Definition of Life

Thank you. Its a great forum. Stability is relative, so perhaps life is too - eg I am more alive than my cells, atoms are more stable (alive?!) than protons, etc?This thought process is under development.Cheers.

Hmm. I don't think I agree. I feel much less stable than an atom. Wait? Don't atoms just change links here and there if I die, and then if I rot in the grave/riverbed? I mean, it would seem that my atoms are MUCH more durable than myself .J0N

Yes that's hard to disagree with... Perhaps if we consider the life form (species) rather than the individual: a species of bacteria lasts longer than each individual, so the form has stability provided by reproduction; so reproduction is one method of gaining stability (of the form). I'm not sure of my facts, but I'm guessing that: at the atom level, individual protons are less prone to being reduced to quarks and anihilating when they are hiding inside atoms, as are quarks inside protons. I know it's a different type of stability but is it not these layers of stability that build up to produce the complexity we call life?I'm not at all sure, so glad to be learning from this forum. All the best.Edited by 42, : Messed up the quoting

A slight correction about the decay of protons and neutrons.It is neutrons which decay rapidly when they're isolated: an isolated neutron will decay into a proton, a neutron, and an anti-neutrino, with a half-life for the decay of the neutron of about twelve minutes. It is neutrons, not protons, that are unstable when isolated.Whether protons decay at all, ever, is, if I recall correctly, a controversial question, but if they do, they have an enormous half-life.

I think you are on to a winner [Off-topic... please do not respond]

Thank you. I thought neutrons and protons were more similar than they are!
All the best.

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Human Evolution in 42 Steps

start at Message 1 - the discussion is on what is, and what is not, life.Enjoy.

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Join the effort to unravel {AIDS/HIV} {Protenes} and {Cancer} with Team EvC! (click)

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we are limited in our ability to understand
by our ability to understand

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RebelAAmericanOZen[Deist
... to learn ... to think ... to live ... to laugh ...
to share.

I am not sure if this discussion thread is still alive, but if it is, my contribution could be an essay that is devoted to ORIGINAL OF LIFE question."...The explanation of the living system phenomenon is proposed that Consciousness is the fundamental property of MATTER that is just not observable in the non-living systems. Consciousness property of subatomic particles is not recognized by fundamentally incomplete quantum mechanics theory. Consciousness property of Matter also is not observable in the thermodynamically equilibrium systems. However if a system steered far enough from an equilibrium and past a critical point, a non-equilibrium system will emerge. The further development of these systems in the direction out from equilibrium will reveal the property that causes the phenomenon we call - LIFE".To learn more about this idea you can go to http://www.iscid.org/...rs/Iosim_ComplexSystemSimplicity.pdf
Introduction to this essay was posted also in this EvC forum in Biological Evolution section under topics "THE SIMPLICITY OF THE COMPLEX SYSTEMS"MarkEdited by miosim, : No reason given.

Reviving this old thread in response to an issue on the Self-sustained Replication of an RNA Enzyme thread: The simple answer is that there is no clear definition of life that always distinguishes life from non-life.There are examples that we can all agree belong to the category "life" and there are examples that we can all agree belong to the category "non-life" ... and then there are examples where we cannot agree that they belong in "life" or in "non-life" categories, and there are no currently known criteria that can make this distinction. Personally, I think the best working definition I've seen, is that life is some physical arrangement of atoms and molecules that is potentially capable of evolution (the change in hereditary traits in populations from generation to generation in response to ecological opportunities) and the formation of nested hierarchies of descent. Note that this allows self-replicating molecules to meet this definition of life, and this falls into the category of {examples where we cannot agree that they belong in "life" or in "non-life" categories}.On this thread we see:

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Message 7
One site I ran across in my research into abiogenesis is
Psychozoan: The Definition of Life
It discusses the different parts of the definitions with pros and cons. Rather interesting, if not too practical in the long run -- the definitions are too frought with problems when they:
(a) includes things that are not (normally) considered alive
(2) excludes things that are normally considered alive

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and

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Message 25
See wikipedia
Life - Wikipedia
particularly the "conventional definition
Life - Wikipedia

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While there is no universal agreement on the definition of life, scientists generally accept that the biological manifestation of life exhibits the following phenomena:

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1. Organization - Living things are composed of one or more cells, which are the basic units of life.
2. Metabolism - Metabolism produces energy by converting nonliving material into cellular components (synthesis) and decomposing organic matter (catalysis). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
3. Growth - Growth results from a higher rate of synthesis than catalysis. A growing organism increases in size in all of its parts, rather than simply accumulating matter. The particular species begins to multiply and expand as the evolution continues to flourish.
4. Adaptation - Adaptation is the accommodation of a living organism to its environment. It is fundamental to the process of evolution and is determined by the organism's heredity as well as the composition of metabolized substances, and external factors present.
5. Response to stimuli - A response can take many forms, from the contraction of a unicellular organism when touched to complex reactions involving all the senses of higher animals. A response is often expressed by motion: the leaves of a plant turning toward the sun or an animal chasing its prey.
6. Reproduction - The division of one cell to form two new cells is reproduction. Usually the term is applied to the production of a new individual (either asexually, from a single parent organism, or sexually, from at least two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth.

(bold in the original)

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(note the second wiki link above works, but it takes you to the same place as the first wiki link and should be replaced by
Life - Wikipedia)To my mind, basing a definition on the existence of a cell is begging the question -- the first criteria is basically saying that life is something that has the basic units of life. This is a fairly standard definition of life, and it was reviewed by Joseph Morales (see above), and he ended by concluding that there are degrees of life, different levels that apply.Enjoy.Edited by RAZD, : link updateEdited by RAZD, : color

The trouble with any definition of life that includes reproduction is that excludes the many examples of things we'd call alive but don't reproduce: the sterile, the elderly and the unlucky. So you end up having to be rub in a side order of "potential to reproduce" or "from a class that reproduces" which again muddy the issue and, even then, reproduction is, itself, not exactly a trivial concept to clearly define.

Hi Mr Jack, Exactly. You could say that {life} is capable of reproduction or is a product of reproduction. Especially if you want to exclude replicating molecules from your defined class of life ....Enjoy.Edited by RAZD, : clrty

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When it comes to multicellular organisms the definition says it can be broadly applied to include growth/development (ie cells dying and new cells being generated). The elderly are still hotbeds of reproducing cells, even if they lack any viable germ line cells.

What definition is that? Including growth and simple cell replication in reproduction is a very non-standard notion of reproduction, indeed.

It's from the conventional definition that RAZD posted above:

quote:

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Reproduction - The division of one cell to form two new cells is reproduction. Usually the term is applied to the production of a new individual (either asexually, from a single parent organism, or sexually, from at least two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth.

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I referenced this earlier upthread, and recently re-read it. Fascinating discussion of pros and cons to many definitions\approaches. I can summarize his conclusions as follows (italics mine for clarity): The Definition of Life by Joseph Morales, 1998

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We will be searching for a definition of life that is useful. In order to be useful, the definition should meet the following criteria, so far as possible:

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• Sufficiency. It should provide the sufficient conditions that enable us to specify whether something is living or not.
• Common Usage. These conditions, when applied to "easy" examples, should classify those examples in the same way we normally do. Easy examples include obviously living things such as people, animals, plants, and bacteria; things that were alive but are now dead; and things that we would never normally consider alive, such as rocks, screwdrivers, and growing crystals.
• Extensibility. It should be possible to apply these conditions to "difficult" examples with some kind of coherent result. Difficult examples include viruses, mules, fire, simple feedback systems (such as those with thermostats), Gaia, extraterrestrial creatures, and robots.
• Simplicity. The definition should be as simple as possible, with a minimum of ifs, ands, or buts.
• Objectivity. The definition should refer to measurable and objective properties of the organism. That is, the definition should be specific enough so that different people can be counted on to apply the definition in the same way when they encounter a new "difficult" example.

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Once we have determined the sufficient conditions that something must satisfy in order to be considered living, we can go on to ask what additional properties follow from the fact that something is alive. ...

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Our procedure will be to review some of the definitions that other authors have given, beginning with the most naive and progressing to the more satisfactory. Then we will try to improve on the best existing definitions.

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... suggests a new definition of life, which I shall formulate as follows:

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Living things are systems that tend to respond to changes in their environment in such a way as to promote their own continuation.

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When I say "new," of course, I mean only "new to me." Such a simple and obvious definition has probably been proposed by someone before. Yet in my reading on the subject, I have never run across it.

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Do we need to incorporate the concept of feedback into our definition of life? Well, it is important to recognize that feedback per se does not necessarily promote life. Positive feedback can increase the fluctuation in a system and cause it to self-destruct. Even negative feedback can be inappropriate, if it preserves the stability of some subsystem at the expense of the system as a whole. The central point seems to be that the ensemble of inner processes tends toward homeostasis, the maintenance of the overall pattern of the system.

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Remember that we are searching for the minimal criteria to identify life. From this point of view, it is sufficient to amend our definition by adding one simple clause:

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Living things are systems that tend to respond to changes in their environment, and inside themselves, in such a way as to promote their own continuation.

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A Matter of Degree

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Why are there "difficult" examples to address in definitions of life? Take the example of viruses. We have seen, so far, that Barrow and Tipler regard the virus as alive, whereas Poundstone and Margulis and Sagan regard it as not alive.

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In other words there are disagreements in where we can draw the line and stay that "life" begins here (this thread discussion is regarding the definition of life as it pertains to abiogenesis, and the beginning of life).He promised more to follow, but has not published\posted it yet as far as I can tell (going to his baharna home page).He discusses the shortfalls of microbiological definitions of life, in particular that they only apply to the cell and not to life-forms composed of multiple cells.He also discusses the "mule problem" in regards definitions based on reproduction vs autopoieses:

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Can we use autopoiesis as a single sufficient criterion for identifying life forms? ... Margulis and Sagan consider non-replicating examples such as mules:

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Replication is not nearly as fundamental a characteristic of life as autopoiesis. Consider: the mule, offspring of a donkey and a horse, cannot "replicate." It is sterile, but it metabolizes with as much vigor as either of its parents: autopoietic, it is alive. ...

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Now consider the case of the candle flame. ... All the molecules in the flame are regularly replaced, yet the flame itself persists. Is the candle flame an autopoietic system? Seemingly it is. Thus it would seem that things can be autopoietic without being alive.

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It would seems that replacement and repair of components is a part of life, from the cellular level to the multicellular level, but that that alone is insufficient to define life.And he discusses the "honey bee" problem in regards to genetic continuance:

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Now consider the honey bee, which gives its life by stinging a bear that is attacking the hive. The case is different here, for a worker bee can never have any offspring. Only the queen and a small number of drones play any role in reproduction. ... So in a general way the bee must defend the whole hive to ensure its own continuance, and thus it makes sense for it to give its life by stinging the bear.

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The examples of suicide missions and rescuing strangers are far less straightforward. The sociobiologists, such as Edmund O. Wilson, have proposed that altruism can be a genetically motivated trait, even in humans. However, we can also see that social forces can and do encourage altruistic behavior in all of us. Whether the causes are genetic or cultural, the underlying logic is much the same. Human beings are generally social creatures and our individual survival depends on our being part of a viable social group. Actions that benefit other members of the group can thus have an indirect future benefit either for the actor, or for relatives or descendants of the actor, or at the very least for members of the same species. ...

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Sterile people can act to benefit family, nation, species ... but they don't have to, so this behavior fails as a requirement to the definition of life.At this point he comes to Homeostasis ("... the property of a system in which variables are regulated so that internal conditions remain stable and relatively constant. ... ") and makes the modification to his definition noted above. After this he discusses various factors, but makes no further critiques of other definitions nor make a further change to his definition, discussing instead aspects of his definition. So his final version is: One problem I have with this definition is that it doesn't address growth, one of the standard elements of the standard definition of life. It seems to me that any good definition should be able to derive or default to the standard definition when looked at in detail.EnjoyEdited by RAZD, : .Edited by RAZD, : .