So while this post is going to be about viruses, it's not about SARS-CoV-2. And while the title asks the admittedly click-baity question "are they alive?" I'm not actually going to try to answer that question. I'm more interested in taking a look at what we mean when we say "alive." I love to poke at what happens when we find things outside the neat categories humans try to put things into. "Alive" may be the category nearest and dearest to our hearts and possibly one of the easiest to poke holes in.
What exactly is a virus? I think we can safely define them like this: they are organic (regardless of whether they are alive or not) structures that can replicate using either DNA or RNA (a molecule similar to DNA used by most living things). To do so, they "infect" a cell, basically breaking through the cell's membrane or outer layer of defense against the outside world. They then hijack the molecular machinery the cell uses to replicate and begin to generate copies of themselves.
Like most things in biology (and probably all science), defining "alive" depends on who you ask. But it seems that most of us agree on a few characteristics: 1. It can reproduce (on it's own) 2. It has cells 3. It obtains and uses energy 4. It grows and 5. It can sense/move in response to/adapt to the environment.
There are a couple of go to arguments against viruses being alive based on this list of characteristics. Let's get the easy one out of the way: viruses don't have cells. Cells always have some sort of barrier that creates an "inside" and separates it from the "outside." For a lot of folks this is a dealbreaker: no "inside" and you can't be "alive," you're just a part of the environment. I've got a few issues with this, two I'll handle now and one I'll come back to. First, as organisms get more complex what is "inside" becomes less clear. Think about this: there is a passage that goes straight from your mouth to your anus. You aren't a a cylinder, you're a tube. Secondly, though maybe this is more opinion than fact, I just feel that saying "cells or bust" is a bit arbitrary. Imagine we found something that could reproduce, takes things in, spits things out but didn't have a cell (spoiler, we'll meet one of these things in just a bit). I'd be inclined to at least consider that these things are alive. Especially if you consider using DNA or RNA as a form of coding which not just allows it to reproduce itself but allows for mutations (changes due to the environment or a mistake in reading the code), that allows for changes across generations and therefore evolution. I would not be at all surprised if when we find life on another planet it did not have cells.
Probably the most popular argument against viruses being alive is that they can't/don't reproduce on their own. Again I've got a bit of a quibble with this: parasites. Most parasites cannot survive, let alone reproduce without their hosts. I have never heard anyone argue that because a parasite cannot survive or reproduce on its own this disqualifies it from being alive, yet it seems that many throw this line of argument out for viruses and expect the case to just be closed.
If we take a look back at the list of characteristics of life I also think that 4. It grows may also be a bit arbitrary. In fact single celled life doesn't really grow, at least not in the sense that multicellular life does. Bacteria and other single-celled organisms reproduce by dividing one cell into two. While there are stages in the processes that you might call "growth" it's definitely not the same as an organism being born, taking in nutrients, going through development and its body getting bigger.
Let's go back to the idea that to be alive you need to have a cell. Pretty recently we've found proteins that can, at least in lab settings, reproduce and create amino acids (molecules that are the building blocks of proteins which make up, well, most of everything alive). I'm not going to argue that these proteins are living things but I am going to argue that they represent a possible glimpse into the beginnings of life on earth. We know that in the history of earth there was a time when we went from no life to having life and that there was some process that allowed organic molecules like amino acids and proteins to eventually become "alive." What I will argue is that this is a spectrum. Rather than "alive" and "not alive" I think we can look at this as an evolution from inorganic molecules to organic molecules to RNA and DNA to cells to more complex organisms. Even in the most complex vertebrates like ourselves, everything happening in our bodies is chemistry. I don't necessarily know that you can draw a sharp line between chemistry and biology here. [To be clear: I'm not trying to argue here that organisms like us with lots of cells and tissues and organs are "better" or "more advanced" necessarily; we just came later in the story]. So going back to viruses: they also contain organic molecules like RNA and DNA and so, I think, could be considered a part of this spectrum of organic structures.
I think it's really interesting when one category starts to bleed into another like this: when inorganic becomes organic, when chemistry becomes biology. Our human categories really start to break down almost every time we look at a transition in evolution between two groups. When you look at the evolution of plants, for example, you have mosses which are considered plants by most but don't have stems or leaves which are generally considered things all plants have. But we don't always see these rule breakers at the evolutionary transition points. The platypus is a famous example of a mammal that does not give birth to live young. It isn't an evolutionary descendant of other mammals, it's just a different branch on the tree. Even the notion that all animals require oxygen has recently been overturned with the discovery of a species living in a completely oxygen-devoid part of the Mediterranean.
As we've seen we can form categories for living things by coming up with a list of characteristics, looking to see if we've got all of them and then if it does, put it in that category. As we've also seen, that doesn't always work. The other way we can form categories is through evolutionary relationships. If two things share a common ancestor (they evolved from the same species or, maybe in some cases, the same molecules) then we can put them in the same category. That's why we consider platypuses to be mammals: they don't give live birth but they do share a common ancestor with other mammals. Just like you and your cousins, platypuses and other mammals share the same stock, they're part of the same family regardless of characteristics. Recently, some analysis of the shapes of proteins in viruses indicate that they may in fact share common ancestry with cellular life. This is super preliminary but if more evidence starts to stack up and we become more confident that viruses and cellular life evolved from the same early organic chemical reactions on earth, I would be willing to say that viruses and cells are in the same category. Maybe still not alive. Maybe, the cousin of alive.
Sources:
https://bmcbiol.biomedcentral.com/articles/10.1186/1741-7007-8-30
http://www.bbc.com/earth/story/20170125-there-is-one-animal-that-seems-to-survive-without-oxygen
https://www.popsci.com/new-evidence-that-viruses-are-alive/
https://www.sciencedaily.com/releases/2015/09/150925142658.htm
https://advances.sciencemag.org/content/1/8/e1500527
https://www.sciencealert.com/amyloid-protein-self-replication-abiogenesis-contrasts-rna-world
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