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What is Life ?

  1. Aug 25, 2004 #1
    That is the title of the famous Schrödinger's book (1944), which inspired to scientists as Crick.

    Now, after the great discoveries in Biology, mainly in Molecular Genetics, the question remains.

    We know essential aspects of living organisms: need of some type of informative auto-replicating molecule and an isolated medium where biochemical reactions occur in a stationary state far from equilibrium.

    But the great diversity of life forms in our planet, exemplified by extremophylic organisms, make the question yet non-trivial: What are the minimal conditions inherent to any real or potential living structure?

    For example, is Carbon an essential atom to life? Or individuality of systems with membranes?

    And the old question on virus: are they living forms? Or a transposon? And a prion?
  2. jcsd
  3. Aug 27, 2004 #2
    Or, posed in other form:
    Is it conceivable life formation from Nanotechnology?
  4. Aug 28, 2004 #3
    I know some of you dont like too much Frank J. Tipler (or at least his ideas), but, in his book The Physics of Immortality, he explain some "curious" thoughts about this subject.

    Here is a guy who is not too happy with Tippler:

    This definition of life [that life is information preserved by natural selection] has some counterintuitive consequences. ... [It means] automobiles are alive. They self-reproduce in automobile factories using human mechanics. Granted, their reproduction is not autonomous; they need a factory external to themselves. But so do male humans: to make a male baby, an external biochemical factory called a ‘womb’ is needed. ... The form of automobiles in their environment is preserved by natural selection: there is a fierce struggle for existence between various ‘races’ of automobiles. Japanese and European automobiles are competing with native American automobiles ..." Surely Tipler’s definition of ‘life’ does not have the counter-intuitive consequence that automobiles are alive. Even if these is some loose analogy between automobiles and biological species, it is false to claim that automobiles evolve by natural selection. It simply isn’t the case that new models of cars arise because of random mutations in genetic material -- and yet this is part of what is required for any process to count as evolution by natural selection.[40] Perhaps Tipler might insist that we ought to understand ‘natural selection’ more broadly here: "The key feature of the ‘living’ patterns is that their persistence is due to a feedback with their environment: the information coded in the pattern continually varies, but the variation is constrained to a narrow range by this feedback" (p.125). But this ‘broader’ definition is plainly unsatisfactory: no reasonable definition of ‘living thing’ can allow that any manufactured entity which involves a feedback loop -- e.g. a thermostat -- is a living thing. (Part of the problem here is that the persistence of these kinds of things involves ‘unnatural selection’ -- namely, selection by living things on the basis of their interests, etc. Genuinely living things must have more independence.)

    Personally, I find very interesting Tipler efforts in arguing why silicon-based life (as emulation of reality in a computer) is as good and as REAL as carbon-based one. Maybe the key is the conciousness.

    I think an ameba has it (in the simpliest form of course). A car has not IMHO.

    Tipler thinks humans are finite state machines and because of this we can be emulate on a computer in exactly the same state. Since we have conciousness in carbon form, what does stop us to think we are not able to have it in silicon form?

    Actually, quantum mechanics spoils "a bit" this fact... :rolleyes:
  5. Aug 28, 2004 #4
    I didn't read the Tipler's book. I have just visited his webpage, where I find interesting arguments. Nevertheless, I cannot avoid an association with the Teilhard's omega point (interesting from a theological,non-scientific, viewpoint).

    It is interesting the relationship life - information - computing, with the use of Turing.

    About your assert "Maybe the key is the consciousness"... Why? When is there consciousness? In an amoeba?
  6. Aug 28, 2004 #5
    Well, I think that life is every carbon-based being that has some kind of conciousness. From humans to the before mentioned amoeba. This primitive being has few but very specific instructions. Where those algorithms of behaviour come from? This is what, IMO, separate animated from inanimated matter.

    I remember a tv program about the last research regarding the conciousness developed by Penrose and a prestigious anesthesist.

    Por cierto, soy de Alcalá de Henares, aunque ya habrás intuido algo leyendo mi inglés verdad...? :biggrin:
  7. Aug 29, 2004 #6
    I think that your concept of consciousness is very wide, and in this form could also apply to an artificial nanomachine. :rolleyes:
    No intuí nada porque tu inglés es mejor que el mío :biggrin:
  8. Oct 11, 2004 #7

    Does anybody know the records of salt concentration, pH, temperature or pressure for earth biological systems?
  9. Oct 11, 2004 #8


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    Strain 121, an archaea found where volcanic activity, can grow at 121C in the laboratory and can survive a few hours at 130C

    Psychotrophile can also steadily grow between 0C and 12C and 4C is the optimum temperature.

    Halophiles (Halobacterium) can grown between 155 to 30% salt solution. 25% is the optimum.

    Barophiles (MT-41 - found in marine trench) can grow in condition of 500 to 1000 atmosphere. 700 is the optimum.

    Acidophiles (Picrophilus) can grow in in solution with a pH ranging from -0.06 to 4. 0.7 is the optimum.

    Alkaliphile (Natronobacterium) can grow at pH ranging from 8.5 to 12. 10 is the optimum.
  10. Oct 12, 2004 #9
    Hello Iansmith. Thank you for your interesting notes on extremophils.
  11. Oct 12, 2004 #10
    I think it's safe to say that what defines 'life' is the ability to learn. Not just change its parameters (due to feedback loops), but to comprehend even entirely new situations, offered to the subject via its senses, from which it's then able to distill formerly unknown elements and remember their properties.

    If we look at an infant, then this is exactly what happens. During the first couple of years after its birth, its memory is virtually empty. It then learns to recognize elements in its environment, as well as how they relate to each other.

    This learning process can quite easily be defined, but the real difficulty lies in comprehending and replicating the actual learning process, i.e. the 'storage' of 'data' (sensory and other types).

    If we are to accept the above as being correct, then this would consequently place the limit of what we can call 'life' by anything which has something resembling a biological neural network in functioning.
  12. Oct 13, 2004 #11
    perhaps something like: life is that which consists of matter and that can replicate autonomously
  13. Oct 14, 2004 #12
    But that don't seems to occur in the most "simple" forms of life, as bacteria or isolated eukaryotic cells growing in culture. Could you refine your concept of "ability to learn"?
  14. Oct 14, 2004 #13
    In a wide sense (materia as a whole of ordered sequence of electronic status in a silicon matrix), a software virus could adjust to your hypothesis. Yes?
  15. Oct 14, 2004 #14


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    Coming up with one single definition of life is pretty difficult, because it is so complex.

    Some common characteristics typically used include:
    1) Reproduction - the ability to produce more of the same type of organism
    2) Growth and/or development - even single-celled bacteria can grow in size
    3) Order - life has complex organization
    4) Utilization of energy - conversion of energy into work, use of energy to maintain order, use of energy for growth
    5) homeostasis - ability to maintain a constant internal environment despite a changing external environment
    6) response to the environment - examples: plants bending toward light, a mammal moving toward a heat source, a bacterium moving along a chemical gradient in media.
    7) evolutionary adaptation

    The examples I've given are from Cambell's Biology text, but would be similar in any biology textbook.
  16. Oct 14, 2004 #15


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    I don't know what the above paragraph is attempting to dispute, but it's very flawed. The most glaring flaw in the reasoning is that the production of a car in a factory is analogous to reproduction of humans in a womb. Humans are still reproducing humans, you don't have SUVs getting together and producing compact cars that will someday grow into an SUV. Humans build cars, cars don't reproduce and grow and develop. They also can't maintain homeostasis. If you forget to put coolant in the radiator, it will overheat and break down, it won't instead use its fuel source (gasoline) to synthesize its own coolant. It also can't respond to its environment, or obtain its own food (darn, I wish my car would take itself to the gas station, but it won't).
  17. Oct 16, 2004 #16
    Yes. That's a good description. But what would be the minimal conditions?
    For example, could we talk of life about a virus? And what's about a prion? Or, in general, on parasites. In such cases, the number or points seems to be lower than the seven you cited.
    I think that in these cases and in other very different (social ants and so on) it would be useful the concept of individual. Even in the case of cells and subcellular structures (mitochondria, ribosomes...)
  18. Oct 16, 2004 #17
    I would not consider a software virus as consisting of matter, that would be similar to saying that an idea consists of matter because it is some pattern in the electrical activity within our brain.

    I guess, I think that software could never be alive. It always runs within a computer, but the computer could be alive if it would have the required software and physical parts that it would need to replicate itself.
  19. Oct 20, 2004 #18
  20. Oct 21, 2004 #19
    A biological virus does consist of matter, you could put some of them in a tube, you cannot do that with computer virusses. Computer virusses do not consist of matter, If you claim that they do I think you should also claim that characters in a computer game consist of matter.

    (I know that my proposal would also count biological viruses as life but I actually would like a definition that does not count them as life, my "feeling" about what is life and what is not does not group virusses among the living things... but well i could not think of a nice short definition that would exclude them)
  21. Oct 21, 2004 #20
    I can also put a chip containing computer viruses in a tube.
    I cannot take a viral sequence without its DNA or RNA support, neither a computer virus program without its material basis. Of course, in both cases, I can write the sequences as something "non material" language.
  22. Oct 21, 2004 #21
  23. Oct 21, 2004 #22
    Yes, but the difference is that the chip is not the virus (it contains the virus), the chip is material but the virus is not. You could delete the virus from the chip, you cannot delete the biological virus from its material "body". The biological virus really is this material body.

    The biological virus is not some kind of program in DNA/RNA, the virus is the complex of DNA/RNA and some proteins.
  24. Oct 21, 2004 #23


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    Prions aren't living organisms, they are proteins. Viruses don't fit all of those criterion. They land squarely in the gray area.
  25. Oct 21, 2004 #24


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    If viruses are "alive" then scientists have already "made life" in the lab. Because they have made viruses from chemicals.
  26. Oct 21, 2004 #25
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