The definition of life

CACTUSJACKmankin

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I have a problem with the current definition of life. The "definition" that we were all fed in school is essentially a laundry list of things that cells do. It's way more complicated than it needs to be, it's against occam's razor. Let's try to think of what fundimentally makes living systems different from other kinds of chemical reactions. In other words, what sets biology apart from chemistry? It's evolution! Evolution defines life. Evolution should be the focus of the definition of life, not just an "also". Using evolution as the definition for life is the simplest definition. It would however include things like viruses.

Put it this way: if you drink a poison and take the antidote (barring individual reactions such as allergy) you will get better each time you take the poison and later take the antidote. The poison itself doesn't become something that the antidote can't deal with. But if you have a bacterial infection, you can take treatment for it and it may take care of the infection but, that bacteria can become resistant to treatment. That's a growing problem in the medical field. The same kind of thing is behind why you keep getting colds. Colds keep evolving into strains that our bodies haven't fought yet. Just as when you eliminate the bacteria you can deal with only the resistants are left, when you can kill off all of one cold strain the only ones left are a different strain you can't kill yet.

Agree or disagree and why.
 
Good luck. It's the same trying to define a planet.

Planetary bodies and life forms fall on a continuum. Where you define the change from planet to non-planet and life to non-life is where you arbitrarily draw the line.

Even using evolution has drawbacks since you are essentially saying DNA and RNA molecules, they 'evolve' IE change over time via random errors in copying, but don't have to be life forms. They could be segments free floating in cells or within mitochondria. Are you going to say mitochondria are separate life forms?
 
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What about going one simpler, where we classify life as a chemical system that can avoid entropy.
 
chance said:
What about going one simpler, where we classify life as a chemical system that can avoid entropy.
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The laundry list I learned at school was move, breathe, feed, grow, excrete, reproduce and respond to stimuli. I agree its not a definition, more a description that uses ungrammatical English.

However, I do not think scientists could use a definition of life based on evolution if only because they would then be accused of trying to define their way out of the evolutionism/creationism "controversy." Chance seems to me to have the right general approach but the wording would need to be improved.
 
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CACTUSJACKmankin said:
I have a problem with the current definition of life. The "definition" that we were all fed in school is essentially a laundry list of things that cells do. It's way more complicated than it needs to be, it's against occam's razor. Let's try to think of what fundimentally makes living systems different from other kinds of chemical reactions. In other words, what sets biology apart from chemistry? It's evolution! Evolution defines life. Evolution should be the focus of the definition of life, not just an "also". Using evolution as the definition for life is the simplest definition. It would however include things like viruses.
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I disagree. As a thought experiment: suppose you had an asexually reproducing fish (they do exist) that had the unusual trait of a super-accurate copy mechanism such that the likelihood of any of the fish's offspring having a mutation which differentiated it from its parent or siblings was statistically insignificant. Basically the fish has stopped evolving and at some point will probably go extinct when conditions change for the worse.

If evolution defines life, are any of those fish alive?
 
agreeing with skeptigirl...

The fundamental unit of natural selection (arguably, a la "the selfish gene") is not the organism but the gene. I am not certain how that effects your argument--would you say that it is DNA that is alive, or that the methods by which DNA copies itself (organisms) are alive?

There are other examples of selection by consequences, too--operant behavior for one, culture for another. Same process (natural selection) but with different replicants (behavior and customs)--but if we define life by "things that evolve", then the fuzzy category now includes things vastly different than we currently think of as alive.
 
Mojo said:
A better definition might be "things that replicate themselves", perhaps.
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Dennett calls those "replicants", and includes genes and memes in this. Skinner would include operant behaviors as well.

Not all would agree with either of them, but there ya go...
 
John Hewitt said:
The laundry list I learned at school was move, breathe, feed, grow, excrete, reproduce and respond to stimuli. I agree its not a definition, more a description that uses ungrammatical English.
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I learned the same list, as well as a list of the exception for each trait - either something not alive has it, or something alive lacks it (or both).

I'll make a futile attempt to kill the thread by saying life is a fuzzy grouping based on arbitrary criteria, and every attempt to nail it down has failed.
 
aerosolben said:
I'll make a futile attempt to kill the thread by saying life is a fuzzy grouping based on arbitrary criteria, and every attempt to nail it down has failed.
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I agree. (so much for killing the thread.)

My mom once told me about a sermon she had just heard--it granted that there was tremendous variability in the world, extraordinary variability among forms of life, but "one thing is certain--the difference beween life and non-life". She was certain I'd find the larger message agreeable, and was surprised a bit when I disagreed with his bedrock assumption. Of course, since she used to teach biology, I was able to bring up viruses and prions, as well as computer-based "life".

Fuzzy indeed...
 
I doubt that it matters, but when I took genetics in college, the definition of life we were given then was something that:

1) Could replicate itself
2) Could metabolize something for energy to fight entropy (this is possibly the 'bad' use of entropy)
3) Could differentiate itself from other things (typical example was a cell membrane)

This definition allowed us to agree that things like crystals, catalytic chemicals and viruses were not living and move on.
 
Chance said:
What about going one simpler, where we classify life as a chemical system that can avoid entropy.
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A diamond avoids entropy longer than a typical organism.

Give it up, it's hopeless. Let's define free will instead.

No, not really. Only kidding.

~~ Paul
 
Which definition of entropy are we using here?

The "things become less organized over time" one? I thought it was unpopular to use that one anymore because it isn't what the second law of thermodynamics says.
 
Life is a chemical reaction that hasn't finished yet, and started a long time ago. Life is a very simple analogue that only drew serious breath when computers came along. Does anybody else out there remember Life ?
 
The definition I follow in school is good ol' MRS GREN: Movement (even if it's just translocation of chemicals across a membrane), Reproduction, Sense (respond to environmental stimulus), Grow, Respire (use energy), Excrete, Nutrition (take in and change chemicals).

This is crude but an effective starting point.

Of course, assuming that there is a line between biotic and abiotic is a false dichotomy.

Athon
 
Wavicle said:
If evolution defines life, are any of those fish alive?
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Presumably life means all the things that are alive today, the so-far-so-good lines of descent. You make an important point, that slavish reproduction from generation to generation is not going to prosper in the long-term. The mechanisms of life, cellular biology and so on, have evolved by selection to have a non-zero error-rate in reproduction.
 
Wavicle said:
I disagree. As a thought experiment: suppose you had an asexually reproducing fish (they do exist) that had the unusual trait of a super-accurate copy mechanism such that the likelihood of any of the fish's offspring having a mutation which differentiated it from its parent or siblings was statistically insignificant.
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That's little different from bacteria, which develop resistance and certainly evolve and are definitely alive. No matter how super-accurate the copy mechanism, mutation will occur some of the time. No matter how slowly, evolution will happen and they are alive.

Here's a thought experiment, what if we find life on another planet and it happens not to be cellular? Do we ignore it as non-living or is it the ultimate proof that we're not alone? This is why we need as open and basic a definition of life as possible, apart from that it is philosophically better have a simpler definition.

In a futile attempt to make a coherent definition ==> Genetic molecule that is encapsulated and has a method of copying.
 
CACTUSJACKmankin said:
That's little different from bacteria, which develop resistance and certainly evolve and are definitely alive. No matter how super-accurate the copy mechanism, mutation will occur some of the time. No matter how slowly, evolution will happen and they are alive.
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Well, the reason we call it a "thought experiment" is because we are going to posit impossible or impractical conditions in order to explore a question further. You wouldn't discard Einstein's thought experiments with trains moving near the speed of light because air resistance would always keep the train at sub-relativistic velocities. The question is: if an asexually reproducing fish somehow had a perfect copy mechanism, would the proposed definition now exclude them from living?

CACTUSJACKmankin said:
Here's a thought experiment, what if we find life on another planet and it happens not to be cellular? Do we ignore it as non-living or is it the ultimate proof that we're not alone? This is why we need as open and basic a definition of life as possible, apart from that it is philosophically better have a simpler definition.
Click to expand...

Most definitions would have no issue with this. If it reproduces and metabolizes; it's most of the way there.

CACTUSJACKmankin said:
In a futile attempt to make a coherent definition ==> Genetic molecule that is encapsulated and has a method of copying.
Click to expand...

But wouldn't that definition exclude a virus, since it has no method of copying? I'm not opposed to exluding virii (I don't consider them living) but I thought you wanted a definition which included them.
 
The thing is, life isn't a neat definition. It describes phenomena we see. Hence if we came across something that fit most of our description for life excluding one, we would need to discuss how this impacted on our understanding of things. It might turn out that the mechanics of evolution (such as random variation) could 'freeze up'. We would need to evaluate what this means for life, and reevaluate.

Therefore such a thought experiment is a useless proposal. We might as well say 'what if we came across X which did not do Y' for any category within science. Categories are merely tools with which we analyse what we observe.

Another way of putting it; if such a fish as that which you suggest is found, what would our redefining of 'life' accomplish? If it is more useful to redefine it to include that fish, we would do so. If it is more useful to create a new categorisation, then we would do that.

It's the Pluto discussion all over again. And honestly, it matters little, apart from the fact there has to be some consensus.

Athon
 
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