Where Did Viruses Come From?

 

In 2020, the existence of the entire human race was threatened by this little tiny thing called coronavirus. Interestingly, this thing is just one out of the diverse family of viruses which are just less than 200 nanometers (nm) in diameter. What is more fascinating is that we arguably cannot even classify this thing as a living thing or be regarded as being alive. They are obligate intracellular parasites i.e. they require infecting a host in order to carry out basic life activities like replication or metabolic processes to generate energy for themselves. Aside this, they are just typical crystal compounds outside any biological host and can be said to be the bridge between living and non-living things. The structure of a virus is very simple. It typically consist of a protein capsid head that houses genetic materials (DNA or RNA) and a tail.

However, the question on the origin of viruses or where they come from is an interesting topic in biology because it gives us a better perspective on the evolution of life on Earth. Virologists and cell biologists alike have posited three major hypothesis which are: the progressive (or escape) hypothesis, the regressive (or reduction) hypothesis and the virus-first hypothesis.

The progressive hypothesis suggests that viruses originate from mobile genetic materials that possess the ability to move within cell genomes which gained the ability to exit the cell and enter another cell. In other words, viruses are once genetic materials that can move within a cell genome and later evolved to be able to leave a cell and enter or infect another cell. This hypothesis was supported by the observation in retroviruses family in which new viral RNA that were formed in the host cell after replication are able to assemble themselves and exit the cell to infect another cell and continue the process.

Regressive hypothesis suggests that viruses may have existed through a reduction process. This process posit that viruses may be free living organisms that lost some of their important genetic information in the course of their adaptation for replication through parasitic approach. Therefore, the possibility that viruses might have evolved from a complex free living organism ancestor. The genomic analysis of Mimivirus family support this hypothesis as they do not differ significantly to some parasitic bacteria like Rickettsia prowazekii. 

Whilst the progressive and regressive hypothesis suggests that cells existed before viruses, the virus-first hypothesis claim that virus existed before cells and are not descendants of the latter. With recent understanding that the very first replicating molecules consists of RNA and RNA has enzymatic properties to catalyze reactions, perhaps self replicating RNA existing before the first cell emerged evolved to infect the first cells. Thus, RNA viruses.

So, which of the hypothesis is true? Well, perhaps not only one of these hypotheses are correct as there are significant evidences for each of them. Moreover, there is a possibility that viruses originated from multiple mechanisms and could be major contributors to the emergence of life as we know it.

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