The past events of COVID19 pandemic were enough to prove that viruses play a key component in shaping human health and global economy. They had played and are still playing crucial role in the evolution of both cellular and acellular life. But when did they appear in evolution? Who were parents of first virus? Is there any theory available for origin of viruses? Which came first bacteria or viruses? These are some mind-boggling questions with no clear answers. Please do not think its not important to paint the correct picture of evolution of viruses. At present million of dollars are being poured to study the science and evolution of viruses due to their highly infectious nature and increased disease burden in human population across the globe.
When and Who discovered viruses?
Two scientists, Dmitri Iosifovich Ivanovsky (Russian botanist) and Martinus Willem Beijerinck (Dutch microbiologist and botanist) independently discovered Tobacco mosaic virus in 1982 and 1898 respectively.
Ivanovsky showed the extracts of infected leaves of tobacco plant (infected with mosaic disease) retained the capability of infection post filtration by Chamberland filter-candle (pore size 0.1 – 1 micron). These filters were designed by Charles Chamberland which could completely remove bacteria and cells (known at that time) from the liquid solutions.
Why it was named as VIRUS?
Beijerinck was the first scientist to call it a ‘virus’ in 1898.
(Source: Wikipedia)
(Source: Wikipedia)
If you ask me the exact meaning of the word ‘virus’ then I can tell you that it has its roots in Latin which means ‘slime, poison, venom’. So why did Beijerinck name the infectious agent (extracts of infected leaves) a – virus? He was able to demonstrate that the incitant (infectious agent) of the tobacco mosaic disease was slimy or somewhat liquid in its physical state (‘contagium vivum fluidum’), hence the name.
To his surprise he realized that the infectious agent could multiply in living organisms (host) but cannot be cultured outside. Therefore, the infectious agent could not be a bacterium (contagium fixum).
It is speculated that Ivanovsky could not comprehend the impact of his discovery back in 1892.
What are viruses?
For ease of understanding I will try to explain ‘what are viruses’ in layperson language. Interestingly, viruses have split personality disorder like Brad Pitt in the movie Fight Club, as they too have two different life cycles and characteristics 1) intracellular (inside the cell) 2) extracellular (outside the cell). Intracellularly they reprogram the genetic machinery (DNA) of the infected cell (host) and produce its own copies (virions) and extracellularly the virions escape the infected cell and remain in the environment in metabolically inert phase (inactive state).
They can either have DNA or RNA as their genetic machinery. For example, coronavirus, SARS-CoV-2 (causative agent of COVID19 pandemic) is an RNA virus and Herpes simplex virus (causative agent of herpes simplex) is a DNA virus.
When did viruses came into existence ?
By now we have an understanding that virus means the evolution of parasitism. If you do not know what parasitism is, then take a careful look around yourself. You might spot many parasites like ticks, fleas, lice, roundworms, tapeworms etc. Parasitism is a negative type of relationship between two individuals in which one gains the benefits at the cost of the other one. To survive and evolve in this living world viruses need cellular factories (live cells) therefore we can easily rule out their existence prior to the cellular world. However, there are several hypotheses floating in the scientific world to explain the time of their arrival and by which biological process they evolved.
According to Forterre P. (2005), Viruses originated from ancient cells. Now what on earth are ancient cells? These cells existed before the last universal common ancestor (LUCA) diversified into modern cells. Modern cells were further branched out as the three super kingdoms, Archaea, Bacteria, and Eukarya.
To support the theory, scientists have shown that the origin of archaeoviruses (virus of archaebacteria), bacterioviruses (virus of bacteria), and eukaryoviruses (virus of modern cells) seems less likely to happen as these viruses have conserved protein folds (three-dimensional protein structural folds) which are crucial in virion synthesis and other important functions of a virus. This indicates that viruses may have originated and evolved prior to the splitting of LUCA into modern cells.
How did viruses emerged?
As of now there is no clear answer to this question. Few researchers believe that ancient evolved into viruses through a hybrid model;
Step 1. Viral genetic material evolved in a pre-cellular world
Step 2. Viral propagation using cellular machinery initiated once the cells popped up in evolution.
However the other group do not agree with them and alternatively proposed a reductionist approach of viral evolution; For some reason, ancient cells discovered advantages of parasitism and multiplication through released particles (in this case-virions). With time these ancient cells devolved as released particles (virions) were able to propagate independently.
Any improvements in the evolutionary studies of Virus?
The current molecular and phylogenetic methods are not advanced enough to study the origin of viruses. However, structural biology (3D protein structure) can be a potential alternative to solve this long-standing question. This is not an assumption but a ground truth; the changes in secondary, tertiary and quaternary protein structures are extremely slow (3-10 times) as compared to the alterations in protein sequence (the primary structure). Since protein folds and structures are highly conserved across all the species, there is a possibility that the protein folds which we see today were also present in pre-LUCA cells.
My thoughts
In the light of above-mentioned observations and theories I can say that the viruses are primitive, emerged after parasitism came into existence and mostly in pre-LUCA cells but their trajectory in the evolution is still unclear to me.
What are your thoughts? Let us know in the comments below.
Reading sources
- Discovery of the first virus, the tobacco mosaic virus: 1892 or 1898? C R Acad Sci III. 2001
- The two ages of the RNA world, and the transition to the DNA world: a story of viruses and cells. Biochimie. 2005
- A phylogenomic data-driven exploration of viral origins and evolution. Sci. Adv. 2015
- Origin of viruses: primordial replicators recruiting capsids from hosts.Nat. Rev. Microbiol. 2019
- Investigating the Concept and Origin of Viruses. Trends Microbiol. 2020