What Came First - Cell Or Virus?
Are you curious about the origins of viruses? Which came first - cells or acellular viruses? Keep on reading to uncover the fascinating answer!
Viruses! They are all around us. Some of them bother us a lot; it can come as a common cold from the playground, a deathly pandemic, or more recently, a handy tool in gene editing. But have you ever wondered where these pesky little fellas came from? To be honest, nobody really knows, but scientists have come down to three most plausible explanations for their origin: The virus first hypothesis, The escape hypothesis, and the Reduction hypothesis. Hopefully, by the end of this, you can win a debate with anybody who says they are extra-terrestrial (some people really believe that). So, let’s break them down one at a time.
THE VIRUS FIRST HYPOTHESIS
At the dawn of life, there were only RNA molecules that could qualify as a precursor to life. They were capable of self-replication and possessed enzymatic activity. These molecules invaded a proto-cell type of structure. Let’s call them viruses here, shall we? Now, these viruses predate cells and contribute to the rise of cellular life. But that sounds like a paradox because the pre-existence of cells is necessitated for the replication of the viruses, as they don’t have machinery of their own! This is exactly the fallacy that led scientists to question this hypothesis and challenge the existence of an independent viral world. This, however, gives us an insight into the deep-rooted relationship between viral and cellular molecules. It illustrates the emergence of a cellular world from an ancient RNA world by showing the transition of primordial RNA molecules to cellular life forms.
THE ESCAPE HYPOTHESIS
Viruses were originally integrated into the genetic material of host cells but managed to break free from the cellular system. Over time, they underwent evolutionary changes by acquiring genes through horizontal gene transfer (HGT). HGT is a process where genes are transferred between different organisms. This hypothesis is consistent with this phenomenon. The DNA and/or RNA molecule could’ve been a part of plasmids or transposons. Once these fragments obtained a protein coat, they transformed into autonomous entities with the ability to infect the cells they had previously eluded. The virus supposedly underwent a gradual change from genetic materials inside the cell to them being separate entities capable of infecting other cells. However, this hypothesis becomes wonky as it fails to explain how the virus acquired structures unique to it that are not found in cells (such as a capsid). It is also unclear about the source and nature of the origin of a virus. Furthermore, a case of a self-assembling capsid, through evolutionary mechanisms, is rare in nature.
THE REDUCTION HYPOTHESIS
This hypothesis supports the idea that cells actually came before viruses and that viruses emerged from small primordial cells. Except for replication machinery, those cells lost their cellular components gradually with evolution. This is supported by the recent discovery of giant viruses (called….Girus) like mimivirus and megavirus with overlapping genomic and physical features when compared to parasitic bacteria. This is a relatively easy concept to imagine. An RNA cell living inside another RNA cell as an endosymbiotic parasite is likely to lose its own protein-synthesising and energy-producing systems. For example, The replication of DNA in both human adenovirus and Bacillus subtilis bacteriophage Ф29 involves an unusual protein-priming mechanism not found in the cellular realm. Additionally, they both encode a distinctive DNA polymerase belonging to the B subfamily. Furthermore, large DNA viruses like mimivirus, vaccinia virus, and pandoravirus exhibit a greater resemblance to prokaryotes (bacteria and archaea) in their approach to handling newly synthesized genetic material than they do to other viruses. Despite being a scientist’s favourite, this hypothesis has shortcomings too. It cannot explain where the diversity in their structural and genetic complexities comes from.
CONCLUSION
In case you were wondering, where does this leave us? How do any of these hypotheses explain life in the biosphere as we see it now? While there may be differing views, viruses are widely recognized as significant contributors to the evolution of cells. For instance, viruses might have played a role in facilitating the shift from RNA to DNA during the genomic evolution of cells! Source So do we not have one solid hypothesis of where viruses came from? Unfortunately, we don’t. It is still in the "how did dinosaurs die” phase, and scientists frequently use a concoction of all three theories to explain the emergence of such a mystery. In the end, all of it boils down to one thing, life finds a way.
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