Life with a capital L? (Like Zimmer with a capital Z?)

Over on Facebook, David Hillis, an evolutionary biologist at the University of Texas, took up my question as to whether anyone can define life in three words. His short answer was no, but his long answer, which I’ve stitched together here from a series of comments he wrote, was very interesting (links are mine):

Like all historical entities (including other biological taxa), it is only sensible to “define” Life ostensively (by pointing to it, noting when and where it began, and following its lineages from there) rather than intensionally (using a list of characteristics). This applies to the taxon we call Life (hence capitalized, as a formal name). You could define a class concept called life (not a formal taxon), but then that concept would clearly differ from person to person (whereas it is much less problematic to note examples of the taxon Life). So, I’d say that I can point to and circumscribe Life, and that it the appropriate way to “define” any biological taxon. A list of its unique characteristics is then a diagnosis, rather than a definition. So, I’d argue that any intensional definition of Life is illogical (does not recognize the nature of Life), no matter how many words are used.

Defining Life (the taxon) is like defining other particular historical entities. We don’t “define” Carl Zimmer or the United States of America by listing out their attributes. Instead, we point to their origin and history. The same should be true for Life. If we ever discover a Life2, we’ll have a new origin and history to point to.

The question people actually want to ask is “Are there entities in the universe that are similar to the Life we know about here on Earth?” The answer, of course, depends on what people mean by the arbitrary meaning of “similar”. One person might answer “I mean ‘self-replicating with variations’.” Then, the answer is yes: humans have created imperfectly self-replicating systems (“artificial life”) here on Earth. But then someone else says “But that is not what I meant by similar…I meant that they had to have metabolism and cellular structure and a nucelic-acid-based genetic system.” OK, then we have to keep looking to find something that similar. But then someone else says “But that’s pretty arbitrary…I’d still consider it alive if it didn’t have cellular structure.” Exactly…it is indeed arbitrary to argue over how similar something has to be to consider it “similar” to Life. So, in the end, we can ostensively define Life (by referencing its origin and history), and we can do the same for other historical entities that some people might also want to say are alive, but there can be no simple “right” answer that will satisfy everyone about which entities should be considered alive, because we all emphasize different characteristics in defining an arbitrary class concept of “life”.

36 thoughts on “Life with a capital L? (Like Zimmer with a capital Z?)

  1. Interesting. And I reckon Hillis would say that debates over whether viruses are a form of life is just like other taxonomic debates (how many kingdoms, etc.)?

  2. Spoken like a true phylogenetic taxonomist! I’m having quals flashbacks with the diagnosis/definition stuff.

    Since the phylogenetic definition of a taxon is, under phylocode, a listing of the tip clades which specify a common ancestor, I would say our current best definition of life, phylogenetically speaking, is:

    Bacteria. Archaea. Eukaryotes.

    Conveniently, three words! Really, one should use three species, but if we can assume each domain is a clade (there is substantial evidence for this, but also some contradictory evidence), then the definition would hold.

    If we discover something that branches more deeply than these three groups, well then, we make a new name for that taxon. “Lifeialans” or something.

    (Yes, I’m leaving out viruses and the whole debate about whether or not a “tree” structure even works here. But he nice, I’ve got only three words to work with!)

  3. Actually, Nick, you do include viruses in that “phylogenetic definition” of Life, since the many separate lineages of viruses are almost undoubtedly also all descendants of the common ancestor of Bacteria, Archaea, and Eukaryotes, whether they are escaped “parts” of these organisms or simplified parasites. So, I’d certainly consider viruses a part of Life…they are just a part of Life that is dependent on cellular organisms.

  4. I think that self replicating,metabolism and so on…. all are just features of life but not definition. Definition of life should be a molecule or a complex to be found in cytoplasm which initiate ‘life”. This is proven by the fact that viruses are not more than a DNA or RNA and not alive until they enter the cytoplasm then they start replicating and start ‘life’. The same is with prions that they are just protiens, and only when they enter the cytoplasm they start”life”. The same is with the sperm it leaves it’s cytoplasm outside the ovium and just when entered the ovium cytoplasm it starts ‘life’.

  5. Self-reproduction with variation is ‘true at Walmart’. It may be less true as you travel to campus but so is almost anything else. Pragmatic win.

  6. By the way, Life is indeed ostensively defined in this manner in the Tree of Life Appendix in “Life: The Science of Biology,” 9th edition (2011, Sinauer and W. F. Freeman) and in “Principles of Life” (2012, same publishers). The appendix of these introductory biology college textbooks also provides a brief diagnosis (list of distinguishing characteristics) for Life and many other major biological taxa, and shows how they relate to one another on the Tree of Life. You can access an interactive version of this appendix on the web at
    Click on “View the Dynamic Tree of Life”. Adjust the size to your screen, and click on names within the tree to see brief descriptions of the taxa. Then click on the names within those description to see photos, maps, keys, and a wealth of other resources for each taxon. The printed versions are easier for some people to use and follow, but the interactive online version is fun to explore with.

  7. Lots to possibly talk about, but I’ll stick to one thing. The idea of defining “Carl Zimmer” is a curve-ball. Individuals do not have definitions, they have personal identity, which includes some sort of “essence”. Questions about what constitutes “Carl Zimmer” or “personal identity” have deep historical roots. An important figure is Locke. Locke would say something like, ‘Carl Zimmer is the person has has a continuous memory of Carl’s life’; this is a definition related to a psychological property, and a property that involves consciousness. This is already messy. Others might combine this with a body structure that forms a continuous chain in space and time. I guess this could be called an “ostensive” definition. But it also precludes the possibility of finding a second Carl Zimmer. In short, using personal identity as the clarifying example doesn’t make things clearer.

  8. I respectfully disagree, John. My point is that Carl Zimmer is a particular historical entity, just as Life is a particular historical entity. We do indeed define Carl Zimmer ostensively, by pointing to his genealogy (his parents), his birth, and (at some point in the future) his death. We don’t expect to find another Carl Zimmer…we know that he is unique in time and space. Life (the taxon) is also unique in time and space. We can find another thing “similar” to Carl Zimmer, and define that thing as another person, but it won’t be another Carl Zimmer. In a similar manner, we can find another thing “similar” to Life, and define that historical entity (we might call it Life2), but it won’t be another Life.

    At the same time, there are characteristics of Carl Zimmer that we use to recognize him. In the same way, there are characteristics of Life that we use to recognize living organisms. But we should not confuse this with a “definition” of Life.

    As for using some other name for Life (like Biota): That would be OK, but my point is that when many (most?) people use the word “life,” they actually mean it in the sense of the taxon Life. Biology is the study of Life (the taxon; not life, the arbitrary concept). If you don’t believe me, pick up any college introductory textbook on biology, and after reading for 1,200 pages or so, ask yourself what the book has been about. I doubt you will have read a single page that was not explicitly about Life (the taxon).

  9. More thoughts on the ‘Carl Zimmer’ example. Because there can only be one Carl Zimmer, this is a bad example. Perhaps a better analogy would be searching for a human, on earth or elsewhere. If a putative human were found on Mars, what evidence would one look for to say this is, indeed, a human? Would it be a set of defining criteria (DNA, ability to breed with humans, a particular type of conscious experience)? Or would it be tracing evolutionary roots to a common ancestor? Or a combination?

  10. Yes, there can only be one Carl Zimmer. My point is that there can also only be one Life. We can search for more things “similar” to either one, but we’d have to say what we mean by “similar,” and I don’t see many people agreeing on what they mean by that when it comes to Life.

    When (or if) we do discover another system that resembles Life enough that some people want to call it “alive”, there will undoubtedly be a big debate. It will likely have many similarities to Life, but also many differences as well. Then, I think, many people will understand that Life (in our usual meaning of the word) is a particular thing, confined in time and space (just like any other historical entity or individual thing). We will probably call the new thing something like Life2, to distinguish it from the Life that we know so well.

  11. David, I appreciate your latest comments, but I still think what Thomas Holtz said makes ample sense. Strictly from a purely scientific perspective, should we discover life on, say, Mars or Europa, it does sound better to refer individually to Terran, Martian and Europan biotas.

  12. John: Or perhaps Earth Life, Martian Life, and Europan Life. Which, if the others exist, I expect will be so different from Earth Life that there will be endless debates about whether or not they are really “alive” or not, with different people weighing in on every side. We already have this debate here on Earth, with some people arguing that artificial systems like Avida are “instances” of life. I’d say that they are clearly not a part of Life, but that we can learn about some principles of self-replicating systems by studying them. I prefer to call Avida “Avida” and call Life “Life”, which I think is what most people mean when they use the word “life”. I can’t see it as being any different if we discover a Europa Life: it will be a different thing from Life As We Know It, even if it does share some similarities.

  13. Hi, Ostensive approach makes total sense to me. But still we are limited by our perspective. Life5 could appear inanimate to us, or not appear at all and vice versa. And I wonder if ‘self-reproducing’ as a shared trait is parochial too, if senescence doesn’t happen or is imperceptibly slow. Just thoughts…

  14. Isn’t that the end of intensional definition? Seems there is a definite lack of non-historical entities to be defined intensionally out there..

  15. “The idea of defining “Carl Zimmer” is a curve-ball.”


    “Yes, there can only be one Carl Zimmer.”

    Much as in Highlander. I deduce that Carl Zimmer has eliminated all rivals through decapitation.

    “David, you’re too kind. What about the clone army I’m engineering in my crawl space?”


  16. David, I’m buying into your overall argument, but human identity (Carl Zimmer) is not a simple case.
    Although I (now) think its useful to think of life as earth-life, this avoids the fascinating speculation of what makes (generic) life special. Still an fascinating question.

  17. I like Hillis’ analysis, it clears up some things for me. I was striving for that people want identifying characteristics rather than a proper definition, but I didn’t think “diagnosis” was a proper term.

    [I have sit in on astrobiology seminars, and too many want to “define life” to be able to sort out potential characteristics and the mechanisms to get there.

    “See here, you started here, a reasonably known state, and you ended here, a reasonably known state. Constraints are good, but how do you know those constraints are good?”]

    Though when we get to “origin and history” we get back to processes by way of pathways. I am a firm believer that evolution won out over most static individual systems because they adopted variation, procreation (populations) and death (generations). Even today the rumored eternal individuals (reverting to embryo jellyfish?) are few.

    So the hypothesis would be that the pathways of life is exemplified by (biochemical) populations undergoing evolution. I can see that we don’t have to include that to one day describe Life2.

    I prefer to call Avida “Avida” and call Life “Life”, which I think is what most people mean when they use the word “life”.

    And here I thought biologists liked to be inclusive, they don’t throw out the term Evolution 1.0 for Evolution 2.0 “now with added mechanisms and user friendliness”. Compare “Newton gravitation” (Gravity 1.0) with “General Relativity” (Gravity 2.0).

    [Never mind that Einstein was like Gates, GR is actually less user friendly and more resource demanding in its characterization. =D]

    So we have to choose “origins and history” at some point, whether we drag processes into it or not. But yeah, we could wait.

    The problem is that then the NASA “definition” ‘wins’, because astrobiology needs something to work from. As long as they don’t forget about fossils or biosphere characterization…

  18. John Kubie wrote

    “…human identity (Carl Zimmer) is not a simple case.”

    I certainly agree with that. But I’d also say that statement is true of all individuals and other historical entities, including Life. Even though individuals and other historical entities are bounded in space and time, those boundaries are fuzzy (note that this is NOT a criticism of Carl!). But this is the reason that humans argue so endlessly about the exact point that Carl Zimmer or other individuals began to exist, or the exact point that he will cease to exist, or exactly when a part of Carl ceases being a part of Carl (donated blood, for example). And yet, we have no problem with recognizing Carl, and most people are pretty comfortable pointing him out. I think all those things are true of Life as well. Its exact beginning is fuzzy and contentious, and people get all worried about what to call the little spin-offs of cellular organisms (like viruses), even though their relationship to Life is pretty clear. So, to me, it helps to understand that with Life, we are talking about a particular historical entity that deserves a proper name. That doesn’t take away any interest from looking for similar systems elsewhere…it just makes it a little clearer (at least to me) what it is that we are looking for. And perhaps gives us more realistic expectations about what we might find.

  19. Hi David! I like your inclusion of viruses in Life, due to their probable descent from the LUCA. But under your arguments for both Life and Zimmer, I wonder if you think the general term “organism” (which is not a taxon) should also be understood by ostensively referring to individual examples, rather than defining the term “organism” as a particular type of entity with certain qualities. Organisms, of course, are also historical phenomena (like Carl Zimmer). How can we point to examples of historical entities like single organisms unless we have defining criteria allowing us to classify them as such? Does this world view have a place or use for the term “organism”?

  20. My guess is that you might ostensively view an Organism as something like an irreducible branch or tip of Life (without defining qualities) — but that is about as satisfying to me as a concept of a “star” with no defining characteristics other than the fact that it is a component of the historical
    entity known as the “universe”.

  21. David, the main reason that the “Carl Zimmer” example is difficult is not that the definition of Carl is as a specific entity (proper noun, rather than a common noun), it is that most definitions of a human identity deals with continuity of both body and conscious experience. Adding a mental dimension to ‘life’ may be interesting, but very complicated.

  22. John, I just see consciousness as one more characteristic of some individuals, and therefore part of their diagnosis (rather than part of their ostensive definition). If I become unconscious (in a coma, for example), most people would still recognize me as David Hillis. Many people would argue that I became an individual long before I gained consciousness. But my point applied to all individuals or historical entities that are bounded in space and time (as opposed to unbounded classes of things that are defined by their attributes, like helium, or predators), and Carl was simply a handy example that would be familiar to readers. If a human example of an individual seems difficult, then substitute any other particular thing, and the point is the same.

  23. David: Many philosophers consider ‘personal identity’ something quite different from object identity. Calling this ball the same ball as the one I left on my desk an hour ago is not the same as saying its the same Carl. According to this notion, your identification of Carl Zimmer is not the defining characteristic. Its whether Carl Zimmer has a conscious experience, that includes a set of conscious-accessible memories that are “ostensive”; that is, that form a semi-continuous chain extending back towards birth (and an imagined chain extending forward towards death). John Locke is an early philosopher who addressed the differences between personal identity and object identity. An example illustrates Lockes’s notion of personal identity: that if a person has a form of amnesia that separates two parts of that person’s life, then the person is two persons.

    The second point is that understanding consciousness from the outside is not the same as consciousness from the inside. We may recognize that sufficient conditions for consciousness — certain firing patterns in cortico-thalamic neurons — without getting a handle on the two spooky features of consciousness — free will and the subjective “feeling” of experience. Although we can infer whether the anesthetized Carl is Carl or is conscious, only Carl knows for sure.

  24. Honestly, I doubt that I’ll live to hear a final definition of life. I remember when bacteria was accepted as life, but the humble virus was a heated debate. A cancerous cell is considered life.
    One definition was, consuming, excreting and reproduction. But, if that is true, one could call a prion alive, as it consumes compounds and excretes itself (as well as apparent toxic effects) and most certainly reproduces.
    It’s widely accepted that prions are not life, but merely a malfunctioning protein that causes disease.
    If we have difficulty with ascertaining what life is and defining it, how CAN we define a complex organism like a Carl Zimmer? Indeed, one goes into some of the heavier philosophical questions that are thousands of years old.
    And when we go there, we always end up with, I think I think, therefore, I think I am.
    I think.

  25. Hi David! I like your inclusion of viruses in Life, due to their probable descent from the LUCA. But under your arguments for both Life and Zimmer, I wonder if you think the general term “organism” (not a taxon) should also be understood by ostensively referring to individual examples, rather than defining the term “organism” as a particular type of entity with certain attributes. Each organism, of course, is also a historical phenomenon (like Carl Zimmer). But how can we refer to examples of historical entities like organisms unless we have defining criteria allowing us to recognize them as such? Does this ostensive world view have a place or a use for the term “organism”?
    My guess, David, is that you might view an Organism as something like an irreducible branch or tip of Life – but that is about as satisfying to me as a concept of a “star” with no defining characteristics other than the fact that it is a component of the historical entity known as the “universe”. Thoughts?

  26. OK, but the example still seems to hold…I wasn’t discussing identifying the consciousness of either Carl Zimmer or of Life, but rather their physical existence as real things. As such, Carl Zimmer still serves as a useful example of an individual.

  27. No definition is as yet entirely satisfactory. Ian Stewart tries his hand in “The Mathematics of Life” and then considers some of the difficulties.

    By the way, Koonin in his “The Logic of Chance” also considers viruses as life forms; doesn’t mentions prions one way or the other.

  28. Professor Hillis (in Carl Zimmer’s formulation) writes:

    “Like all historical entities (including other biological taxa), it is only sensible to “define” Life ostensively (by pointing to it, noting when and where it began, and following its lineages from there) rather than intensionally (using a list of characteristics). This applies to the taxon we call Life (hence capitalized, as a formal name).”

    In the comments Professor Hillis recommends we view the companion website to ‘The Principles of Life’ at where we can see brief descriptions of taxa. Doing so one discovers, by clicking on “the taxon we call Life,” this text: “Characterized by a nucleic-acid based genetic system (DNA or RNA), metabolism, and cellular structure.” (The taxon of Life “characterized by,” and not “life” “defined by,” I am quick to point out.)

    I am innocent in these things, but I wonder how much of Professor Hillis’s preferred reticence about speaking of “life” (as opposed to “Life”) flows from an implicit presumption that the extended creature we know on this planet as Life is most likely, not only in the peculiarities of its ramifications, but in its basic mechanisms, Earth specific. The implication seems to be that any other “similar” creature we come across in another setting will be a creature of a different sort. (We leave aside the possibility of still undiscovered taxa distinct from “Life” inhabiting this planet.)

    It seems to me that it is this sense of the uniqueness of terran Life that leads Professor Hillis to otherwise disdain speaking of life generally. And in this sense, as a particular creature with a distinct history and character, Life is indeed a singular thing – like Carl Zimmer. But so, of course, by definition, is every other of the myriad creatures, entities, or configuration or constellation of entities in existence. All actual “things” are in this sense “bounded” or “historical.” The dust mote floating past me as well as the Milky Way bending across the night sky are bounded historical entities not amenable to “definition.” “Things” are not amenable to definition, and Life (as taxon) being an evident thing, cannot be so treated.

    This, of course, was no one’s intention, as I far as I can tell.

    But “life”? This is “unbounded” – it is a notion, a class, a set, an abstraction. It, in the terms here, is certainly formally amenable to definition, but, alas, we seem to have only one instance of it, that being Life, the terran creature. How does one define the set rules for a set which has only one member? Is that a pointless exercise? Can we only point at this unique creature, reduced to watching and describing it being born, feed, metabolize, reproduce and die?

    Two thoughts:

    One regards what seems to be Dr. Hillis’s operant assumption that “similar life” if found will be significantly distinct from terran Life. This does indeed seem to be an outcome more imagined now than it was only a few generations ago, but that is, it seems, speculative. Given the knowledge at hand I am not aware how this likelihood could be demonstrated to be any more probable than the likelihood that there are presently millions (or billions) of planets currently occupied by creatures “characterized by a nucleic-acid based genetic system (DNA or RNA), metabolism, and cellular structure.” Not that I am necessarily inclined to the latter likelihood, but only noting that there is no presently known evidentiary indication contradicting the equivalence of earth Life with any supposed universal life.

    Second is simply that an obvious approach to defining the set rules for a set which has only one member is merely to query the uniqueness which has led, even tentatively, to the construction of that set. As the wise man said “One of these things is not like the others, one of these things just doesn’t belong.” What are the attributes that led to the contradistinction of “life” in the first place, the setting aside of the notion of “the living”?

    This is what defines the class as we know it. It may not be a useful definition for either discovering the “life-like” in other contexts, or otherwise have great value outside the vernacular, but it certainly isn’t some sort of categorical impossibility. And in those terms the results of Trifonov’s sieve – self-reproduction with variation – doesn’t seem so far off the mark to me.

    In the end, while knowledge of the relevant possibilities or probabilities may still be well beyond us, we are nonetheless not constrained, by either biological fact or semantic procedure, from tentative definition. We are likely to be mistaken, as we usually are, and there’s a good chance that our thinking may serve as much as a stumbling block as an opening, but we need not remain mute, merely pointing to what, in any event, we still do not yet understand.

  29. Should nanobes ever become established as a real form of life we would have to add a fifth group, after viruses (unless you are willing to say that the viruses that infect eukaryotes, eubacteria and archaea are themselves different lineages that just share trimmed down parasitism (and what about giant viruses, or viruses that infect other viruses, and viroids???).

  30. In response to C.L. Dorsch: Note that my main point was drawing the distinction between two different meanings of the word “life” that are often confused. One is the biological taxon we call Life with a capital “L”, which, like other such entities, can only be “defined” ostensively. I think this is the meaning that most people (or at least most biologists) have in mind when they use the word “life”. Like other biological taxa, Life can be diagnosed or characterized, but that is not the same as a definition, and those attributes are not necessarily requirements of its recognition. [Consider another example of a “characteristic” that is not a requirement: we say that limbs are “characteristic” of the taxon Tetrapoda, but many tetrapods (like snakes) have lost their limbs. I’d say that the same thing applies the “characteristic” of Life as cellular; viruses are parts of Life that have lost cellularity.] Hence, there is an ostensive “definition” of Life in the Tree of Life Appendix I referenced (pointing to the largest monophyletic group on the Tree of Life), followed by a list of the characteristics (a diagnosis) we commonly use to recognize Life.

    I also noted that it is certainly possible to define an unbounded (in space and time) class concept of life (with a small “l”), which may apply to other entities that are similar to Life on Earth. But, I’d argue, any such concept is bound to be an arbitrary class, rather than a natural class, so I doubt that any definition will satisfy everyone. In the end, different people will recognize things that are more or less similar to Life, and call them living, or not. My point was that there is not a right or wrong answer…there are just entities (known or imagined) that are similar to Life in varying degrees. The sooner we recognize the subjective nature of this problem, the sooner we can put this “problem” behind us. Then, we can realize the potential for a whole range of possible entities that are similar to Life in varying degrees. If we discover a part of Life (big L) on another planet (which I’d say is unlikely, but possible), then that will be relatively easy to determine, and certainly surprising (unless we’ve put it there). Much more likely is that we’ll discover something SIMILAR to Life…and the fun part will be learning HOW similar it is, not arguing about whether it should be called alive or not. The first is discovery and science, whereas the second is quibbling over semantics. Personally, I think finding anything even remotely similar to Life on another planet would be exciting. If and when we do make that discovery, I suspect that this entire discussion will become a lot clearer to many people.

  31. Manda Clair Jost: Reading back through the responses, I see that I missed a question that you posed. I’d say any particular organism (like Carl Zimmer) is an individual that can only be defined ostensively. But the concept of “organism” is a class that can be defined intensionally. Like the class concept of life, different people may define this term differently, so as to arbitrarily include or exclude things like viruses as organisms.

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