Viruses and the definition of “life”

One of the things we often debate here is the definition of “consciousness,” but consciousness is far from the only concept that is difficult to define.  Others include religion, democracy, free will, and biological life.

Life has a number of definitions, many of which are suitable for particular purposes.  If I recall correctly, NASA, for purposes of detecting it in extraterrestrial environments, defines life as anything that reproduces and undergoes evolution.  But many biologists insist that homeostasis is crucial aspect.

Cells are often referred to as the basic unit of life.  That certainly seems true for the homeostasis definition.

(Interestingly, cells may be the basic unit of life, but they’re not the basic unit of capability.  For that, we have to drop down to proteins, molecular nanomachines that do the actual work in biology.  A simple cell has tens of millions of proteins.  More complex cells, such as neurons, reportedly have tens of billions.  It pays to remember that life began as, and remains, a molecular enterprise, with even cells being vast ecosystems.)

But here’s a question.  Are viruses alive?  It depends on which definition we use.  They reproduce and undergo evolution, producing new species and strains all the time, as most of us are only too aware of these days.  But they don’t have homeostasis.

Outside of a cell, in their “virion” phase, they consist of some genetic material (RNA or DNA) surrounded by a protein coat, both for protection and containing penetration machinery to get through cellular membranes.  Once they’ve invaded the cell, they hijack that cell’s reproductive mechanisms for their own use, reproducing themselves.

Viruses appear to be evolutionarily ancient, perhaps co-evolving along with the earliest cells.  Both cells and viruses may have evolved from pre-cellular biology.  It’s tempting to think of the cells as farmers putting up a fence to protect their mechanisms and resources, and viruses as the raiders coming in to hijack and ransack those mechanisms and resources.

Thinking back to Aristotle’s three layers of soul, he would almost certainly have attributed a nutritive soul to cells.  But I’m not sure he necessarily would have for viruses.  He might have seen them as soulless parasites.

But is that entirely fair?  It seems like viruses simply use the resources available to propagate, just as all heterotrophic life consumes other life to survive and propagate.  In evolution, they also provide an important means of horizontal gene transfer, increasing genetic diversity in non-sexually reproducing species.  And they’re often used today as vehicles for delivery gene editing therapies.

It’s worth noting that viruses are not the smallest infectious agent.  Viroids are even smaller.  Composed of a short circle of RNA with no protein coat, they only infect plant cells.  Their existence seems to have inspired the idea that life began in an RNA world.  Viroids may be leftovers from the earliest primeval life, or proto-life forms.  If viruses are alive, then it seems hard to exclude viroids.

There are even smaller replicating agents: prions.  These are proteins that have become misfolded, with the disturbing ability to transmit their misfolding, leading to a number of neurodegenerative conditions, such as “mad cow” disease.  Are prions alive?  Most biologists say no.  They do reproduce, but I haven’t seen any assertions they undergo evolution.  They may be more like crystals or clay, albeit of a more organic variety.

So what is life?  Does it require a cell?  Or do viruses and viroids count?  Thinking in terms of extraterrestrial life, it seems possible to conceive of mechanisms we might discover that would challenge any definitions we come up with.  Suppose we encountered a sort of organic clay somewhere.

Reality often seems to delight in ruining our little categories.

23 thoughts on “Viruses and the definition of “life”

  1. All of my life we have been arguing about where the lines are to be drawn between categories we created. (Is toothpaste a solid, a liguid, or a gas?) No matter how many times we do this we can easily find examples that sit right on the fence between two categories (virri may be alive, may not be).

    We would be a lot better off if we automatically assumed that all such schemes of categories are approximate and for our convenience only. Then we could dispense with the interminable arguments re fence sitters: what side of the line are they on?

    Liked by 2 people

  2. In the spirit of getting information about viruses correct … [dons immunologist hat]. The following does not change anything about your excellent post. It just provides some more info.

    I can think of at least three kinds of viruses. All of them are just vehicles for getting genetic information into a target cell, but only one kind is “some genetic material (RNA or DNA) surrounded by a protein coat”. Another kind is more like a tiny cell, but without any machinery inside. That is, when it leaves the cell, this kind of virus takes some of the cell membrane, the lipid bilayer, of that cell with them to form their own mini-cell. The coronavirus is this second kind. (That’s why soap is effective against this kind, because soap breaks up the lipid bilayer.) The third kind, instead of being surrounded by a protein coat, has a fairly elaborate mechanism to store the genetic material in the “head” and an apparatus to inject that material into the cell. Bacteriophages are this third kind, and only infect bacteria, I think.

    What they all have in common is just genetic material on the inside, some sort of protein on the outside that finds a target cell by binding with some target protein on the surface of that cell, and some mechanism to get the genetic material into that cell.

    [doffs immunologist hat]

    Liked by 2 people

    1. Thanks for the additional info! I forgot you had a background in this stuff.

      That actually reminds me of something I’ve been meaning to lookup, but maybe you can answer it. When we acquire immunity from an infection, at a high level, what are the mechanisms of that immunity? Where does the “signature” of the infection get stored and how is it recognized in the future? Is it just that a small number of Memory B and T cells specific to that infection remain around forever? Just curious.


      1. Simple answer, pretty much. The only thing I would add that wasn’t clear is that before the infection you have a small number of responsive B’s and T’s, and the infection will cause the responsive ones to multiply. Then they tend to hang around a long time, but not necessarily forever.

        For those watching at home (and to simplify a bit), the B’s make the antibodies. Any one B-cell will make just one kind of antibody it’s whole life. There are different types of T-cells, and they do a few things. One thing is to tell the B-cells it’s time to ramp up antibody production and to reproduce. Those are helper T’s. Some will kill cells that look like they are producing viruses (when the virus proteins start showing up on the surface of the cell). Those are killer T’s. There’s lots of other T’s doing various stuff.

        [again, based on knowledge from about thirty years ago]

        Liked by 2 people

        1. Interesting. Thanks!

          [And I know what you mean about pulling knowledge from thirty years ago. Earlier this week I had to explain an accounting concept to someone, which I have a background in, but just about as far in the past.]


  3. Even the Solar system turns out to be hard to categorize.

    I was just reading a set of essays by Carlo Rovelli, and in one of them he points out that, although most systems have billions+ of variables (quantum numbers associated with each particle of the system), we only interact with them based on a few. Gas pressure and temperature, for example, are gross collective properties. The myriad micro properties of the particles just aren’t a factor.

    When we categorize, we seem to select those gross properties of interest that allow us to fit things in categories. But when we look more closely, we tend to see individuality. People are a good example: the mob versus the person.

    Liked by 1 person

    1. Excellent point. Our conception of things is often about what they mean to us. Science can often helps us get outside of that, but our starting point is in terms of those affordances, and I’m not sure we ever completely escape them. Even the scientific view is anchored in them. It makes you wonder how differently a true alien might see things. (Although assuming they evolved somewhere themselves, it doesn’t seem like it could be that different, but that may simply be a failure of imagination on my part.)

      Liked by 2 people

    2. I think this is an important point, and I’ve also been following Rovelli’s recent essays. Of course, my interest stems from my recent fixation on mutual information and unitrackers. When we “categorize”, we create a unitracker associated with “gross properties of interest”, and when we subcategorize, we create a new unitracker by associating additional gross properties with the first unitracker.



      1. What fascinates me about it is that, based on my observations of dogs, they don’t categorize anything like we do. It’s a key difference between a fairly “conscious” animal — one with apparent thoughts and even apparent opinions — and us.

        To my dogs, each tennis ball is a distinct entity and I can’t secretly swap one for another during play. That always reject the substitute. To me, those are three damn near identical tennis balls, all from the same can, all with the same wear.

        I don’t think dogs have a category for “tree” for example. Each is distinct based on their smells and location. But I have a tree category and sub-categories for pines (and sub-pine categories!), oaks, maples, elms, lindens, sumac, etc.

        To me, our ability to abstract into categories is a primary characteristic of higher intelligence.


        1. I actually do think dogs can categorize, or perhaps more accurately, form associational concepts. But with a couple of very significant differences.

          The first is that smell is going to be a much larger part of their categorization than it is for us. Primate smell is fairly atrophied compared to most mammals. The balls that look identical to us probably smell very different to the dogs. And dog vision is fairly limited compared to ours.

          But the second is they lack the ability to metacognitively realize they’ve formed a category, attach a symbol to it, and subsequently use it for additional reasoning. That type of abstract symbolic thought, it seems, is unique to humans. But maybe that’s what you meant by “categorize”?


          1. Yes, that’s what I meant by categorize. 🙂

            Forming an association, to me, is something else (a link or a relation). By category I mean a grouping based on some abstraction. Mathematically, a set with a member function.

            A canonical example: Way back in grade school, those worksheets with pictures of tall skinny things, trees, telephone poles, stop signs, etc. We were supposed to circle the ones that were trees. We were learning the category “TREE”.

            And now it’s a CAPTCHA where you’re shown a bunch of photos and told to click the ones that contain cars (or trees or whatever). Testing your human capability to decode pictures and correctly categorize their contents.


          2. I see categorizing as combining concepts. And I expect dogs have categorized trees along with bushes and hydrants as “things likely to have interesting smells”, interesting being mostly due to other dogs. I think what separates humans is the capacity to combine arbitrary categories into one with ease. It’s the arbitrariness that leads to things like selves (and so, self-awareness) and words (and so, language).



          3. Male dogs do have a very general category: “vertical surfaces I can pee on.” This covers everything from fire hydrants to human legs. Female dogs are aware of this and recognize vertical surfaces as “message boards”.

            For them it’s all very concrete and specific. Only the most general categories. As you say, and per my original comment, we recognize all sorts of properties as index markers. (I’m not sure I’m down with “arbitrary” so much as “abstract” since there is usually something behind the index.)


  4. If you want to encounter “organic clay”, read Alfred North Whitehead”s, “Modes of Thought”—the prose is impossibly sluggish, yet one still detects a hint of liveliness.

    Liked by 1 person

    1. Thanks! That was an interesting article. I think I’d read it before, but a long time ago, and it was good to review it again. Giant viruses seem like another threat to our little categories. There was also this more recent article:

      It doesn’t seem quite right to say the first life were viruses. Modern viruses depend too much on cells. It seems more accurate to say that they co-evolved with cellular life, and in many ways they may be closer to early pre-cellular replicating molecules that, as the article mentioned, utilized free floating resources.

      Liked by 2 people

  5. I think life is messy and our categories (symbols, language, etc) are precise, but precise only in terms of the paucity of their component parts. If you open up (decompose) life or reality and look inside, you discover infinite regression (or at least a humongously large finite number), Everything we model (life, reality, jet planes, …) include only the major factors (in our less than humble opinions). Great post and comments. Kudos to all.

    Liked by 1 person

    1. Thanks Mike!

      The regression usually comes to an end at the molecular level. (I say usually because quantum physics may have a role in some processes, such as the “quantum walk” of photosynthesis.) At that level, we see the organic chemistry underlying everything. Not that it’s simple by any stretch. The book I read on microbiology was among the most complicated things I ever attempted.


  6. Starting with the definition of “life”, this interesting discussion has revolved around the notion of categorizing. Categories pick out features that are important, from some perspective. The basic function of definitions is to clarify these categories – and the perspective – at the meta-level of communicating the sort of category we believe we’re thinking with.

    Steven Ruis says “We would be a lot better off if we automatically assumed that all such schemes of categories are approximate and for our convenience only. Then we could dispense with the interminable arguments re fence sitters: what side of the line are they on?” This is the perspective of “science” – that our words and their categories are simply arbitrary terms used to focus our discussion on features that are key (defiing) from some perspective. Bio-semiotics uses this perspective to view all of life as a process of categorizing – icons, indexes and (for humans) symbols.

    But the perspective of ordinary life and ordinary language is hesitant to view our categories as arbitrary. Wittgenstein links our ordinary categories – such as “tree” – with our ways of life. From this perspective, the question I’d like to propose is … How does the discussion using the word “life” from a scientific perspective (find the keys to control of phenomena – causation or evolution) relate to the discussion using the word from an ordinary language perspective? What is it that we want from our category “life” in our ordinary lives? Yes, we want some control over life (health issues); and yes, we want some understanding of the possibilities of different kinds of (extraterrestrial) life. But is that what life means at its core? Are we just categorizing machines? or is there something about our experience of life that takes us beyond our categories? Can the ordinary language word “life” convey the concept of seeking meaning in the expression of our social selves, rather than in categorizing the world? Shouldn’t this expressive function of language be at least a required point in discussion of the meaning of “life”? It would be great to hear your views on the meaning of “life” from this perspective. Thanks for the stimulating conversation – though I’m not sure I’ve contributed effectively.

    Liked by 1 person

    1. Hi rqparks,
      Thanks for commenting, and welcome!

      I think you’re addressing a broader conception of “life”, one that can always come up in these kinds of discussions. I focused on biological life in the post, which could be seen as exploring the lower boundary of the concept of life. Your question, I think, is exploring more the “upper” boundary, the idea of a living autobiographical self.

      Or we could look at it in terms of Aristotle’s three types of souls: the nutritive or vegetative soul, the sensitive soul, and the rational soul. The nutritive soul seems shared by most living things. (Although as I noted in the post, viruses, not having their own homeostasis, can be seen as falling outside of that conception.) The sensitive soul seems to include most animals. Some of these animals are social species, and will have a form of social life.

      But with humans, we get the rational soul, the ability to reason using symbolic thought. It’s here that we have, at least form our viewpoint, the fully developed autobiographical self.

      I don’t know if this is along the lines of what you were thinking, or if you’re more interested in exploring the way to live, or some other broader aspect of this kind of life.


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