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Thread: Why life needs a liquid (tho maybe not water) - explanation by Chris McKay

  1. #61
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    Quote Originally Posted by swampyankee View Post
    I don't know how serious biologists are about trying to find a definition of life, as that may be more a philosophical than scientific question, but it's been recognized for at least 50 years that a complete and consistent definition of life is likely impossible: if you need to explicitly exclude or include something like viruses or prions, the definition is neither complete nor consistent.
    The first of the definitions mentioned in Wikipedia's page on Life says that living things "maintain homeostasis, are composed of cells, undergo metabolism, can grow, adapt to their environment, respond to stimuli, and reproduce." Possibly needs refining to say that not every individual living thing has to be able to reproduce, as long as it's a member of a species that does. Other than that, how is the definition not complete/consistent?

    Requiring specific chemistry is almost certainly flawed,
    I suppose it could be argued that biology is about identifying characteristics of species by examining specimens, and so a definition of life can and should include all the characteristics which all the specimens we've examined have in common, including their chemical basis.

    The counter argument is that scientists are more likely to find interesting new specimens elsewhere in the solar system if they engage in speculation about how other feeding/growing/reproducing/evolving things might differ from currently known ones — speculation which is informed by knowledge of chemical and thermodynamic principles, and knowledge of physical conditions on other planets and moons. And therefore we need a definition of life which includes what we might find as well as what we have found.

    and the remnants of flawed or excessively restricted definitions show up in many areas of science: the Arrhenius acid/base system means that "acid" or "base" can only be defined in aqueous systems and oxidation was originally restricted to reactions with oxygen.
    Interesting examples! Chemists seem quite happy to have multiple definitions of acidity and basicity — there are "Arrhenius acids", "Brønsted acids", and "Lewis acids".

    Getting back to the question of life and liquids, I've seen it argued that water is ideal for life because it has neutral pH, i.e. it is right in the centre of the acid/base spectrum.

    However (as chemists realised long ago) the supposed neutrality of water is simply a consequence of Arrhenius defining acidity and basicity in terms of the ions H+ (or H3O+) and OH-. If we are considering chemical reactions where the solvent is ammonia, then it is more logical to define acidity and basicity in terms of the ions NH4+ and NH2-, and by that definition water is distinctly an acid.
    Last edited by Colin Robinson; 2017-Apr-10 at 08:00 AM.

  2. #62
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    Quote Originally Posted by swampyankee View Post
    I don't know how serious biologists are about trying to find a definition of life, as that may be more a philosophical than scientific question, but it's been recognized for at least 50 years that a complete and consistent definition of life is likely impossible: if you need to explicitly exclude or include something like viruses or prions, the definition is neither complete nor consistent. Requiring specific chemistry is almost certainly flawed, and the remnants of flawed or excessively restricted definitions show up in many areas of science: the Arrhenius acid/base system means that "acid" or "base" can only be defined in aqueous systems and oxidation was originally restricted to reactions with oxygen.
    I agree. I don't think anyone has ever said that the definition of life necessarily includes oxygen or water.

    I think most people would say that a cell, metabolising CHN-based molecules in liquid methane, actively growing and reproducing cells of the same species would be classed as "alive".

    In fact to say otherwise is almost certainly some kind of "-ism".

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    Quote Originally Posted by kzb View Post
    I agree. I don't think anyone has ever said that the definition of life necessarily includes oxygen or water.
    And yet, there is no evidence that non-oxygen/water based chemistries display functions consistent with the definition of life.
    (The definition cited previously by Colin will do).

    Quote Originally Posted by kzb View Post
    I think most people would say that a cell, metabolising CHN-based molecules in liquid methane, actively growing and reproducing cells of the same species would be classed as "alive".

    In fact to say otherwise is almost certainly some kind of "-ism".
    No .. in keeping with the intent of the OP, it would be worded as a progression of the OP hypothesis, but made easily testable, ie: 'that only oxygen based chemistries display functions consistent with the definition of life' ... (And that hypothesis is already supported with an overwhelming abundance of independently verifiable test results).

    Assertions made pertaining to your first statement above however, would be only opinion-dependent, (as per the OP intent), as the hypothetical entity you describe, has zero objective evidence supporting its existence in the first place.

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    Quote Originally Posted by Selfsim View Post
    And yet, there is no evidence that non-oxygen/water based chemistries display functions consistent with the definition of life.
    (The definition cited previously by Colin will do).



    No .. in keeping with the intent of the OP, it would be worded as a progression of the OP hypothesis, but made easily testable, ie: 'that only oxygen based chemistries display functions consistent with the definition of life' ... (And that hypothesis is already supported with an overwhelming abundance of independently verifiable test results).
    On how many different planets were these tests carried out?

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    Quote Originally Posted by Colin Robinson View Post
    On how many different planets were these tests carried out?
    Irrelevant as far as the stated hypothesis is concerned. (Ie: 'that only oxygen based chemistries display functions consistent with the definition of life').
    Its falsifiable too .. but not by citing hypotheticals.

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    Quote Originally Posted by Selfsim View Post
    Irrelevant as far as the stated hypothesis is concerned. (Ie: 'that only oxygen based chemistries display functions consistent with the definition of life').
    Its falsifiable too .. but not by citing hypotheticals.
    This thread is all about hypotheticals.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright
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    Quote Originally Posted by Selfsim View Post
    Irrelevant as far as the stated hypothesis is concerned. (Ie: 'that only oxygen based chemistries display functions consistent with the definition of life').
    Life on Earth is usually described as "carbon-based" rather than "oxygen based". After all, life molecules have backbones made of chains of carbon atoms, not oxygen atoms. In nearly all cases, they contain more hydrogen atoms than oxygen atoms, so perhaps there is a case for describing them as "carbon-hydrogen based"...

    Do you really think it's irrelevant that biologists have studied only one planet — a planet where most of the atmospheric carbon is in the form of a carbon-oxygen compound (CO2), and where most of the hydrogen is in the form of a hydrogen-oxygen compound (H2O), and where H2O is also the main surface liquid?

    Isn't the chemistry of life on Earth also consistent with the hypothesis that carbon-hydrogen based life uses whatever other elements are readily available in the atmosphere and in liquids, especially elements found already linked to atoms of carbon and/or hydrogen ?

    Its falsifiable too ..
    Yes. One hypothesis or the other may well be falsified by sending further probes to Titan for a more thorough look...

    but not by citing hypotheticals.
    Citing a hypothesis is not the same thing as testing it. But in order to test it, you have to cite it first.

    Quote Originally Posted by Noclevername View Post
    This thread is all about hypotheticals.
    Very true. Every hypothesis is hypothetical, because "hypothesis" and "hypothetical" are two forms of the same word.
    Last edited by Colin Robinson; 2017-Apr-12 at 09:45 AM.

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    'that only oxygen based chemistries display functions consistent with the definition of life' ... (And that hypothesis is already supported with an overwhelming abundance of independently verifiable test results).

    OK, following the same logic, the only proof that life exists anywhere in the universe is here on Earth. So why is anyone bothering searching for life elsewhere?

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    Quote Originally Posted by kzb View Post
    'that only oxygen based chemistries display functions consistent with the definition of life' ... (And that hypothesis is already supported with an overwhelming abundance of independently verifiable test results).

    OK, following the same logic, the only proof that life exists anywhere in the universe is here on Earth. So why is anyone bothering searching for life elsewhere?
    Fifty years ago, there was no proof of the Higgs boson. One hundred years ago, there was no proof of the neutron. Two hundred years ago, there was no proof micro-organisms caused disease.

    Among other places that life has been found, after being thought impossible, on Earth is several kilometers within the crust, within halite deposits, within hot springs, and thermal vents on the ocean floor. When the Trieste reached the bottom of the Marianas Trench, the expectation was that nothing would be living there. Dismissing the possibility of life in non-terrestrial environments is premature.

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    Quote Originally Posted by Selfsim View Post
    Irrelevant as far as the stated hypothesis is concerned. (Ie: 'that only oxygen based chemistries display functions consistent with the definition of life').
    Its falsifiable too .. but not by citing hypotheticals.
    Yes it is falsifiable -but this experiment would require surveying every single suitable planet in the universe !

  11. #71
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    Quote Originally Posted by swampyankee View Post
    Fifty years ago, there was no proof of the Higgs boson. One hundred years ago, there was no proof of the neutron. Two hundred years ago, there was no proof micro-organisms caused disease.

    Among other places that life has been found, after being thought impossible, on Earth is several kilometers within the crust, within halite deposits, within hot springs, and thermal vents on the ocean floor. When the Trieste reached the bottom of the Marianas Trench, the expectation was that nothing would be living there. Dismissing the possibility of life in non-terrestrial environments is premature.
    I was trying to show up the error in Selfsim's argument.

    His logic seems to be:

    All Earth life has molecules containing oxygen

    Therefore, there is no evidence for life that does not require oxygen

    Therefore, life without oxygen is impossible.

    From what you've written I doubt you would agree with that position.

    The truth is we have a very intriguing possibility that completely alien-chemistry life exists on Titan. Hydrogen and acetylene are produced in the upper atmosphere but are being consumed somehow at the surface. No-one can think of a good reason why that would happen at the temperature on Titan.

    Added to that, there are an interesting range of unsaturated organic molecules in the atmosphere which are ideal for making polymers. In other words, cell membranes and other structures could be made.

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