The Beginning of the End
By Sukanya Chakraborty
Viruses. The conquerors of life. Microscopic agents that can transform the face of the earth into a remarkable dystopian version of itself and make upheaval and unpredictability daily realities.
A virus has the potential to exploit an astonishing variety of mechanisms to infect and hijack host cellular machinery, sometimes causing irrevocable damage. Be it antigenic variations, mutations or zoonotic spillovers, these simple entities have mastered them all. And yet all they are constructed of, are pieces of genetic material wrapped in protein. How did they arise then, in the first place, and why do they exist? These questions have plagued the scientific community for several years and the quest to find the origin of viruses is still fervently ongoing.
Despite the debate currently surrounding the history of viruses, there are widely accepted hypotheses regarding the same. The “progressive hypothesis”, bases the creation of viruses on the ability of mobile genetic elements (elements which can “jump” from one place to another within genomes) to leave one cell and enter another. This can be very well applied to the Retroviruses (such as the infamous Human Deficiency Virus) which integrate their genetic material into that of the host after reverse transcribing it into DNA from RNA. The eukaryotic genome components, called retrotransposons closely resemble this process. They often encode the enzymes reverse transcriptase and integrase, using which they can convert into RNA and get incorporated in new locations within the genome. It is believed that such elements might have acquired a few structural proteins and entered a new cell, forging an infectious agent. Astoundingly, more than 40% of the human genome is composed of these elements.
On the other hand, the “regressive hypothesis” attributes the origin of viruses to a degenerative process. The idea suggests that as free-living complex organisms gradually adopted more parasitic lifestyles, they lost certain genetic components, and thereby transformed into viruses. According to proponents of this hypothesis, a pair of independent organisms initially developed a mutualistic relationship. Over time, the relationship turned parasitic, as one organism started to rely more and more on the other for its survival. As the once free-living parasite became more dependent on the host, it lost previously essential genes. Eventually it was unable to replicate independently, becoming an obligate intracellular parasite, a virus.
The last school of thought, the “Virus-First Hypothesis” or the “Virus World Hypothesis”, posited by Koonin and Martin in 2005, deems viruses to be the first entities to have existed, even before the origin of cellular life. The RNA World Hypothesis which has received great impetus in the past few years, states that RNA was the first biomolecule to have been formed. In synchrony with this therefore, some of these information carrying and replicating RNA molecules may have infected the first cells. These relics of pre-cellular life possibly evolved and developed later into the manifold forms colonising nature today. The discovery of the exotic “giant” viruses led many researchers to endorse the Virus World hypothesis. These novel viruses, containing thousands of genes, heralded an entirely new class of these organisms at the convergence between the living and the inanimate.
Irrespective of their origin, however, one cannot ignore the salient fact that viruses participated in and modelled evolution and natural selection by coexisting with unicellular and multicellular organisms for billions of years. The relentless attacks and counterattacks by viruses on hosts through gene rearrangements, recombinations and mutations characterises the concomitant evolution of viruses and their host cells. The advantages that several viruses display in their replicative cycle may have been conferred to them by acts of so-called “molecular piracy”. This very process weaves a great deal of difficulty in isolating the ancestry of viruses, as their genome is often an amalgamation of many types of intermeshed host and foreign genes.
Another barrier in studying virus inception is the incredible host range spanned by them, from humans to horses, seaweed to bacteria to even other viruses! And of course, they do not leave any fossil evidence, making morphological phylogeny quite unreliable. Decoding this “patchwork” genome, is however, not an impossible feat, and scientists have gained several insights from sequence alignment studies. Analysing contemporary viral genomes and mutation rates allows us to approximate the time of divergence from other groups, and trace plausible common ancestors.
Of course we cannot say which hypothesis is correct. It may not come as a surprise if we discover that the different classes of viruses arose by different mechanisms, or if they arose multiple times during the course of evolution of myriad life forms. Maybe, there are completely unknown and covert mechanisms still waiting to be discovered in the saga of these perfect parasites.
1] Forterre, Patrick, and Morgan Gaïa. “Giant viruses and the origin of modern eukaryotes.” Current opinion in microbiology 31 (2016): 44-49.
2] Moelling, Karin, and Felix Broecker. “Viruses and evolution-viruses first? A personal perspective.” Frontiers in microbiology 10 (2019): 523.
3] Wessner, D. R. (2010) The Origins of Viruses. Nature Education 3(9):37