of these complex free-living organisms first became symbiotic in nature. Slowly over time, one organism became more dependent on the other and the relationship became parasitic in nature. This led to the loss of essential genes in the organism and it became unable to replicate independently.
The Virus-first Hypothesis
Both the progressive and regressive theories assumed that viruses originated from pre-existing cells. In the virus-first hypothesis it was questioned that what if viruses existed first? It was postulated that viruses existed as self-replicating units in the previous times.
Conclusion
There is no specific conclusion to the above postulated hypotheses. The possibility of viruses arising from progressive manner or regressive manner is large. Perhaps, the third hypothesis about viruses being the first ones to originate also has intriguing implications. It could be that viruses arose multiple times through multiple mechanisms or they originated by a method which we are still to discover.
Pioneers of Virology
Louis Pasteur, a French microbiologist speculated the existence of a submicroscopic organism/particle when he couldn’t find the causative agent for rabies. Charles Chamberland, a fellow scientist of Louis Pasteur then invented a filter, known as the Chamberland filter in 1884 that consisted of pores smaller than the size of bacteria.
Adolf Eduard Mayer, a German chemist, published his work showing the mosaic disease of tobacco. He showed that the disease could be transferred from an infected plant to a healthy plant by rubbing the liquid extract and thought it to be a bacterial disease. Dmitri Ivanovski, a Russian botanist then used the Chamberland filter for the first time to demonstrate that the leaf extract of plants infected with the tobacco mosaic disease remained infectious even after filtration.
In 1898, Martinus Beijerinck, a Dutch botanist repeated the experiments of Adolf Mayer and came to a conclusion that the disease was caused by a new form of an infectious agent. He observed that the agent only multiplied in living cells and called it contagium vivum fluidum (soluble liquid germ) and named it as virus. He was convinced that the agent has a liquid and soluble nature. Ivanovsky and Beijerinck’s work was published as “One Hundred Years of Virology” in 1992 to honour their work.
In 1898, two German scientists Friedrich Loeffler and Paul Frosch, discovered the second virus Foot and mouth disease virus (FMDV). They are believed to be the true discoverers of the virus as they concluded that the virus was a particle, not a soluble liquid. Yellow fever virus was the first human virus to be discovered.
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