This week I was asked to write a blog about viruses and virus-like particles. My professor decided to go easy on me and only wants me to discuss the evolutionary origin theories of viruses and which I think is most likely.

Welp.

Let’s start with a description of what viruses are, and how they are different from other organisms. If you read my last blog,[1] I mentioned that viruses aren’t considered ‘living’ by certain standards because they can’t replicate or produce energy without a cell’s assistance. They are basically a string of RNA or DNA with a protein coat that go around parasitizing cells (animal, plant, bacteria, protozoa, etc.) and reproducing. While some have more “accessories” like proteins that can help them replicate their genome, most lack the complex machinery that even simple cells have for energy production, metabolism, and other life processes. Nothing else on earth seems to look like viruses, so of course in our classification system that likes to group and link things together based on their similarities, viruses are just out there on their own.

There are three prevalent ideas that have been the main theories for where viruses come from. They are the reduction hypothesis, the escape hypothesis, and the virus-first hypothesis.

The Reduction Hypothesis starts with the presumption that everything started out as primitive cells. During the course of evolution, these cells somehow lost most of their machinery that made them cells, until all that was left was the genome (be it RNA or DNA), a protein coat to protect it, and some elements to help them reproduce that genome once they entered another cell. The fact that there aren’t any organisms that seem to be ‘intermediates’ between cells and viruses seemed to defeat this theory, as it is expected that there would have to be some sort of middle step that the cells went through on their degeneration to viruses. There’s also the question of how cells went from a lipid membrane (the outside-most layer of the cell made up of fatty molecules called lipids) to a protein coat in viruses (the protein capsid that protects the genome). But there is support for this theory in the recently discovered mimivirus and megavirus, both of which are larger than some small bacteria (a very unusual trait, as every other virus discovered up till now was too small to be seen even on a microscope). These viruses also have a very complex genome, and mimivirus in particular has enzymes and proteins that were only expected to be found in true, living cells.[2] Thus, these two viruses can be seen as the necessary intermediate for the reduction hypothesis.

The Escape Hypothesis also starts with cells. The idea is that pieces of cell genome—again, either RNA or DNA—‘escaped’ the primordial cell and decided they wanted to live on their own (figuratively speaking, of course). This theory came from similar genome pieces that we actually see in biology. Plasmids, for example, are small pieces of bacterial DNA that are separate from the cell’s genome. They code for special goodies (like antibiotic resistance) that are not necessary for life, and use their own machinery to replicate those genes. Plasmids are incredibly mobile, and easily passed between bacteria, even bacteria that aren’t related. Thus the idea that something like these could have escaped a bacteria and decided to parasitize the host’s machinery for replication isn’t so far-fetched.[3]

The Virus-first Hypothesis assumes that viruses were the original form, and as cells started to arise, they began to parasitize them and evolve along with them. There are some questions for this theory. If current viruses require cells in order to replicate and survive, what did these ancestral viruses use? If they started out as more primitive than the current viruses, where did they get all the complex components that they have today? If the natural evolutionary process leads to something large and complex like mimiviruses, why are there still small and simple viruses around that didn’t get the memo? There are also supports, however. Early viruses may have found some sort of ‘soup’ of nutrients to grow in, then transitioned to cells once they arose, losing their methods of autonomous replication in the process.[4] There are also, for lack of a better term, biologic components—virus-like particles—that seem to be examples of what these pre-viruses were like. Viroids are the smallest infectious agent known. They are nothing more than a short, circular strand of RNA; no protein coat, no particular complexity, just a little piece of genome floating around infecting cells, like a rogue plasmid. The idea is that these viroids are very similar to what the primordial viruses were like.[5] That still begs the question why so many viroids were completely left behind in the evolutionary journey, but it is unlikely to receive an answer any time soon.

Now on to the second part of the assignment: which theory do I find most likely. It may sound cliché, but I see no reason to choose one to the exclusion of the others. Perhaps there were primitive viroid-like organisms that existed before proper cells developed, then evolved with them to become the viruses we know today. But that explanation doesn’t seem to fit every known virus, such as the mimiviruses which look like intermediates between cells and viruses. Maybe they once were cells, then started to regress until they are now something akin to a virus, but larger and more complex than most any other known. Still other viruses may have arisen from “gene escape events,”[6] like the human hepatitis delta virus which contains coding genes very close to the ones found in human cells.[7]

Science is very limited when it comes to testing things for which we have no current comparison. Using the laws of the present to try and discover the laws of the past can only reveal so much. Thus, the question of the origin of viruses cannot be clearly discovered by what we know now; obviously what happened back then is no longer happening, or at least not in the same way. However, it is still interesting to see the incredible variation among these absurdly simple components, and marvel at how something not even considered to be alive has such a great impact on the world and its inhabitants.

---

[1] https://vetmedone.health.blog/2019/04/07/how-to-stop-a-virus/

[2] https://www.sciencedirect.com/science/article/pii/S0168170205002376?via%3Dihub

[3] https://mmbr.asm.org/content/74/3/434.long

[4] https://www.sciencedirect.com/science/article/pii/S1879625713001028?via%3Dihub

[5] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4609113/

[6] Ibid.

[7] Ibid.