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Author Topic:   Definition of Life
RAZD
Member (Idle past 1404 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


Message 69 of 77 (560232)
05-13-2010 9:46 PM


A Simple Definition of Life ...
Reviving this old thread in response to an issue on the Self-sustained Replication of an RNA Enzyme thread:
Message 24: Which is what makes it so much fun to delve into an actual definition, because this seems to be such an easy question to answer at first.
And yes, I do have an answer, a fairly simple one.
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:
quote:
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
and
quote:
Message 25
See wikipedia
Life - Wikipedia
particularly the "conventional definition
Life - Wikipedia
While there is no universal agreement on the definition of life, scientists generally accept that the biological manifestation of life exhibits the following phenomena:
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)
(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 update
Edited by RAZD, : color

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Replies to this message:
 Message 70 by Dr Jack, posted 05-14-2010 10:56 AM RAZD has replied

  
RAZD
Member (Idle past 1404 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


Message 71 of 77 (560387)
05-14-2010 8:29 PM
Reply to: Message 70 by Dr Jack
05-14-2010 10:56 AM


Re: A Simple Definition of Life ...
Hi Mr Jack,
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.
Exactly.
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 ...
You could say that {life} is capable of reproduction or is a product of reproduction.
... and, even then, reproduction is, itself, not exactly a trivial concept to clearly define.
Especially if you want to exclude replicating molecules from your defined class of life ....
Enjoy.
Edited by RAZD, : clrty

we are limited in our ability to understand
by our ability to understand
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... to learn ... to think ... to live ... to laugh ...
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• • • Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click) • • •

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Replies to this message:
 Message 75 by RAZD, posted 12-02-2015 10:42 PM RAZD has seen this message but not replied

  
RAZD
Member (Idle past 1404 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


Message 75 of 77 (773505)
12-02-2015 10:42 PM
Reply to: Message 71 by RAZD
05-14-2010 8:29 PM


Another Simple Definition of Life ... by Joseph Morales.
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
quote:
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:
  • 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.
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. ...
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.

... suggests a new definition of life, which I shall formulate as follows:
Living things are systems that tend to respond to changes in their environment in such a way as to promote their own continuation.
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.

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.
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:
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.

A Matter of Degree
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.
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:
quote:
Can we use autopoiesis as a single sufficient criterion for identifying life forms? ... Margulis and Sagan consider non-replicating examples such as mules:
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. ...
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.
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:
quote:
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.
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. ...
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:
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.
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.
Enjoy
Edited by RAZD, : .
Edited by RAZD, : .

we are limited in our ability to understand
by our ability to understand
RebelAmerican☆Zen☯Deist
... to learn ... to think ... to live ... to laugh ...
to share.


Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click)

This message is a reply to:
 Message 71 by RAZD, posted 05-14-2010 8:29 PM RAZD has seen this message but not replied

  
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