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Author | Topic: Definition of Life | |||||||||||||||||||
RAZD Member (Idle past 1432 days) Posts: 20714 From: the other end of the sidewalk Joined: |
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: and
quote:(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, : colorby our ability to understand Rebel☮American☆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)
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RAZD Member (Idle past 1432 days) Posts: 20714 From: the other end of the sidewalk Joined: |
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 Rebel American 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) • • •
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RAZD Member (Idle past 1432 days) Posts: 20714 From: the other end of the sidewalk Joined: |
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: 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: 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: 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, : .by our ability to understand Rebel☮American☆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)
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