About Rationally Speaking

Rationally Speaking is a blog maintained by Prof. Massimo Pigliucci, a philosopher at the City University of New York. The blog reflects the Enlightenment figure Marquis de Condorcet's idea of what a public intellectual (yes, we know, that's such a bad word) ought to be: someone who devotes himself to "the tracking down of prejudices in the hiding places where priests, the schools, the government, and all long-established institutions had gathered and protected them." You're welcome. Please notice that the contents of this blog can be reprinted under the standard Creative Commons license.

Tuesday, August 12, 2008

Life, what is it?

Big question this morning! Of course, plenty of philosophers and scientists have wondered about the question of what is life, and the answer has proved more slippery than everyone thought. We all think we know how to tell if something is alive or not, and yet it is remarkably difficult to come up with a precise definition of the concept (just like American Justice Potter Stewart once famously said of pornography).

When I was in college, popular definitions of life included things that grow, metabolize and reproduce, although by that definition fire is alive. My professor of biophysics at the University of Rome, Mario Ageno, wisely refused to engage in this discussion, but defined (somewhat jokingly) death as a sudden increase in entropy -- which would make life a system that maintains a locally low level of entropy. Richard Dawkins defines life as anything subjected to Darwinian evolution, but by that token one has to admit that computer viruses and memes (whatever they are) are alive, a step that I, for one, am not prepared to make.

What about real viruses? There too the debate has raged ever since I can remember, with few biologists claiming that viruses are living organisms, and a majority of life scientists opting to view viruses as intracellular parasitic organic machines, but not quite alive (they do not have metabolism, they don’t really grow as much as they “assemble,” and their reproduction is entirely and intimately dependent on the cells they attack).

Things have just gotten a bit more complicated with the publication in Nature (4 August 2008) of a paper by La Scola and collaborators, who have announced not only that they have found the largest virus known, but that the organism in question (a term used on purpose by commentator Curtis Suttle of the University of British Columbia) is made sick by another, smaller virus, known as Sputnik. In other words, it turns out that viruses can be attacked by other viruses, in the same way in which they attack bacterial and other cells, a behavior that was previously thought to be limited to living organisms.

The giant virus is known as mamavirus, and it normally infects amoebas. It has been isolated from a cooling tower in Paris, and is part of a family of very large viruses discovered only in 2003. These viruses were initially mistaken for bacteria because of their size, with a genome of about 900 protein coding genes. This is about three times as much as the largest previously known virus, and larger than some bacteria. But what sets aside the new giant virus is the fact that inside it researchers found a tiny virus, Sputnik, with only 21 genes. Sputnik takes over the protein-making machinery of mamavirus, just like mamavirus takes over the cellular machinery of an amoeba, to reproduce its own particles.

Even more interestingly, three of Sputnik’s genes are very similar in sequence to those of the mamavirus, suggesting the possibility of horizontal gene transfer between viruses, just as it commonly occurs among bacteria -- yet another difference between viruses and bacteria that is disappearing under our nose because of new research. Moreover, giant viruses are likely not exceptional at all, as a genomic survey of marine waters has produced many sequences similar to those of the few known giant viruses, implying that perhaps these organisms are common parasites of plankton, something that may imply an important indirect role of giant viruses in marine biological cycles and, ultimately in the shaping of planetary climate.

So, are giant viruses alive or not? The answer depends on what you are willing to count as essential properties of life, but certainly if giant viruses make the cut, then so too do smaller viruses like Sputnik, since there are no qualitative differences between the two groups, aside from the sheer size and complexity of the genome. Anyone out there who would like to submit a working definition of “life” and argue why (or why not) viruses should be excluded from it?


  1. It is interesting to me that we have a very chauvinistic view of what life is despite the fact that we cannot define it.

    I submit that what we call 'life' or 'alive' is an instance of ontological suicide.

    If one man in the world is 'alive', that is, without any other living thing around, for how long will he be alive? Not very. It seems to me that an (one) entity, by itself, cannot be alive. I would almost say that only an ecosystem is alive, and that the seemingly independent organisms within said ecosystem are the very things that make it so.

    An analogy can be made with an economy. One business does not an economy make. For to whom would he be selling? From where would he get new product? Any person from whom he buys as an investment would very easily arguably also be a business.

    I know there are probably plenty of things wrong with this comment, but at least I'll learn from criticism. :-)


  2. This is interesting to, in the quest for an understanding of our planet's abiogenesis. That philosophers of science have such trouble with a scientific definition of the threshold between non-life and life, surely makes the creationist claim that abiogenesis is impossible that much more laughable. So, when was the breath of god made necessary?

  3. I don't think we need a precise definition of life any more than we needed a precise definition of planet. A set of operational definitions to be used in different contexts is all we need. Change them as seems useful.

  4. Are we going to define "life" by what it is or what it does? (Or is that a meaningful distinction?) Because it seems to me that mamavirus came into the tent because it is infected by Sputnik (what it "does"), but then Sputnik came in too because it can't be distinguished "qualitatively" from mamavirus (what it "is"). I have no answer (heck, I'm a criminal defense attorney), only the question.

    ("What it is, bro?"
    "Ain't nothin'.")

  5. i won't try to answer your question, i rather want to ask one.

    from what i've read, the fact that the mamaviruses get sick is the great argument to consider them alive. what is getting sick? in this case, i believe it means that the virus doesn't replicate as well when it's infected by the sputnik virus. is a rusty engine sick? is it then alive?

    i'll venture a bit of an answer - it seems to me that the organism would need at least cellular machinery to perform some main functions to be considered alive.

    my main question: is the mamavirus fundamentally different from other viruses, or is it just big and can harbor other viruses in its casing?

  6. ...the organism would need at least cellular machinery to perform some main functions to be considered alive.

    I lean in this direction, too, but now I think we'll need a list of minimum-complement of cellular machinery.

    For starters:
    Self-replication tools
    1. Genes
    2. Gene-duplicators (e.g. DNA polymerase in all its multi-component glory)
    Homeostasis tools
    1. Physical enclosure in space and time (e.g. cell membrane or similar)
    2. Regulation of conditions around some mean value; feedback loops
    Metabolic tools
    1. Absorb energy and matter from environment
    2. Elimination of waste
    3. Conversion of molecules into other molecules, especially those related to the other functions described above.

    I'm surely missing something...

  7. Max, Richard Dawkins actually wrote that whatever the definition of "life", it will need to include a genetic component, which excludes fire.
    Nick: If only systems can be alive, and not individuals, does that mean I can murder you with impunity, because by your definition, you alone as an independent entity do not have a life to take...

  8. Life can be defined by revealing its essence: strike a match and, without reciting its attributes of heat and light etc, tell of its essence. You, nor I, cannot do such a thing. At best we can continue to study the attributes and learn, infinitely, more aspects of that which will always approach the essence but never become it, as the essence of anything is always separated from its attributes.

  9. "Richard Dawkins defines life as anything subjected to Darwinian evolution, but by that token one has to admit that computer viruses and memes (whatever they are) are alive, a step that I, for one, am not prepared to make."

    Yea for Dawkins.

    "Subjected" just means a thing could not go on to reproduce without food, a given set of genes, etc. But IF dependency is key to defining life, (or even evolution) then the original source of the "first life" is not just marginally important. It is, in essence, the ONLY ISSUE.

    The fact that some will deny that the role of evolution is, at best, minuscule, does not do away with the more obvious fact that life as we know it is dependent on mostly non-living, non-evolutionary components in the universe.
    Light, heat, water, physics, chemistry and on and on...

    And evolution shaped these particular factors exactly HOW?


  10. I have no illusions that I am qualified to define what is life and what is not but I am fascinated by discoveries of objects/organisms that are in the grey area between the two conditions.

    Cal wrote: "And evolution shaped these particular factors exactly HOW?"

    Cal, go (back?) to school and get a degree in evolutionary biology. Don't expect to have the whole thing explained to you on an internet blog.

  11. Life is defined in detail here:


    Put some together, win $1 Million.


  12. Late to the part, as usual these days...

    Mamma mia! Is that the Italian version of the mimivirus, eh? :O)

    Or does it have two names? I actually am not very familiar with these guys. I remember their name since they infect the amoeba group I studied for my PhD (phylogeny of Acanthamoeba), so they caught my attention some day in the past.

    And I remember that their genome is about 1.2 million bases. That's very big indeed; as Massimo pointed out, beats many bacteria. Hell, we sequenced a bacterium this week that is quite independent (vaginal pathogen, I don't know if it's an obligatory parasite), and its genome is a meager 1.65 million bases.

    So, it appears to me that many people commenting here are leaning towards an "organizational" definition, is that it? By that I mean: it's not what the organism is composed of, but how it is organized and whether it can do some (minimal) things.

    These definitions are trouble, as pointed out before. Parasites are the thing here. OK, viruses are quite strange and easy (well, easier) to dismiss. But what about other, more complex organisms that cannot close the life cycle by themselves, but need (like a virus) to parasitize another organism? Some wasps reproduce like that, injecting their eggs into a paralyzed spider (or was it some insect?). Then the eggs hatch, and the young feed on the still living host until they can go live by themselves. So, if there is suddenly no host, then isn't the wasp alive? Sure, it has the potencial to reproduce and etc., but so does the virus. But nobody would hesitate to consider the wasp alive.

    What I'm going at is the cutoff level of the necessarily arbitrary definition. Specially since we keep discovering that, as Massimo pointed out, things are getting blurrier by the day with new discoveries, and it is certainly all along a continuum. So where along this continuum do we cut? But then, to me it's artificial and "academic". It's not living because we defined it so, not because of some really intrinsic qualitative difference. Kinda like saying (wrongly, by the way) that snakes are not tetrapods because tetrapods have four legs. It's classifying by absence of something. A recipe for trouble in systematics, and probably here too.

    I myself do not like to consider viruses as non-living, but the problems that considering "at least somewhat" alive are considerable indeed. Like the computer virus problem, mostly. But then I think we are doing something very wrong too by dismissing a definition just based on that problem! I mean, we start off with the list of living things and THEN go on to find the sentence that fits them all, and only them, but nothing that "makes us uncomfortable". Is that acceptable, philosophically and/or logically speaking?

  13. What about this holistic-technical definition?

    whatever kind of thing that is able to self-organization (as much of the inert matter) and is composed at least by some nucleotides.

    Just think of ethical and political implications of a broader definition!.

    I think is better think of life as a complex term that should be defined depending on the subject that We are studying. All of us know what is life about, but a broader technical definition is more complex depending on the subject that what life apparently is.


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