The definition of life

[CACTUSJACKmankin]

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.

[skeptigirl]

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?

[chance]

What about going one simpler, where we classify life as a chemical system that can avoid entropy.

[John Hewitt]

What about going one simpler, where we classify life as a chemical system that can avoid entropy.
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.

[Wavicle]

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.

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?

[Mercutio]

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]

A better definition might be "things that replicate themselves", perhaps.

[John Jackson]

A better definition might be "things that replicate themselves", perhaps.


Such as crystals? :eek:

[Mercutio]

A better definition might be "things that replicate themselves", perhaps.

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...

[aerosolben]

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.
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.

[Mercutio]

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.
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...

[Wavicle]

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.

[Paul C. Anagnostopoulos]

What about going one simpler, where we classify life as a chemical system that can avoid entropy.
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

[Wavicle]

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.

[CapelDodger]

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 ?

[athon]

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

[CapelDodger]

If evolution defines life, are any of those fish alive?

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.

[CACTUSJACKmankin]

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.
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.

[Wavicle]

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.

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?

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.

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

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

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.

[athon]

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

[Wavicle]

The thing is, life isn't a neat definition.
I'm quite sure I am not disagreeing on that point.

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.

Ummm, no. Hasty generalization.

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.

The thread started because CACTUSJACK felt that commonly used definitions for life were unnecessarily complicated. My point was that using evolution as a minimalist definition of life is inadequate. Not sure where exactly you're coming from. I thought we were just philosophizing about a particular proposed definition.

[Dr Adequate]

Life is chemicals which given the right environment can catalyse their own synthesis.

* adopts Early Christian Martyr look *

[Wolfman]

This was a popular debate between myself and my friends back in my university days; it tended to be inspired by science-fiction type fantasies, trying to come up with the most bizarre possible forms of 'life' possible.

But one thing that became quickly apparent -- and which is apparent in most of the discussions above -- is that our "definition" of life, as such, is not so much a definition of all possible forms that life could take, but rather a convenient means of differentiating all forms (living and non-living) that we've had personal contact with.

Think of our classification system for life (family, genus, etc.). There is nothing implicitly "right" about this system; it is, rather, a convenient way for us to classify all life that we've found on the planet. If we were to discover life on another planet, it would not fit anywhere within this classification scheme; we'd have to create an entirely new classification for the new life.

It is my argument that our definition of "life" is the same. It considers only those forms with which we've actually had experience, and seeks to classify them as "living" and "non-living". Even within our own world, excluding all other possible life forms in the universe, we've had to re-think our definition of "life" several times, with the discovery of things such as viruses, prions, etc. For the most part, we tend to do that by saying "...this is an exception", but there are those trying to come up with better definitions.

We face the same problem in trying to differentiate between "plant" and "animal"; as our knowledge of the world around us grows, we find more and more examples of life that does not fit tidily in either category.

But to get to my point -- our "definitions", as such, are pretty much always retroactive. We don't start by considering all possible forms that life could take, and then creating a definition based on that. Rather, we start by looking at all examples of life that we are familiar with at the present time, and trying to find a definition that best fits with those examples. And, as our knowledge changes/grows, we need to make adjustments to that definition.

I would not include something like DNA in a definition of life, for example; I would include it in a definition of terrestrial life, but consider it possible that life could form elsewhere that used a different mechanism. I would not include evolution as a definition of life; within a stable system, evolution is neither necessary nor even necessarily beneficial. There are, in fact, creatures living on our planet today which have remained pretty much unchanged for millions of years; are we going to relegate them to non-living status?

Replication -- so far as I can consider, I'd tend to think of replication as a necessity for the extended survival of life; but not, in the strictest terms, as a necessary definition of life. To take a rather extreme (and quite possibly, but not certainly, impossible) example, consider a gas cloud floating around in space that managed to, over millions/billions of years, develop some form of internal organization and cohesion. It takes in energy, uses that energy to maintain itself, even produces by-products. But it lacks material for reproduction (in this case, reproduction would mean splitting itself into two smaller portions, neither having the capability to grow larger). I know I'm wandering into realms of rather wild speculation here, but just want to demonstrate that reproduction may not be an absolute requirement.

I would tend to look more at issues like organization. Every living creature that exists on our planet, and every living creature that I would be able to envisage in my imagination, would require the ability to organize matter/energy in very specific patterns/forms. I don't know exactly how to describe this (there are others here with a much greater specific vocabulary than myself in this regard), but essentially this is pretty much the same argument as the entropy/metabolism arguments used by others already (although I'm not sure that it could be argued that viruses/prions engage in metabolism).

However, much of my argument is likely irrelevant to the point at hand. The only "life" we have to deal with at time, or attempt to categorize, is that on our own planet. And it is understandable that we focus specifically on that; since a truly "universal" definition of life that could encompass every possible form that life might take in our huge universe would be pretty much impossible, bound up in debates and disagreements that could never be resolved.

[CACTUSJACKmankin]

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.
What viruses have is A method of copying. It's not internal but it is a method of copying. If there weren't a method of copying we wouldnt have these viruses anymore. That's the point. The reason they are around is because they made copies, and those copies can be different enough from the "parent" that it can thrive in circumstances that the "parent" cannot. Anything that can do that, to me, is life.

[Soapy Sam]

Life is a chemical reaction that hasn't finished yet, and started a long time ago.

I agree. A reaction with increasingly complex manifestations, including self-awareness and the ability to define itself, at least tentatively.

[chance]

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

A diamond can only go in one direction (after it’s removed from it’s formation environment), towards decay. Life IMO is a ‘condition’ where decay is held a bay by the exploitation of resources.

One could consider many forms of ‘life’ using this definition:

a. “life as we know it” qualifies.
b. A virus qualifies (long periods of ‘hibernation’ inactiveness, do not disqualify).
c. Robotic life could qualify with a bit of a stretch (symbiosis with other systems (us) is it’s method of avoiding decay). But I think this should be more correctly be a consciousness type question, where you can have consciousness without life.

However I see your point about a diamond, it too could be considered a chemical system (during the creation process, presumably interspaced with vast periods of inertness), but I wonder if a diamond (or any crystalline growth for that matter) is more correctly classified as ‘changing state’. Is it possible that life is a form of crystalline growth, and by crystal I dont mean in the strict mineral sense, but a more loose chemical bond sort of thing. Certainly is a grey area.


"free will" chance runs screaming out of the room...........

[CapelDodger]

I agree. A reaction with increasingly complex manifestations, including self-awareness and the ability to define itself, at least tentatively.
It's quite remarkable, isn't it? I mean, who saw that coming :) .

[Paul C. Anagnostopoulos]

A diamond can only go in one direction (after it’s removed from it’s formation environment), towards decay. Life IMO is a ‘condition’ where decay is held a bay by the exploitation of resources.
Ah, so it actively fights entropy. Surely a self-repairing robot is alive, then.

~~ Paul

[Soapy Sam]

No. Life is a continuous process. A robot is assembled from inert parts, then switched on. A living entity is alive at every point of its existence until it dies.
It's germ line is immortal. This is not true of an artifact, however complex its behaviour. To my view this is the fundamental difference between life and non-life.

(This does not mean an artifact cannot mimic life in every way. But to my view it is not alive.)

[RecoveringYuppy]

No. Life is a continuous process. A robot is assembled from inert parts, then switched on. A living entity is alive at every point of its existence until it dies.
It's germ line is immortal. This is not true of an artifact, however complex its behaviour.
I think there are important exceptions to even that though.
Many seeds and spores spend quite a bit of time inert, not manifesting the metabolic parts of the definition. Some lifeforms can even be frozen for extended periods.

[Paul C. Anagnostopoulos]

No. Life is a continuous process. A robot is assembled from inert parts, then switched on. A living entity is alive at every point of its existence until it dies.
But then it could repair and enhance itself. An egg is assembled from inert parts, too.

~~ Paul

[CapelDodger]

No. Life is a continuous process. A robot is assembled from inert parts, then switched on. A living entity is alive at every point of its existence until it dies.
There's a semantic problem here : are we trying to define life as a physical phaenomenon, or life as an attribute? I'll stick to the former, the comfortable "process" definition, the other is a can of worms :) .

Arguably self-fueling, self-repairing, self-sufficient robots (or self-sufficient communities of robots) could be considered as life in the chemical-reaction sense. Conceived and created by, for instance, humans who are a part of that chemical reaction. There'd still be chemistry involved, after all. It could even be regarded as an extension of life, which has not yet worked out how to exploit tungsten. Tungsten bones and teeth, imagine, but a bit of a leap for natural selection. We humans, with our consciousness and understanding and compulsion to tinker with things, can make that leap.

Life would take on a whole new cohort with more tungsten than calcium, more silicon than carbon. It may well have happened already, somewhere else.

[Soapy Sam]

The OP is asking about definitions, after all, so we can all toss our hats in the ring.

For me, what makes life distinct from such things as self-repairing automata is not the chemistry (which could conceivably be imitated) , but the continuity.
This is an old, old process, subject to selection and environmental constraints. I define things as living which can be traced continuously to that process, without breaks.
Yes, we could define a robot as living, but it's discontinuous from the actual process , so I would not. Some broad minded souls might define a car as alive. I would not. Or a robot.

By "not continuous" I mean that the process was broken. The chain of chemical activity goes from human to gamete to human, not human to robot, except in a metaphorical sense.

I'm not staking any fortunes on this. It's just a working definition.
(I'm restricting this to life here on Earth and now or in the past. The future may be something else).

[CapelDodger]

However I see your point about a diamond, it too could be considered a chemical system (during the creation process, presumably interspaced with vast periods of inertness), but I wonder if a diamond (or any crystalline growth for that matter) is more correctly classified as ‘changing state’.
I think so, it's an isolated event in particular circumstances, just like the formation of many minerals. It leaves no legacy to influence the future. Life comprises the chemical reactions that do influence the future, and in a way that promotes their repetition. The formation of diamond or serpentine doesn't qualify.

Life surfs on entropy-flow, it can't stand still. The moment it does it's chalk.

[CACTUSJACKmankin]

In response to the crystal and robotic questions, i would ask what the genetic medium is. I do think, while unlikely, it does not necessarly have to be DNA or RNA. Remember the movie evolution? what if there are planets where life isn't even carbon based? Even if such a thing were so, it would still have to act very much like DNA and RNA. It would have to be able to store a code that could be converted into the makeup of the organism. How does this apply to robots and crystals?

[chance]

Ah, so it actively fights entropy. Surely a self-repairing robot is alive, then.

~~ Paul

I don’t really have philosophic problem with that, certainly it’s a different “type of life”.
A form of symbiotic or parasitic life perhaps.

It’s avoiding entropy by using or coopting resources, is that not that what we are doing, a robot is just using a different method.

[chance]

No. Life is a continuous process. A robot is assembled from inert parts, then switched on. A living entity is alive at every point of its existence until it dies.
It's germ line is immortal. This is not true of an artifact, however complex its behaviour. To my view this is the fundamental difference between life and non-life.

(This does not mean an artifact cannot mimic life in every way. But to my view it is not alive.)

Abiogenesis is a process where inert chemicals came together, and was then.. (well not switched on but was able to avoid entropy).

I’m not so sure the demarcation is so clear, if Frankenstein’s monster was a reality, it would have been created also. I don’t think how, it or we got here, is a criteria.

[RecoveringYuppy]

Ah, so it actively fights entropy. Surely a self-repairing robot is alive, then.

~~ Paul
And if "fighting entropy" was a good definition we'd have to wonder why life hasn't figured out a way to simply deep freeze itself.

[chance]

In response to the crystal and robotic questions, i would ask what the genetic medium is. I do think, while unlikely, it does not necessarly have to be DNA or RNA. Remember the movie evolution? what if there are planets where life isn't even carbon based? Even if such a thing were so, it would still have to act very much like DNA and RNA. It would have to be able to store a code that could be converted into the makeup of the organism. How does this apply to robots and crystals?

For a robot, it’s method in avoiding decay is it’s intellect, it has no physical substance to pass on to the next generation. For all intents and purposes it is using “meams” in place of DNA.

By this method it could effectively evolve by design (intelligent evolution anyone?)

[skeptigirl]

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.So what do people with HIV have? A replicating toxin?

[skeptigirl]

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.Currently genetic molecules that replicate do so by using other organisms' structures. RNA and DNA can replicate if bathed in the right chemicals but to my knowledge, it is extremely inefficient without cell structure.

[Soapy Sam]

Abiogenesis is a process where inert chemicals came together, and was then.. (well not switched on but was able to avoid entropy).

I’m not so sure the demarcation is so clear, if Frankenstein’s monster was a reality, it would have been created also. I don’t think how, it or we got here, is a criteria.

But by the very definition of the word, abiogenesis does not involve life. The end product of abiogenesis is life.
It's at that point the question in the OP comes into effect.

The similarity of terrestrial life implies one source. Could be a single abiogenic event, could be the only one of many that resulted in sustained life. Once that start has happened though, there is a continuous process, in which everything is alive at every stage.

The distinction is clear once past the initial event.
If, so happen life on Earth did not arise through abiogenesis, but was imported from elsewhere, it simply shoves the start of the process back.

[CACTUSJACKmankin]

Currently genetic molecules that replicate do so by using other organisms' structures. RNA and DNA can replicate if bathed in the right chemicals but to my knowledge, it is extremely inefficient without cell structure.
Obviously, cells weren't the first thing to pop out of the soup. There must have been many pre-cell forms (perhaps viruses were among them) and they must have had ways of replication before they incoporated all of these assistant chemicals and cell structure. Therefore, these are more like evolutionary innovations to hasten the process rather than requirements.

[Wavicle]

So what do people with HIV have? A replicating toxin?

HIV cannot replicate itself. This was from an upper division course in genetics, the question of whether or not a virus is alive, considering what it does, was of particular interest. Afterall, the virus contains genes. The definition meant that for the purposes of genetics, a virus is dead because it is nothing more than a gene wrapped in a few proteins. Its only functional organelle (if you want to call it that) is the capsid.

A virus is alive the same way a chain letter is alive. Neither can do anything without a host to copy it.

[Wavicle]

Currently genetic molecules that replicate do so by using other organisms' structures. RNA and DNA can replicate if bathed in the right chemicals but to my knowledge, it is extremely inefficient without cell structure.

I think you're thinking of PCR (http://en.wikipedia.org/wiki/PCR). Not sure I'd call it extremely inefficient though.

[CapelDodger]

In response to the crystal and robotic questions, i would ask what the genetic medium is. I do think, while unlikely, it does not necessarly have to be DNA or RNA. Remember the movie evolution? what if there are planets where life isn't even carbon based? Even if such a thing were so, it would still have to act very much like DNA and RNA. It would have to be able to store a code that could be converted into the makeup of the organism. How does this apply to robots and crystals?
Robots would store and retrieve information in silicon or some such medium. It's own design, and programs for repairing and perhaps reproducing itself, could easily be included.

I wouldn't actually go for a single self-sustaining robot, I'd design a linked community. Robots for different tasks - mining, manufacture, hunting down humans, whatever - that, as a whole, sustain the community. And a master-program that is dedicated to keeping it running.

[CapelDodger]

And if "fighting entropy" was a good definition we'd have to wonder why life hasn't figured out a way to simply deep freeze itself.
However deep the freeze entropy would still increase, decay would occur. It's like giving up work and living on your savings. You can only do that for so long. Life keeps working and balances expenditure with income, which is why it's still here. Some is temporarily quiescent, but if it doesn't get out for some repair and reproduction regularly it's a genetic dead-end.

[CapelDodger]

For a robot, it’s method in avoiding decay is it’s intellect, it has no physical substance to pass on to the next generation. For all intents and purposes it is using “meams” in place of DNA.

By this method it could effectively evolve by design (intelligent evolution anyone?)
The possibility of robot evolution would theoretically be a design option. If the programs that direct the process are hard-wired it can't evolve. Monstrous amounts of error-checking could be incorporated to root-out those occassional cosmic-ray assaults on data-integrity. The robot community would just do what it was designed to do - provide for itself and do whatever else we want it for, such as provide for us. Leaving us free to concentrate on our animal side :) .

[CapelDodger]

I think you're thinking of PCR (http://en.wikipedia.org/wiki/PCR). Not sure I'd call it extremely inefficient though.
In comparison to the rate cells rip along at, anything would be extremely inefficient. PCR within the volume of a typical cell would crawl along in comparison to the reality. Cells are very vulnerable when they're dividing - all the senior management is offline, the rest of the system is coasting. There's a lot of selection-pressure to get efficient at it :) .

[chance]

But by the very definition of the word, abiogenesis does not involve life. The end product of abiogenesis is life.
It's at that point the question in the OP comes into effect.

The similarity of terrestrial life implies one source. Could be a single abiogenic event, could be the only one of many that resulted in sustained life. Once that start has happened though, there is a continuous process, in which everything is alive at every stage.

The distinction is clear once past the initial event.
If, so happen life on Earth did not arise through abiogenesis, but was imported from elsewhere, it simply shoves the start of the process back.

I agree, that is why I stated that how life arises (abiogenesis, or created) is not a criteria in the definition of life. If this interpretation is acceptable it leaves the door open for Frankenstein’s monster, and robotics (within reason) to be classified as alive in some sense.

Where is see the logical position being if avoiding entropy is the sole definition, is a classification of types of life, e.g.

Classification, method of avoiding entropy
a. Animal/plant DNA,
b. robotic MEAM (symbiotic with ‘a’),
c. android SYNTETIC DNA
d. virus DNA (parasitic upon ‘a’)
e. electronic MEAM (symbiotic with ‘b’)

[CapelDodger]

Where is see the logical position being if avoiding entropy is the sole definition ...

Entropy can't be avoided. What life does is excrete entropy systematically. Perhaps that (or similar) should define life : a physical system that is able to excrete entropy at least as fast as it accrues it, averaged over a critical timespan. The critical timespan being related to the system becoming so degraded as to be beyond recovery.

A system that excretes entropy faster than it accrues it will store up savings. Life on Earth, for instance, has survived mass extinctions because it could liquidate a lot of species to cope with the short-term entropy-flow problem, while remaining a very viable concern.

[chance]

Entropy can't be avoided. What life does is excrete entropy systematically. Perhaps that (or similar) should define life : a physical system that is able to excrete entropy at least as fast as it accrues it, averaged over a critical timespan. The critical timespan being related to the system becoming so degraded as to be beyond recovery.

A system that excretes entropy faster than it accrues it will store up savings. Life on Earth, for instance, has survived mass extinctions because it could liquidate a lot of species to cope with the short-term entropy-flow problem, while remaining a very viable concern.

I think we are more or less on the same page, re ‘excrete’ or ‘avoid’.

[skeptigirl]

Obviously, cells weren't the first thing to pop out of the soup. There must have been many pre-cell forms (perhaps viruses were among them) and they must have had ways of replication before they incoporated all of these assistant chemicals and cell structure. Therefore, these are more like evolutionary innovations to hasten the process rather than requirements.
My understanding is the replicating molecules were mostly RNA. There may be other theories or hypotheses. Viruses would not have yet developed and I am not aware of any viruses that replicate without a host cell. Viruses may have evolved after cells did as fragments of genetic material spilled in and out of cells.

Mitochondria may have been extracellular at first then were incorporated into surrounding membranes. I have heard that hypothesis.

[skeptigirl]

HIV cannot replicate itself. This was from an upper division course in genetics, the question of whether or not a virus is alive, considering what it does, was of particular interest. Afterall, the virus contains genes. The definition meant that for the purposes of genetics, a virus is dead because it is nothing more than a gene wrapped in a few proteins. Its only functional organelle (if you want to call it that) is the capsid.

A virus is alive the same way a chain letter is alive. Neither can do anything without a host to copy it.
You say potato I say patato.

The question of whether viruses are alive or not is the same as the question about whether Pluto is a planet or not. Make your definition and then decide. But not everyone agrees with your definition.

[skeptigirl]

I think you're thinking of PCR (http://en.wikipedia.org/wiki/PCR). Not sure I'd call it extremely inefficient though.PCR is a modern laboratory process. I was not referring to PCR.


The replicating genetic material I was referring to were viruses. But organic molecules include RNA and it can replicate strictly as a chemical process.

[Soapy Sam]

I think there's a slight tendency in the thread to confuse the term "life" with "living organism".

I'd say a bacterium clearly exhibits the process that we call "life". It is a living organism. A virus is rather different. It exhibits characteristics of life, but may not be a living organism. Perhaps robots fill a similar niche.

The question of rate of change may be important. I maintain the life process is continuous, but the counter example has been given of a seed which lies dormant for years. How slow may the manifestations of life be, before something is seen as having stopped rather than slowed life?

To my mind, the seed is living , but marking time awaiting an external trigger. It remains viable.But Paul might say this is true of a pile of electronics on a bench, which await the trigger of being assembled into a robot.

As with all working definitions, we must remember what we need the word for in the first place.

[Wavicle]

The replicating genetic material I was referring to were viruses. But organic molecules include RNA and it can replicate strictly as a chemical process.

For example?

[Wavicle]

You say potato I say patato.

The question of whether viruses are alive or not is the same as the question about whether Pluto is a planet or not. Make your definition and then decide. But not everyone agrees with your definition.

Not sure where Pluto is coming from since you specifically picked out HIV as an incredulous response to the definition coming from my coursework. It's fine if you think HIV is alive but not clear to me how, if that is true, a chain letter is any less so.

[l0rca]

How come nobody is mentioning consciousness here?

Well, I take that back. We'd need to then define that word too.

Instead, I think if we're going to come up with a definition for life, we have to decide on what sort of qualities we would expect a life form to have that make it relevant to consider things as "alive" and "not alive".

"Should they be lifeforms if they can not feel pain?"

This would make it an appeal to empathy.

Perhaps instead: "Should they be life-forms if they are complex systems?"

No?

The idea of "life" taken too simply could make stars living entities, which is why I suggest a certain sense of relativeness when talking about life.

Another question might be: are we on a subject of language-dissection and perspective, or on a subject where, upon approaching certain degrees in quality, that the very state of a material system takes on a unique quality?

[skeptigirl]

Not sure where Pluto is coming from since you specifically picked out HIV as an incredulous response to the definition coming from my coursework. It's fine if you think HIV is alive but not clear to me how, if that is true, a chain letter is any less so.

A Replicating RNA Molecule Suitable for a Detailed Analysis of Extracellular Evolution and Replication (http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=389702&tools=bot)

I'm not arguing for or against viruses defined as living or not. I am saying that life exists on a continuum and some people draw the line in different places.

Crystalline structures and chain letters are not considered living by anyone except Hindus who consider Brahman to be "nondifferentiated from the world and its beings" (Wiki's wording of the concept (http://en.wikipedia.org/wiki/Hinduism).)

But when you have genetic material such as in a virus, it not only replicates, it finds its way to the next cell and the next host so it is a little more than a passive crystalline structure which only grows when the elements needed pass by. Viruses take advantage of another cell's structures but so does a parasite. In fact gonorrhea bacteria are also obligate intracellular organisms. They live inside other cells too.

I don't care if Pluto is a planet or not and I don't care if a virus is a living organism or not. But the fact is not everyone draws the line in the same place. All you can say is you prefer to draw it in a certain place, you prefer to define life a certain way.

[skeptigirl]

How come nobody is mentioning consciousness here?

Well, I take that back. We'd need to then define that word too.

Instead, I think if we're going to come up with a definition for life, we have to decide on what sort of qualities we would expect a life form to have that make it relevant to consider things as "alive" and "not alive".

"Should they be lifeforms if they can not feel pain?"

This would make it an appeal to empathy.

Perhaps instead: "Should they be life-forms if they are complex systems?"

No?

The idea of "life" taken too simply could make stars living entities, which is why I suggest a certain sense of relativeness when talking about life.

Another question might be: are we on a subject of language-dissection and perspective, or on a subject where, upon approaching certain degrees in quality, that the very state of a material system takes on a unique quality?

No one defines life as needing a consciousness that I am aware of. That leaves out too many organisms.

Are stars replicating? Most definitions of life (except for Hindus) that I know of include organic molecules and replication as minimum components.

[Taffer]

Certain classes of transposons encode for the protein which is required to "copy and paste" their genetic sequence into another portion of the genome.

[Cuddles]

The problem with trying to define life is working out what question you are actually trying to answer. The answer may very well be "42", but what is it the answer to? When it comes down to it, all attempte to define life are simply attempts to answer the question of how we can organise things into two groups without ever being completely sure what the question really is. Are we trying to find a definition that can account for all possible life everywhere? Are we just trying to this for Earth life that we are familiar with? Are we simply trying to have an arbitrary grouping that doesn't try to be final but is open to change with new discoveries? I think the reason everyone disagrees with the exact definition of life is because everyone is actually trying to answer different questions.

In my opinion, wondering about potential life on other planets or artifical life here on Earth is all very interesting, but in the end it is largely irrelevant. We have not found anything that could be called life under any definition anywhere apart from here, and we have not created anything that could be called life. This means that any definition that tries to include these things is doomed because it cannot possibly be tested. It might be nice to think about, but it serves no practical purpose. To get a sensible definition of life we really have to think about what we can actually see.

Unfortunately I don't actually have an answer, but one thing I haven't noticed mentioned is complexity. Even fairly complex crystal are actually incredibly simple, they just consist of regularly spaced atoms in a regular lattice and can be described by fairly simple rules. Even the most complex organic molecules are relatively simple. On the other hand, even the most simple things that could be called life are orders of magnitude more complex. The simplest virus is nothing like even the most complex non-living molecules we have seen. I think any definition of life has to take this into account. A non-evolving fish is still an extremely complex thing, while a self-replicating robot is still just a few metallic crystals stuck together.

Of course, things like the precursor molecules to life wouldn't fit neatly here, and neither would AI, but the important point is that I don't think this matters. Defining life is just a practicallity, like defining planets. If we discover something that doesn't fir our definition we can change it, but until we do there is not really any point worrying about it.

[dakotajudo]

FWIW, a few years back I taught a general biology course. Instead of taking a bottom up approach to introduce the concept of "life" (that is, what are the defining characteristics of a living thing?), I decided to take a more cosmological approach, and reviewed an article by Sagan, et al:
http://www.nature.com/nature/journal/v365/n6448/abs/365715a0.html

It simplifies the definition of life - if you can identify a steady-state disequilibrium, something living is probably present.