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Defining Life: Scientists Squirm, Chickens Carry On

The views expressed are those of the author and are not necessarily those of Scientific American.

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It's a rubber chicken, in space, really (Credit: Earth-to-Sky, Bishop Union High School, CA)

What is life?

Simple question, thousands of years of human intellectual torture trying to answer it. The truth is that ‘life’ really does seem to defy easy definition. We can say that it’s a natural phenomenon – yes, OK. Actually it might be better thought of as a number of deeply connected natural phenomenae, OK, that too. But what are the most critical pieces of these phenomenae that allow us to distinguish life from something that isn’t quite life?

I don’t think anyone has a good solution. And it may be that there’s something a bit off with the question in the first place – our attempts to pin down a complex and sometimes extraordinarily variable set of characteristics with a few parameters may just not work well.

Nonetheless, for astrobiology it would be really useful to have some kind of working hypothesis. A practical set of rules that would help identify ‘life’, even if they were not always sufficient to describe it – these would at least let us eliminate false detections. What we could use is a universal life litmus test.

Self-organization, reduction and oxidation, entropy, self-replication and information storage, cellular structure, energy and metabolism, change and mutation, all characteristics that might form part of such test. But the devil is truly in the details.

Now listen to this excellent podcast from Sift, the brainchild of Aaron Bishop Sand, where he gracefully and intelligently torments a whole groups of scientists with just these questions. You will hear me on this, and if you listen to the middle you’ll get to find out what I think chickens have to do with it all…

It’s a tremendous piece, a veritable sound sculpture. Enjoy.

Caleb A. Scharf About the Author: Caleb Scharf is the director of Columbia University's multidisciplinary Astrobiology Center. He has worked in the fields of observational cosmology, X-ray astronomy, and more recently exoplanetary science. His books include Gravity's Engines (2012) and The Copernicus Complex (2014) (both from Scientific American / Farrar, Straus and Giroux.) Follow on Twitter @caleb_scharf.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. Torbjörn Larsson, OM 12:26 pm 07/9/2012

    “Nonetheless, for astrobiology it would be really useful to have some kind of working hypothesis. A practical set of rules that would help identify ‘life’, even if they were not always sufficient to describe it – these would at least let us eliminate false detections.”

    Agreed, definition of individual organisms serves as definitions of individual fossils. Since the process of life is evolution*, it is better definable IMO (not a working astrobiologist mind, but have studied some) as such (the process that takes biological populations to biological populations by heredity).

    That too can be used to identify individuals as members of populations, maybe even more conclusive or at least generic. But not nearly as fast and feasibly unless generation times are comparable with results of isotopic analysis (say).
    * Which by definition [sic!] is the best such process, allowing for more fit individuals differential procreation in biological populations.

    “Biological” is the necessary fuzzifier word here btw. You don’t necessarily need to put it in unless you want to exclude hardware and software from wetware.

    If you put it in, you don’t necessarily want to restrict to cells, since viruses would be interesting signs of life and cellular life in particular. Maybe biochemistry (hydrocarbon chemistry) would suffice.

    [Yes, that means you push the fuzziness to heredity instead. Which go back to trying to distinguish between chemical and biological evolution and the scale of inherited properties. You can't define away quantitative scales by qualitative distinctions.]

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  2. 2. Torbjörn Larsson, OM 12:28 pm 07/9/2012

    “serves as definitions of individual fossils” – serves akin to definitions of individual fossils.

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  3. 3. dadster 5:59 pm 07/9/2012

    Why is it that life has not yet been manufactured from pure non-organic chemicals in a lab yet despite all the scientific advancements, while it is being produced in abudance in Nature ?
    why should bio-scientists concede that the quality of “life” is an emergent phenomena and not a fundamental entity in cosmos ?
    whats the flaw , if any, in the concept that Life energy is different from electromagnetic energy :is it just because physicists insist that it has to be so ?

    Life is not a matter-based energy although it manifests to us through matter just like electromagnetic energy manifests in matter or light is discernible if and only if matter reflects it .But light is not considered an emergent phenomena just because of matter is needed forlight to manifest itself.There are already a number of physical energies like gravity for example,other than electromagnetic energy. Bio-scientists should object to being harnessed with materials physics for funding purposes and strike out on their own with their own distinct paradigms without piggy-backing on the paradigms of materials physics. High time too.

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  4. 4. imrcroft 6:46 pm 07/9/2012

    As with many topics, the dichotomy of “life” and “non-life” is a potentially inaccurate assessment of the reality of things.

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  5. 5. dadster 6:59 pm 07/9/2012

    Life is not evolved .It is information materialized through matter
    Life energy was “born” simultaneously with matter and matter energies out of “cosmic information-system” or out of vacuum energy .Life energy could be conceived as an entity that leaped of of vacuum energy even prior to material cosmos evolved as a later process. There is some from of life in all the so-called matter forms considered now as non-life forms and it could be made evident in different ways and to different degrees and that’s why its so difficult to define what is life ? if we try to define life as “information” (just as matter could also be defined and not as atoms,strings or quarks )then both life and matter become different manifestation of the same entity Viz “information”.Matter becomes condensed electromagnetic energy( e=mc^2; ‘e’being extricable electromagnetic energy )and Electromagnetic energy is made up of lots of “information”. Life becomes condensed “information” becoming a different pure energy-form that is different from electromagnetic energy, which can manifest through matter also.Life can even manifest in empty space even without the presence of matter and could be detected if we have instruments to detect it with or placing a material on its path to get infused with life .

    if all the extricable electromagnetic energy is extracted out of mass’m’ there wont be any mass left ,all the inertial mass in ‘m’ gets destroyed , but ‘information’ emanating out of the process of the total and complete destruction of the mass ‘m’ will only increase! Matter and “life” have the same energy content Viz,”information” for which there is no deterioration ie no entropy .

    The laws of thermodynamics are not applicable to “information” . Hence it is that life-forms ( which is formed directly from’information-energy ” )can self-organize , change ,mutate all of which cannot be done by ‘mass’ which is information converted into the entity’mass’,through the medium of electromagnetic energy.Hence if we consider ‘information’as the most fundamental entity whose different manifestations are “life ” ,”electromagnetic energy” and then “mass” that would perhaps make more sense.Though out of sentimental orthodoxy conservative physicists might not agree readily because they cannot ‘quantify’life energy as they still haven’t developed either proper instruments nor appropriate units of measurement .Hence they prefer to relegate life to the field of meta-physics and deal with only when its presented in matter measuring of which they have developed the tools. Physicists are vary of introducing any other type of energy than their favorite Electromagnetic energy so much so that they want to digitalize gravitational energy as gravitons .Particle physics want energy carriers and, mandate that all energy carriers must be in the form of ‘particles’. Life -energy being a continuous entity cannot be digitalized either setting further problems for particle physicists. hence it is that bio-science should escape from the harness of physicists and seek out different paradigms than mathematical models which is fit for quantized or quantazable energies only .

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  6. 6. Ed Rybicki 3:46 am 07/10/2012

    We virologists have had problems with definitions of “life” or of “organisms” pretty much since we discovered what viruses are, and how they work – because they genuinely are at the interface between biological life and not-life; between the organismal and the molecular.

    Consequently, we have come up with some fairly ingenious ways of defining life that will include viruses – unlike the ones preferred by leaf-and-stem or legs-and-eyes biologists.

    For example:
    “Life can be viewed as a complex set of processes resulting from the actuation of the instructions encoded in nucleic acids. In the nucleic acid of living cells these are actuated all the time; in contrast, in a virus they are actuated only when the viral nucleic acid, upon entering a host cell, causes the synthesis of virus-specific proteins. Viruses are thus “alive when they replicate in cells, while outside cells viral particles are metabolically inert and are no more alive than fragments of DNA.”"

    - Dulbecco R and Ginsberg HS, 1980. Virology, p.854-855 (originally published as a section in Microbiology, 3rd Edn., Davis et al., Harper and Row, Hagerstown).

    They go on, incidentally, to define an organism as:

    “… the unit element of a continuous lineage with an individual evolutionary history.”

    SE Luria, JE Darnell, D Baltimore and A Campbell (1978). General Virology, 3rd Edn. John Wiley & Sons, New York, p4 of 578.

    Of course, this would cover non-biological organisms too – and Steven Hawking apparently believes that computer viruses should count as life: they are obligate parasites which exploit the “metabolism” of the host computer they infect, they replicate in the form of their source code [=genome], and they newest and nastiest can mutate while they do so.

    This sort of thinking has led to some more inclusive definitions, such as the one by Bernard Korzeniowski in 2001:
    “A network of inferior negative feedbacks subordinated to a superior positive feedback.”

    And then there’s mine:

    “Life is the phenomenon associated with the replication of self-coding informational systems”

    I look forward, therefore, to the discovery of sentient plasma clouds; to the emergence of a benign super-intelligence from the internet – and to silicon-based life forms here on or on Mars.

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  7. 7. biggus56 5:23 am 07/13/2012

    “Phenomenae”? Ugh.
    Please: it’s one phenomenon, several phenomena.

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  8. 8. stevewriter 9:47 am 07/13/2012

    Language and human bias confuse people when they try to think about “life.” The following definition is mechanistic.

    Life is a complete mechanism that is capable of living in the same sense that a complete automobile engine is a mechanism capable of running. An auto engine is a complex material shape that does not run by itself. It sits idle until explosions of gasoline releases energy sequentially in its cylinders. Energy tends go in all directions, but in the engine it reflects off the interior shape and is channeled to running purposes. Auto engines and cells have a consistency to their respective patterns that enables them and others of their pattern to perform in the same way.

    Fully equipped and operating cells have the pattern for living and are the only such mechanisms on the Earth. The matter that forms a cell mechanism supplies a shape, but energy must flow through the shape and be channeled by the shape to cause various living purposes.

    We can find parallels between the auto engine and cell even though they have vastly different complexities and scales. 1) Both mechanisms are defined by pattern and function and not by a particular collection of matter. An engine will still run with a replaced alternator or a repaired engine block and a cell will live with a replaced Golgi body or a repaired cell membrane. 2) Repair is necessary for all mechanisms because some misdirected or extra strong energy causes damage to the mechanism’s pattern as it moves through. 3) Material mechanisms function only in a limited range of temperatures. Engines and cells won’t function on the sun because the intense energy would randomize their pattern, and they won’t function on Pluto because there is insufficient energy to drive process.

    Conceptually, because an automobile engine can guide energy to a purpose it could become the pattern for a living mechanism if additional functions were added to its structure. Those functions must enabled it to automatically acquire fuel, and use energy in the fuel to enable the automatic repair to the pattern of its structure. The information that enabled accurate auto repair would also enable reproduction of the pattern. Reproduction can maintain the existence of the living engine pattern for billions of years if it is set at a point that does not reproduce perfect duplicates and one that allows energy damage to cause death. Death is necessary to recycle materials so that the pattern can continue.

    I am not trying to be air tight in the above argument, but to indicate that life has never been about object. It is about pattern and the way energy moves through a pattern to cause specific rather than random events.

    A functionally immortal cell could not survive for billions of years on the Earth, but a mortal cell pattern has survived and evolved to generate huge cooperative colonies that move and think and call themselves human.

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  9. 9. stevewriter 10:16 am 07/13/2012

    @Ed Rybicki: Are there any viruses that do not use cells to reproduce? I think that viruses evolved from cells and not the other way around. Cells evolved the capability of eating the dead, and then the not quite dead, and then preying on the living. When a phage engulfs a living cell, usually the eaten cell dies and is metabolized by the predator. Suppose that the predator eats a prey cell and the prey cell doesn’t die, but turns the tables on the predator. Some mutation allows the prey’s instructions to survive digestion, overpower the DNA of the predator, and take control of its body.

    There is little advantage for a DNA pattern with that capability to maintain a cell body. It might become a virus by an evolutionary process, eventually becoming a package of instructions with insertion mechanisms waiting to bump into and take over some hapless cell.

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