The Definition of Life

http://www.ffame.org/sbenner/cochembiol8.672-689.pdf

The opening discussion:

To decide whether life has a common chemical plan, we must decide what life is. A panel assembled by NASA in 1994 was one of many groups to ponder this question. The panel defined life as a ‘chemical system capable of Darwinian evolution’ [16]. This definition, which follows an earlier definition by Sagan [17], will be used here. This definition contrasts with many others that have been proposed, and avoids many of their pitfalls. For example, some definitions of life confuse ‘life’ with the concept of being ‘alive’. Thus, asking if an entity can move, eat, metabolize or reproduce might ask whether it is alive. But an individual male rabbit (for example) that is alive cannot (alone) support Darwinian evolution, and therefore is not ‘life’ [18]. Further, many efforts to define ‘life’ fall afoul of the fact that no non-trivial term can be defined to philosophical completeness [19]. The general difficulty of defining terms, theoretically or operationally, was one of the discoveries of 20th century philosophy, and is not unique to the definition of ‘life’. It is impossible, for example, to define ‘water’ in a complete way. We can say that water is ‘dihydrogen oxide’, but are we speaking of a water molecule, water as a substance, or water operationally? And what is ‘hydrogen’? Any effort to deal with the definition of life at this level encounters analogous questions, which can easily be paralyzing. The hrase ‘Darwinian evolution’ carries baggage from 150 years of discussion and elaboration. It makes specific reference to a process that involves a molecular system (DNA on Earth) that is replicated imperfectly, where the imperfections are themselves heritable. Therefore, Darwinian evolution implies more than reproduction, a trait that ranks high in many definitions of life. The panel’s definition also avoids confusion from many non-living systems that reproduce themselves. For example, a crystal of sodium chlorate can be powdered and used to seed the growth of other crystals [20]; the crystal thereby reproduces. Features of the crystal, such as its chirality, can be passed to descendants [20]. The replication is imperfect; a real crystal of sodium chlorate contains defects. To specify all of the defects would require enormous information, easily the amount of information in the human genome. But the information in these defects is not itself heritable. Therefore, the crystal of sodium chlorate cannot support Darwinian evolution. Therefore, a sodium chlorate system is not life [21].]. The NASA panel’s definition of life is interesting for other reasons. First, it provides information about what forms of life were believed to be possible, not just conceivable. As fans of Star Trek know, forms of life that are not chemical systems capable of Darwinian evolution are easily conceivable. Besides aliens resembling Hollywood actors with prostheses, the Enterprise has encountered conceptual aliens that do not fit the panel’s definition. The nanites that infected the Enterprise computer in Episode 50 of Star Trek: The Next Generation (‘Evolution’) are informational in essence; their Darwinian evolution is not tied to an informational molecule, like DNA (although they require a chemical matrix to survive). The Crystalline Entity of Episodes 18 (‘Home Soil’) and 104 (‘Silicon Avatar’) appears to be chemical, but not obviously Darwinian. The Calamarain (Episode 51: ‘De´ ja` Q’) are a conceptual life form that is surely energy, not evidently requiring matter. And Q (Episode 1: ‘Encounter at Farpoint’) appears to be neither matter nor energy, flitting instead in and out of the Continuum without the apparent need of either. If we were to encounter any of these other conjectural entities during a real, not conceptual, trek through the stars, we would be forced to concede that they represent living systems. We would be obligated to change our definition of life. We do not change it now simply because we do not believe that the weirder life conceived in the Star Trek scripting room is possible outside of that room. Nanites and the fictional android Data are examples of artificial life. We do not doubt that androids can be created, including androids who (note the pronoun) desire to be human. We do not, however, believe that Data could have arisen spontaneously, without a creator that had already emerged by Darwinian process (as is indeed the case with Data). Hence, we might regard Data as a biosignature, or even agree that he is alive, even if we do not regard him as a form of life. For the same reason, a computer holding nanites would be evidence that a life form existed to create it; the computer is a biosignature, and the nanites are an artificial life form, something requiring natural life to emerge.

Dr. Benner then continues with a discussion on the requirements for life.

Maybe I’m just a trekker but I have to grin like a cheschire cat whenever I see scientists reference Star Trek.

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54 Responses to The Definition of Life

  1. For example, some definitions of life confuse ‘life’ with the concept of being ‘alive’.

    I admit, I have made that confusion my whole life. Silly me, thinking life had something do with being alive! How could I be so niave?

  2. But an individual male rabbit (for example) that is alive cannot (alone) support Darwinian evolution, and therefore is not ‘life’

    Wow, talk abut a world view that begins and ends with Darwin. The philosophy seems to be that Darwinism is life and life is Darwinism. It is pathetic really.

  3. Jehu, I clearly understand how you get to this conclusion. However, I am not as quick to write off the definition as you are. Firstly, though you and I surely agree about the inadequacy of the darwinian model, I also suspect that you see darwinian processes playing out in the microevolutionary realm. Ie, bad mutations happen, natural selection weeds them out — neutral mutations happen, and natural selection doesn’t bother to weed them out.

    Though we both agree that the darwinian process is not adequate to explain the variety of life that exists, if the darwinian process is active in all living things, and the darwinian process is not active in all non-living things, then the detection of a functioning darwinian process should be a good definition of life.

    One must conclude, however, that this definition of life would have to attribute “life” to many evolutionary simulations.

  4. Robert Hazen has produced two publications in the last year on the origin of life and in each he discusses in detail the definition of life.

    In his Teaching Company course he spends a half hour lecture on the topic and I believe he covers it also in his book Genesis.

    Here are some comments from Hazen’s study of life and its origins.

    “Any attempt to define the exact point at which a system of gradually increasing complexity becomes alive is intrinsically arbitrary and really more of a question of perceived value than of science”

    Also

    “Scientists in the early 21st century are in no position to define life. We have yet to articulate many of the theoretical underpinnings of biology; we don’t know if life’s biochemistry is highly constrained or if there are many chemical solutions to life.”

    So essentially he is saying no one know what life really is. But we recognize when we see it.

  5. bFast,

    Firstly, though you and I surely agree about the inadequacy of the darwinian model, I also suspect that you see darwinian processes playing out in the microevolutionary realm. Ie, bad mutations happen, natural selection weeds them out — neutral mutations happen, and natural selection doesn’t bother to weed them out.

    That is natural selection functioning to preserve stability not “Darwninian Evolution.”

    Though we both agree that the darwinian process is not adequate to explain the variety of life that exists, if the darwinian process is active in all living things, and the darwinian process is not active in all non-living things, then the detection of a functioning darwinian process should be a good definition of life.

    I do not agree that the Darwinian Evolution is happening in all living things. The stasis in the fossile record would seem to show that after organisms appear they do not change. I would agree that change has occured in some organisms over time. I believe that this change is almost always either information neutral or information negative. Changes that have resulted in so-called ‘speciation’ have almost always been infromation negative. i.e. the loss of traits and characteristics. I do not consider the loss of informatin to be evolution since to evolve there must be increase in complexity or information.

    http://www.evolutionisdegenera.....inaID=1100

  6. For the sake of discussion I usually say, becoming a Turing Machine is a necessary but not sufficient condition for life.

    That has somewhat been the operational minimum threshhold in various peer-reviewed papers, and it is a tangible specification.

    Life is more than a Turing Machine, but that threshhold is a valuable target to assess the adequacy of origins theories.

  7. Many dyed in the wool darwinians cannot ‘support darwinian evolution’ (at least not pursuasively…); I suppose they’re not alive either? ;-)

  8. “So essentially he is saying no one know what life really is. But we recognize when we see it.”

    Sounds familiar – we recognize design when we see it – even if we can’t always define why we kniow it is design and not chance.

  9. Jehu, you will notice that in my discussion I referred to the “darwinian process” rather than to “darwinian evolution”. This, of course, because I agree with you that the darwinian process is primarily a preserving process. I think it does, however, support some amount of change. Those neutral or near neutral mutations that happen are not rejected by natural selection. The result is that species do take on genetic diversity as time goes by, ergo “change”. Now, the definition of the term evolution is not cast in stone. I recognize that it carries a connotation of “improvement”, but this connotation, as a process of RM+NS is certainly questioned by both you and myself. However, evolution has also been defined simply as “change”. I am sure that you agree that “change”, “increased genetic diversity” within a species, can happen purely by neo-Darwinian means.

    So if you reconsider my suggestion that the neo-Darwinian process happens in all life forms, and does not happen in anything non-living, then we are back to considering that the provided definition of life is not too bad. I agree that we must do so with the caviat that the term evolution not imply “improvement”, only “change”.

    Let me propose a definition of life that you may find acceptable, “a chemical system capable of being changed by a Darwinian process.”

  10. There is an official definition of evolution in NDE, namely “change of allele frequencies in a population.” It is as simple as that and this definition encompasses nearly all discussions of evolution that take place including anything ID would proffer.

  11. So maybe you could define life as anything that is capable of changing allele frequency in a population.

  12. bFast,

    Well, I don’t know really. I am not sure why even reproduction has to be necessary for life, much less Darwinism.

    My definition of life would be an organism which, in its nonfetal stage, possesses it own operating system which allows it to consume food without artificial help in order to maintain a metabolism. In other words, life is characteristic possesed by all things that are ‘alive.’

    As far as Darwinian process goes, it is fasicinating really. It is quite obvious that organisms lose abilities and traits that are not necessary to survival and therefore become less fit over time resulting in extinction when enviromental changes occur that require the use of traits which have been lost. So natural selection really only slows down the rate of change by preventing the loss of traits. Is that really a “darwinian process”?

  13. Many dyed in the wool darwinians cannot ’support darwinian evolution’ (at least not pursuasively…); I suppose they’re not alive either?

    Impotence … “the silent epidemic” :D

  14. So maybe you could define life as anything that is capable of changing allele frequency in a population.

    How about defining life as a quality possessed by something that is alive?

  15. life is what is alive. Define alive.

  16. This seems a rather simple inquiry and I wonder if this post isn’t an example of unnecessary complexity. (I’ve no doubt of its example of NDE circularity!)

    What are the lowest common denominators of those things we label ‘alive’?

    Reproduction? instruction sets? independent metabolization?

    I haven’t a wide enough base to know the LCD, but it seems sensible to start there.

  17. The only consistent definition of life I can think of at the moment is:

    Entities that come into existence due to preexisting programming in (what we call) DNA. These things are alive and NOTHING else is.

    Next.

  18. …until they stop working. Then they’re dead.

    Next

  19. By the way, I had a random thought come to me today that “junk” DNA may harbor instructions that affect *other* species rather than the carrier, that might be transmitted via a bite or a sting, and might alter the victim. Anyone hear of any research along these lines?

  20. Re: 19:

    Watch out! Here comes the Spiderman…

  21. Is a virus alive?

  22. todd

    Exactly!

  23. 23

    bFast:
    One must conclude, however, that this definition of life would have to attribute “life” to many evolutionary simulations.

    Yes, and there have been articles written that begin to consider such topics as the moral obligations of any person responsible for such a situation. Is there a point at which you are obligated to keep it running instead of killing all those creatures? Don’t tell PETA or you will start hearing “An opcode is a virus is a cell is a mouse is a monkey is a boy.”

  24. One of the biggest scientific discoveries of the 21st century will be that life, along with matter, energy and information, is a fundamental entity and is separate from a living organism.

  25. Pick up any introductory biology textbook and read the first chapter. What you will find is a list of the attributes of living things (BTW, this includes textbooks written and published by creatonists, which only omit one of the characteristics listed below).

    Virtually every introductory biology textbook states that, on the basis of our empirical observations of them, living organisms:

    (1) Respond actively to changes in their external and internal environment;

    (2) Respond homeostatically to such changes, maintaining a constant internal environment despite changes in the external environment;

    (3) Take in energy in various forms, using it to power such responses;

    (4) Exchange materials with their surroundings in such a way as to accomplish #1 & 2;

    (5) Have the ability to reproduce (whether or not they actually manage to do so is irrelevant);

    (6) Interact with other living organisms, either directly or indirectly;

    (7) Grow and, if multicellular, undergo development;

    (8) Evolve (BTW, individual organisms can’t evolve, only populations can; individuals develop);

    (9) Are composed of at least one cell, bounded by a plasma membrane composed of a phospholipid bilayer with associated proteins;

    (10) Contain at least one DNA molecule in which is encoded the genetic information necessary to assemble and operate the cell/s);

    (11) Contain a set of ribosomes, enzymes, and other macromolecular biochemical “machines” that accomplish all of the aforementioned processes; and

    (12) Are capable of dying (BTW, unless something kills them, many living organisms – such as bacteria – are essentially immortal; the bacteria in your large intestine have been continuously alive for almost four billion years).

    Note that this list, while long, is not necessarily exhaustive, nor can it be condensed to a single criterion. Furthermore, no single characteristic is sufficient to define life, which is apparently an aggregate phenomenon.

    In other words, the definition of life contained in the opening quotation for this thread is woefully incomplete. Indeed, it would not be innappropriate to refer to it as a “strawman,” quote-mined from an article whose focus was something else entirely, and posted here primarily for the purpose of misdirection.

    P.S. Given this list of characteristics, viruses are clearly not alive (regardless of whether they attack cells or computers), Stephen Hawkings assertions to the contrary notwithstanding.

  26. 26

    Please remember that this NASA exobiology group that came up with this definition just needed something useful and operational. They weren’t trying to answer any big picture questions, just constrain their own scope of work.

    To me the most interesting thing about it is that life is a set of processes. Life is not nouns (DNA) or adjectives (complex). Life is verbs.

  27. Dr. MacNiell, I would assume that one would seek a definition of life that would be useful to see if the threshold between abiogenesis and evolutionary biology has been crossed. As it has become increasingly clear that no DNA based organism could possibly have been first life, having a DNA requirement in the definition of life hardly seems to make a sensible definion of life. The RNA world hypothesis does seem to hold out some hope of a pre-DNA life-form. If such a form is demonstrated to have preexisted DNA based life, I would assume that it is incumbent upon evolutionary biologists to provide a reasonable explanation for how RNA world evolved to become the DNA based world that we now have. Ie, the science of evolutionary biology must begin prior to DNA unless one is content to have the first life be a DNA based life. Such a life, however, would be solidly in the miracle camp.

  28. Allen:

    Thank you for the information – I knew there was a list of observed LCDs out there!

    So viruses aren’t alive? Is that because we cannot kill them? Or because they have no DNA? Or perhaps the qualities of ‘life’ should be reduced to those common to viruses?

  29. Todd,

    Good point about viruses. Here is a theory regarding viruses: The first genetic material was self-replicating RNA molecules floating on bodies of water, fondly known as the RNA world. RNA acted as both genetic material and as a catalyst.

    I was curious how our NDE friends hypothesize about how the RNA molecules first assembled, and how they began to self-replicate without first being destroyed by the elements, radiation, etc. And finally, how, presto, proteins popped up and the whole enzyme thing got rolling, and we switched over to DNA?

  30. bFast,

    Sorry, I realize my question is pretty much synonymous with yours. Oh well, twice is better than none.

  31. The main problem with RNA or DNA originally assembling is huge- nucleotides. Nucleotides are only found IN living organisms. There are currently no known stochastic mechanisms which demonstrate any nucleotides can form outside of living organisms- never mind the five flavors we find in living organism and never mind those flavors linking up to form a chain.

  32. Allen MacNeil:
    In other words, the definition of life contained in the opening quotation for this thread is woefully incomplete. Indeed, it would not be innappropriate to refer to it as a “strawman,” quote-mined from an article whose focus was something else entirely, and posted here primarily for the purpose of misdirection.

    Wow. And here I am thinking it was posted here for reasons of discussion. A discussion that would/ may focus on its inadequacy.

    But I am curious about this bit:

    Allen MacNeil:
    BTW, individual organisms can’t evolve, only populations can; individuals develop

    Umm we know that the variation occurs to the individual. We know that natural selection acts on that variation in the individual.

    IOW it is a bit misleading to say that populations evolve when in actuality the variation and natural selection act on individuals.

    That said it is true that in sexually reproducing populations even the most beneficial genetic mutation has a better chance of getting lost than it does of becoming fixed. And even more so in large populations.

  33. Joseph,

    “… even the most beneficial genetic mutation has a better chance of getting lost than it does of becoming fixed.”

    Just take an example — insects that live in a colony. Insects have been around for 400 million years, give or take. And sure, their evolution has been hypothesized by some, including David Grimaldi.

    Yet, surely some serious questions remain. For example, how did insects begin to practice such amazing teamwork and take to communal living? As you point out, variation occurs to the individual. Say we get a bee that gains some sort of social or communal trait, e.g., it begins to attempt some rudimentary communication with other bees (sorry if the example is inadequate, but you get the point). How are the other bees going to respond? Not with reciprocation, thank you, since they have no such trait. So the lonely socialite dies, and poof, there goes that pioneering trait into the waste basket.

  34. Todd:
    So viruses aren’t alive? Is that because we cannot kill them? Or because they have no DNA? Or perhaps the qualities of ‘life’ should be reduced to those common to viruses?

    I don’t know if the following still stands but I was taught that viruses are not considered living organisms because they cannot survive (for long) nor reproduce on their own.

    Which is pretty much the reasoning that the parasitic bacterium that has been found to have the smallest genome could not have been “the first living organism”- it can’t survive nor reproduce without a host.

  35. For what it’s worth–anyone seen this—Grammar-based peptide fights bacteria?

  36. 36

    Ekstasis,

    Your intuition that a bee with a modified trait would be “lonely” is the problem. Since insects in general have large numbers of offspring, and social insects are practically clones, the bee with the modified trait is going to start life with a number of hive mates who also share that trait. The parent bee might not have survived to reproduce _because_ of this latent ability to communicate better, but now that she has thousands of children they certainly can take advantage of it.

    Today we can see a spectrum of sizes of community in insects and other animals. This variety can guide us in understanding how social species evolved.

  37. Ekstasis- my understanding of insects in colonies (or hives) is that what they are is determined by epigenetics- that is something outside of the genome. Termites, ants, bees all share this.

    “The agent may be a jelly, a hormone, or an ion, and it may cause the arrest or continuation of development or alter some point of bifurcation. Massive far-reaching upheavals in the forms and functions of organisms can be produced without bothering their DNA.”
    Giuseppe Sermonti, geneticist, page 106 of his book “Why is a Fly is Not a Horse”?

    A recent article in “Discover” discusses epigenetics…

  38. Ekstasis,

    “how did insects begin to practice such amazing teamwork and take to communal living? As you point out, variation occurs to the individual. Say we get a bee that gains some sort of social or communal trait, e.g., it begins to attempt some rudimentary communication with other bees (sorry if the example is inadequate, but you get the point). How are the other bees going to respond? Not with reciprocation, thank you, since they have no such trait. So the lonely socialite dies, and poof, there goes that pioneering trait into the waste basket. ”

    The simple response is that the lonely socialite is not so lonely that is cannot reproduce (its social quality having given it the ability to attract mates in unparalleled numbers), at which point a substantial fraction of its children also have this trait, and, working together, out-survive their less social siblings, and hence survive to breed in disproportionate numbers, quickly becoming the majority in the population.

  39. Allen

    The definition of life you offered wraps all the known classes. The article above offers a definition from a NASA panel. Anyone who is familiar at all with astrobiology knows that NASA is a focal point. Astrobiology is interested in not just in the one observed form on earth but any possible form anywhere. Their definition must therefore wrap yours plus any conceivably possible alternatives. Evolutionary biology is a subset of astrobiology. Astrobiology is far more interesting IMO as astrobiology converges with cosmology, astronomy, and space exploration in addition to the usual suspects.

    A lot of money is thrown at it which makes it even more interesting. The next generation of billion dollar telescope proposals almost to the last one have the goal of being able to identify and spectroscopically analyse earth-size planets around other stars. Experiments looking for signs of life or organic precursors of life elsewhere in our solar system from coments to planets are aboard everything that leaves the earth/moon system. SETI has been ongoing for decades.

    For some perspective on it look at how Astrobiology Magazine remembers Francis Crick. Credit for describing DNA is mentioned first but the bulk of the article is about Crick’s Directed Panspermia hypothesis. I happen to agree with Crick about directed panspermia.

    ‘Directed Panspermia’ suggests that life may be distributed by an advanced extraterrestrial civilization. Crick and Orgel argued that DNA encapsulated within small grains could be fired in all directions by such a civilization in order to spread life within the universe. Their abstract in the 1973 Icarus paper reads:

    “It now seems unlikely that extraterrestrial living organisms could have reached the earth either as spores driven by the radiation pressure from another star or as living organisms imbedded in a meteorite. As an alternative to these nineteenth-century mechanisms, we have considered Directed Panspermia, the theory that organisms were deliberately transmitted to the earth by intelligent beings on another planet. We conclude that it is possible that life reached the earth in this way, but that the scientific evidence is inadequate at the present time to say anything about the probability. We draw attention to the kinds of evidence that might throw additional light on the topic.”

    The Miller-Urey experiment generated electric sparks — meant to model lightning — in a mixture of gases thought to resemble Earth’s early atmosphere.

    Crick and Orgel further expanded on this idea in their 1981 book, ‘Life Itself.’. They believed there was little chance that microorganisms could be transported between planets and across interstellar distances by random accident. But a technological civilization could direct panspermia by stocking a spacecraft with a genetic starter kit. They suggested that a large sample of different microorganisms with minimal nutritional needs could survive the long journey between worlds.

    The chemistry of a hypothetical RNA world still has little experimental support despite decades of trying to get complex polymers to self-assemble and remain assembled long enough to do something interesting in any conceivable natural environment.

    Crick as recently as 1993 was still sticking by his guns on the RNA World. It should be noted that Crick was the first to postulate the possibility of ribozymes long before they were discovered and his statement still in support of directed panspermia was made a decade after ribozymes were experimentally confirmed.

    Francis Crick himself has become much less enthusiastic about the RNA world than Watson. In 1973, he and another eminent researcher into the origin of life, Leslie E. Orgel, published a paper advocating the theory of “Directed Panspermia” (6). In 1981, Crick published Life Itself, a whole book about that theory (7). And by 1993 he says, “It may turn out that we will eventually be able to see how this RNA world got started. At present, the gap from the primal ‘soup’ to the first RNA system capable of natural selection looks forbiddingly wide” (8).

    Studies of the galactic habitable zone indicate that there’s been about 8 billion years in an expanding zone where planets suitable for complex life are possible. One of the take-home lessons in this is that our solar system is only 4 billion years old leaving 4 billion years before our solar system even existed for complex life to start elsewhere . As near as we can tell DNA/ribosome based life began here ~3.5 bya leaving scant time in a very hostile environment for the biggest gap leap in evolution (soup to cells).

  40. DvK, Joseph, and SCheesman,

    Thanks for setting me straight regarding genetic and other factors pertaining to insects. I will now sign off and register for Genetics 101!

  41. Exstasis:

    Try here: http://www.dnaftb.org/dnaftb/

  42. To Ekstasis:

    There is so much more! For example if your bee is a worker any and all beneficial genetic mutations it has (ie is unique to it) stops with it. Workers do not get to pass on their genes- only the chosen drones and the queen.

  43. Allen_MacNeill,

    All of those attributes you mention are certainly true of lifeforms, but it’s much more simple to say that all entities generated from DNA and preexisting cells are lifeforms and all lifeforms are generated by DNA and preexisting cells. And nothing else is. Do they have the attributes you list. Yep, but you could knock out an attribute here or there and my the criterion would remain the true of essential common attribute. The DNA/cell system. That system IS life.

  44. By trying to define “life” as what it takes to make a living organism you guys are “missing the forrest for the trees”…

  45. Life: A closed system which processes external energy sources to produce, maintain and repair component parts according to one or more instruction sets; organized, reproduced and perpetuated by inherited instructions.

    How’s that for a forrest :?:

  46. I left out adaption to surroundings, but it might survive without it.

  47. In my opinion, darwinian evolution, insofar as it is suggestive of a nonteleological universe, undermines the very idea that there can be a “definition” of life. That is to say, without an external teleology, I do not see how there is an “essence” to life which can be precisely expressed in language with certainty. We are, as a result, simply left with the question–largely linguistic and pragmatic in nature–which is, “what sort of things are we in the habit of referring to as alive vs. not alive.” While there are a cluster of traits that coincide with those things we think of as alive (Dr. Macneill spelled them out in his post above) our inability to precisely specify the traits of living things vs. nonliving things does *not*, in my view, indicate an inadequate definition on our part that is in need of work. Instead, it indicates a lack of any “essential quality” of being alive. In my opinion, there is a continuum of complexity in nature, wherein it so happens that most things within our human experience fall heavily towards one side (not alive) or the other (alive). We may be able to list a set of traits that almost always are associated with what is called “living,” but we will never possess an inerrant line of demarcation between the living and the nonliving because it simply does not exist. “Life” is just a word we use to indicate a certain threshold of dynamic, organized complexity is present. That threshold is set by human convention. There was a time when I readily accepted the notion of viruses as “nonliving.” Then I became acquainted with EBV. With over 80 genes and mirnas directed at its host, its genomic complexity approaches that of the simple bacterium recently reported. (No doubt there are more sophisticated viral specimens out there waiting to be found.) Suddenly the line separating life from nonlife, which I had been taught since gradeschool biology, appeared rather arbitrary. As with most things, I suspect the more we learn, the more shades of grey we will discover.

  48. 48

    Todd,
    If you can apply it to salt crystals growing on the shore of the Dead Sea, you haven’t captured it.

  49. David: Does my attempt apply to salt crystals? Are they a closed system? Do they reproduce salt? Do they maintain the system using outside energy?

  50. IMHO, chemicals are less important in defining LIFE. I believe INFORMATION is more important. Chemicals are only a substrate…

  51. Todd and Joseph, thanks for the great resources and additional info!

    So, just a fundamental question for evolutionary biologists: how did the original RNA self-replicate itself without any sorrounding cell infrastructure? How would binary fission work, for example? And how and why would it first have taken place? What must be present for the RNA to perform the autocatalysis on itself? How likely is it that the early environment would fortuitously provide such factors?

    Just wondering.

  52. Ekstasis

    Read this

  53. Thanks Dave!

    Frome the link:

    The prebiotic synthesis of nucleotides in a sufficiently pure state to support RNA synthesis cannot be achieved using presently known chemistry. Each of the steps needed to assemble a nucleotide from very simple starting materials was demonstrated early in the development of prebiotic chemistry, but the reactions were inefficient, nonspecific, or both. Some progress has been made in developing more specific prebiotic syntheses, but formidable difficulties remain. This has led some researchers to explore a major new approach to the problem of molecular evolution—the search for polymers that could function as alternative genetic systems.

  54. Eric…

    Thanks for the info. By the way, I am a big fan of your site. Keep up the great work….

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