What is the definition of life?

[KerikBalm]

"You also didn't address if something is alive when it is dormant, and how dormant it can be...."

Sleep = alive. Frozen = not alive. By definition.

Whose definition?

The frozen thing can still be a life form, no?

So we need to ask instead: "Is an animal frozen still classed as alive" or "is an animal dried out/while it's stopped it's metabolism classed as alive"? That is a better question that does not suppose our answer, one that we can test/decide scientifically.

No, it can't be answered scientifically... because its a question of arbitrary definitions.

Your name seems to be a bit of a misnomer... because there are many many technical mistakes in your posts.

[AngelLestat]

There are three key items that, to me, define life for a given entity:

1. It contains genetic information, which is passed from generation to generation

2. A body of highly-similar (genetically) entities evolves its genetic information and behaviours over time (this naturally follows from point 1)

3. It actively maintains homeostasis. The end of this homeostasis means the death of said entity.

Are humans alive? Most definitely .

haha, you are just trying to define us "humans", and anything that is not like us (with our mechanism) then is not alive..

but let me see this from another perspective...

What if something naturally evolve with a different mechanism (this mean "not genes") and is intelligent..

Then you comunicate with this being, is that thing alive?

It would not enter in the "artificial life" definition because it was not created.

So by your definition, a coral would be more alive than this thing?

[PB666]

Actual biologist here.

Those seven items we've all been taught in high school could better be named hallmarks in stead of defining properties. Yes, they are common to life, and if an entity has all seven, we would very likely all call it "alive", but it's not all that is to life. Especially at smaller scales, it tends to break down.

There are three key items that, to me, define life for a given entity:

1. It contains genetic information, which is passed from generation to generation

2. A body of highly-similar (genetically) entities evolves its genetic information and behaviours over time (this naturally follows from point 1)

3. It actively maintains homeostasis. The end of this homeostasis means the death of said entity.

Entities that have merely points 1 and 2 is what I would call not alive but nevertheless biological.

Especially that third item is what I have been missing in this thread. And it is oh-so important. I have seen someone mention "boundary between inside and outside" as a defining property. This forms the basis of homeostasis, but it is not the end of it. Homeostasis means that the entity keeps its internal properties (hence it needs outside/inside border) relatively stable by some means, regardless of its surroundings. There are many forms of homeostasis, and higher forms of live have ever more complex ones. The most simple is osmotic pressure. The concentration of salts and organic compounds (nucleic acids, proteins, metabolites, etc) inside a cell tends to be widely different from that outside, and the cell actively maintains this. More complex forms of homeostasis include such things as blood pressure, blood oxygenation levels, heart rate and structural integrity. Life must spend an arduous amount of energy on maintaining this homeostasis. When it ceases to do so, it will die. E.g. a human dies if blood pressure isn't maintained, blood oxygenation levels drop, heart rate drops, salt balance (i.e. kidneys) is disrupted, you name it.

An interesting property also arises from this homeostasis: living things seem to have locally lower entropy than their surroundings. The necessarily contain and produce higher-order structures. Ironically, these same higher order structures allow for extremely efficient energy transfer (e.g. photosynthesis is much more efficient at converting light into chemical energy than the simple radiative heating of the sun on bare rocks), which in effect increases the entropy of the total system.

So some practical examples:

Is fire alive? No. It doesn't convey genetic information from generation to generation (it doesn't have generations at all), and therefore also doesn't evolve. Very temporary homeostasis might exist for fire, but this is self-limiting, so not truly there.

Is a rock alive? No. It has none of the three properties.

Is a virus alive? In most cases no. It however is quite obviously a biological entity, in the sense that it most definitely carries genetic information and evolves. There are some viruses that skim the border, and seem to have some form of sustained homeostasis.

Is a bacterium alive? In the vast majority of cases, yes. However, just as with larger-than-usual viri, there are bacteria which are so small as to entirely depend on host species to survive, so they skim the border between alive and not.

Are mitochondria and chloroplasts alive? Interesting question! They do have genetic information, which they pass on from generation to the next, and also evolve. Furthermore, it is generally assumed both evolved from endosymbionts. But, AFAIK they cannot survive outside of the host cell, i.e. the host cell provides for most homeostasis purposes.

Are humans alive? Most definitely .

We had to throw a biologist in, now we are really screwed. A physical chemist would have a different POV, first off I posted this LN but it seems to have been lost. 80:20 odds that OP = troll.

Planetary accretion is driven by gravitational processes that ultimately only factor in minor surface processes such as when a star is sufficiently energetic that most surface processes are volatilized. Accretion process produces a large surface potential on planets, particularly those close to the star in which latent heat or EM can be diverted and there is energyflow in day/night cycles. The surface of the planet is in the margin between internal heat turnover, IR radiation, interplanetary interactions (comet and asteroid collisions) and EM absorption. Because of this there is a tremendous amount of chemical potential between non-homogeneously distributed materials and EM gradients, and the potential exist between temperatures where liquid (non-solid, non-gaseous) chemistry can occur. Both states feed into the liquid state (through ionization and dissolution). But are insufficient in activity to dissolve the thermodynamic disequilibrium that is created by planetary formation and external irradiation. To be specific there is no particular thermodynamic state that is stable, since impactors continue to arrive, radionucleotides continue to decay at the planets core creating tectonic turnover and radiation continues to arrive.

Thermodynamic disequilibrium favors catalytic processes that can accelerate toward the most stable state even if the stable state remains distal. Fire can accelerate progression but ultimately is self limited and cannot produce continuity sufficient to sustain an identity and thus cannot but through non-living process create itself (lightning, random chemical interactions). To continually sustain the process a system of information transfer qualifies life. The more efficient forms store information in one form, transmit the information in another form and actualize the information in primary and seconary forms. Because single versions of life ultimately transform one disequilibrium to another disequilibrium (e.g. plants and the snowball earth), the system that is more efficiently catabolic is noted for checking. This qualifies life into biotic states that can be differentiated based on function(s) and inheritance. The checking process occurs as the competition that allows a more consistent flow of energy from the sources of disequilibrium to the ultimate fate (largely infrared radiation). Life generally is multifunctional, meaning for example, multiple functions are required to sustain itself (unlike prions but like viruses).

For example, when a plant places a new leaf in the sun, it captures EM before it hits the ground, absorbs some of it, reflects some of it and transmitts some of it. It can store the energy and use it at night when less EM is incoming. It can trap heat closer to the ground allowing the ground to radiate longer (and creating a more homeostatic environment for itself). The next leaf comes along and grabs the EM sooner, a leaf on a different tree then captures the light sooner (competing). When there are too many leaves a herbivore comes along and degrades the leaf, when there are too many herbivores a predator comes and eats the herbivore, and when the predator dies scavengers eat him and eventually remedial bacteria get the rest. Each process is moving the thermodynamic state toward equilibrium gathering energy and using a small proportion to create order, the remainder to produce waste heat. Each process is keeping other parts of the system in a dynamic equilibrium. This is life, should one part of the system fail, then the system becomes less efficient at its task (and you end up with large amounts of O2 and buried coal). Fortunately for us, the disequilibrium created by the sun suffices to continually drive 02 at a faster rate than buried coal undergoes auto and catabolic degradation. Unfortunately for us we have decided to burn this coal to extract more thermodynamic energy. The informational system is driving this process also, and eventually informational systems will reverse the process and if not life will changed to adapt.

The structure function relationships are key to identify life, unlike simple catalytic reactions or high heat combustion, life ubiquitously relies on a complex set of structure/functional entities (gene/gene products/product effects) to sustain itself. Even simple viruses have multiple genes, each gene product forms one or more functions, and this can be appended by host cell functions the virus borrows to replicate itself. The functions can be divided into

1. Information processing (sensing and transmission)

2. Translative activity (structural genesis)

3. Secondary processing (oxidation/reduction reactions, structural interactions, modifications, cell building, etc).

The dividing lines between each are not strict.

This definition of life solves the basic problem of what life is. Within a conducive zone of energy density, thermodynamic disequilibrium can give rise to increasing complex catalytic processes that eventually can sustain a constant conversion of thermodynamic potential into kinetic potential, in a process required in draining some of the energy to create order which can also be sustained and modulated.

Edited July 14, 2015 by PB666

[John FX]

As one of the things you need to be life is to reproduce and pass information to the next generation, those of you who have not yet had children are alive but you are not life, merely a by-product of it...

A lot of people conflate being alive with being life. This is not always completely accurate.

[sndrtj]

haha, you are just trying to define us "humans", and anything that is not like us (with our mechanism) then is not alive..

but let me see this from another perspective...

What if something naturally evolve with a different mechanism (this mean "not genes") and is intelligent..

Then you comunicate with this being, is that thing alive?

It would not enter in the "artificial life" definition because it was not created.

So by your definition, a coral would be more alive than this thing?

Erm... I think you are misunderstanding the word "genetic" as to only mean something in DNA-form. In fact, is just means any unit of heritable information.

Given the three points I raise, a zillion types of life are possible.

Let's take point one: heritable information. This could be in any form. I don't care whether this heritable information is in the form of DNA, entangled quantum matter, hydrogen plasma, source code or whatever. As long as this information is, in principle, transferable from one generation to the next (again, the mechanism of transfer is irrelevant), it adheres.

So most definitely, if something evolves (and then it transfers heritable information from one generation to the next by definition), and it somehow maintains some form of homeostasis (again, type and mechanism are not specified), I would consider it alive.

Even artificially created things can be entirely alive by these three items.

- - - Updated - - -

As one of the things you need to be life is to reproduce and pass information to the next generation, those of you who have not yet had children are alive but you are not life, merely a by-product of it...

A lot of people conflate being alive with being life. This is not always completely accurate.

You're right there. I should have named bullet point one as "It contains heritable information, which it is, in principle, capable of passing on to from generation to generation".

- - - Updated - - -

We had to throw a biologist in, now we are really screwed. A physical chemist would have a different POV, first off I posted this LN but it seems to have been lost. 80:20 odds that OP = troll.

Planetary accretion is driven by gravitational processes that ultimately only factor in minor surface processes such as when a star is sufficiently energetic that most surface processes are volatilized. Accretion process produces a large surface potential on planets, particularly those close to the star in which latent heat or EM can be diverted and there is energyflow in day/night cycles. The surface of the planet is in the margin between internal heat turnover, IR radiation, interplanetary interactions (comet and asteroid collisions) and EM absorption. Because of this there is a tremendous amount of chemical potential between non-homogeneously distributed materials and EM gradients, and the potential exist between temperatures where liquid (non-solid, non-gaseous) chemistry can occur. Both states feed into the liquid state (through ionization and dissolution). But are insufficient in activity to dissolve the thermodynamic disequilibrium that is created by planetary formation and external irradiation. To be specific there is no particular thermodynamic state that is stable, since impactors continue to arrive, radionucleotides continue to decay at the planets core creating tectonic turnover and radiation continues to arrive.

Thermodynamic disequilibrium favors catalytic processes that can accelerate toward the most stable state even if the stable state remains distal. Fire can accelerate progression but ultimately is self limited and cannot produce continuity sufficient to sustain an identity and thus cannot but through non-living process create itself (lightning, random chemical interactions). To continually sustain the process a system of information transfer qualifies life. The more efficient forms store information in one form, transmit the information in another form and actualize the information in primary and seconary forms. Because single versions of life ultimately transform one disequilibrium to another disequilibrium (e.g. plants and the snowball earth), the system that is more efficiently catabolic is noted for checking. This qualifies life into biotic states that can be differentiated based on function(s) and inheritance. The checking process occurs as the competition that allows a more consistent flow of energy from the sources of disequilibrium to the ultimate fate (largely infrared radiation). Life generally is multifunctional, meaning for example, multiple functions are required to sustain itself (unlike prions but like viruses).

For example, when a plant places a new leaf in the sun, it captures EM before it hits the ground, absorbs some of it, reflects some of it and transmitts some of it. It can store the energy and use it at night when less EM is incoming. It can trap heat closer to the ground allowing the ground to radiate longer (and creating a more homeostatic environment for itself). The next leaf comes along and grabs the EM sooner, a leaf on a different tree then captures the light sooner (competing). When there are too many leaves a herbivore comes along and degrades the leaf, when there are too many herbivores a predator comes and eats the herbivore, and when the predator dies scavengers eat him and eventually remedial bacteria get the rest. Each process is moving the thermodynamic state toward equilibrium gathering energy and using a small proportion to create order, the remainder to produce waste heat. Each process is keeping other parts of the system in a dynamic equilibrium. This is life, should one part of the system fail, then the system becomes less efficient at its task (and you end up with large amounts of O2 and buried coal). Fortunately for us, the disequilibrium created by the sun suffices to continually drive 02 at a faster rate than buried coal undergoes auto and catabolic degradation. Unfortunately for us we have decided to burn this coal to extract more thermodynamic energy. The informational system is driving this process also, and eventually informational systems will reverse the process and if not life will changed to adapt.

The structure function relationships are key to identify life, unlike simple catalytic reactions or high heat combustion, life ubiquitously relies on a complex set of structure/functional entities (gene/gene products/product effects) to sustain itself. Even simple viruses have multiple genes, each gene product forms one or more functions, and this can be appended by host cell functions the virus borrows to replicate itself. The functions can be divided into

1. Information processing (sensing and transmission)

2. Translative activity (structural genesis)

3. Secondary processing (oxidation/reduction reactions, structural interactions, modifications, cell building, etc).

The dividing lines between each are not strict.

This definition of life solves the basic problem of what life is. Within a conducive zone of energy density, thermodynamic disequilibrium can give rise to increasing complex catalytic processes that eventually can sustain a constant conversion of thermodynamic potential into kinetic potential, in a process required in draining some of the energy to create order which can also be sustained and modulated.

Interesting. I don't think what you are saying is at all mutually exclusive with my earlier three points. In fact, you seem to mostly describe (which I would call) homeostasis, i.e. the maintenance of catabolic processes - order, structural genesis - which will be utilized for metabolic processes - destruction of order, energy transfer.

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< also an actual biologist.

For the certainty that post started with. note how it broke down at the end.

"Is a virus alive? In most cases no."

"Is a bacterium alive? In the vast majority of cases, yes."

"Are mitochondria and chloroplasts alive? Interesting question!"

You also didn't address if something is alive when it is dormant, and how dormant it can be.... because there are various levels of dormancy... like a hibernating bear on one hand... and dessicated tardigrades on the other.

A virus outside of a cell vs a virus inside a cell?

There are many cases of life cycles at which at some point, the life form ceases to actively maintain homeostasis, but instead relies on a durable form such as a spore.

We don't call that alive or not alive... but instead invent a new term: cryptobiosis.

FWIW, I never see anyone debate what is alive or not alive in the lab or in scientific journals. These are biological entities with known properties.

Beyond describing those properties, you're just talking semantics.

FYI, responding to this:

"But, AFAIK they cannot survive outside of the host cell, i.e. the host cell provides for most homeostasis purposes."

They can remain viable for a time outside the host cell, long enough to be transplanted... but they certainly can't reproduce or be cultured.

Take the human mitochondria for example... its genome encodes just 13 mRNAs, 22 tRNAs, and 2 rRNAs.

Its proteome has >1000 proteins.

It must import a lot of nuclear encoded proteins.... yet the RNA components for a protein translation system are fully encoded by its genome.

The neccessary ribosomal proteins, DNA and RNA polymerases, many many many other enzymes, are not.

You're missing the point that there are always going to be things skimming the border. This is inevitable.

As for dormancy. Dormant cells or individuals do not cease catabolic and metabolic processes (i.e, homeostasis). They slow it down, sure, but do not cease it. So there is no contradiction here. When we take frozen specimens, you could say that they are currently no longer alive. That said, alive <--> dead does not necessarily have to imply a purely one directional process. Frozen bacteria could be considered as currently-dead bacteria which haven't yet lost the complete capability of being resuscitated (although it will only work in a subset), to give an example.

[KerikBalm]

You're missing the point that there are always going to be things skimming the border. This is inevitable.

"Skimming the border" as with many things, is reality, yes. In reality, things don't fit into our nice, neat, and arbitrary categories.

My whole point is that things skim the border from both sides, making the distinction arbitrary.

As for dormancy. Dormant cells or individuals do not cease catabolic and metabolic processes (i.e, homeostasis). They slow it down, sure, but do not cease it.

When something is fully dessicated, its pretty much stopped.

Most bacterial true spores still have some activity... but endospores are pretty much devoid of activity... they last centuries at least... millions of years if some claims are to be believe.

So there is no contradiction here. When we take frozen specimens, you could say that they are currently no longer alive.

What is "life" and what is "being alive" are not the same thing.

If a life form routinely goes through a stage where it is not "alive"... like your "currently no longer alive" frozen specimens... that does not mean the specimen is not a life form.

A virus particle, I'll admit, does not seem to be alive... but once inside a cell, it does many things that seem to fit the definition of life...

Is there really such a distinction between a replicating virus and a bacteria... and the virus particle and a dessicated bacterial endospore?

Enough to say one life cycle counts as life, but the other doesn't?

[Technical Ben]

Whose definition?

The frozen thing can still be a life form, no?

No, it can't be answered scientifically... because its a question of arbitrary definitions.

Your name seems to be a bit of a misnomer... because there are many many technical mistakes in your posts.

Once we set the arbitrary definition, then it's scientifically testable. Do things fall down? We have to arbitrarily define a thing, say a ball, an atom or a planet, etc. But after that, we can get a non-arbitrary definition of gravity.

So, if we define a colour, we can then test to see what matches the wavelength. That might just be stamp collecting, but it's still science.

So, in answer to your question. A frozen frog is still a life form, right? The problem is, we have 2 questions and 2 definitions to test against. Living things and things that live. Or, a thing that is "alive/living" and a thing that can live, once provided the correct food/energy.

We have dormant life, where it has everything required but the energy/food. It's suspended, but able to live given 1 addition. Or with have living things, that currently are metabolising, moving, etc. We could have a third group I suppose, those that can only live with both additional life and additional metabolism? Would that be true?

A virus particle, I'll admit, does not seem to be alive... but once inside a cell, it does many things that seem to fit the definition of life...

Have we missed a small, but important step? A frozen bacterium has all the parts required for life, but is suspended in action/animation/metabolism. It only requires food/energy to continue. But a virus requires many more parts and metabolism.

There may be more complex viruses that only require a metabolism. But most require not only the building blocks, not only the energy gradients, but the machinery and information they do not possessed themselves to "live" . However, they are very much a form of "life". Would you agree?

I suppose we could say our DNA is "life", but it is not at all "alive" when isolated and on it's own. How much more functional is a Virus in this instance when compared to DNA/RNA?

Edited July 14, 2015 by Technical Ben

[AngelLestat]

Erm... I think you are misunderstanding the word "genetic" as to only mean something in DNA-form. In fact, is just means any unit of heritable information.

Given the three points I raise, a zillion types of life are possible.

Let's take point one: heritable information. This could be in any form. I don't care whether this heritable information is in the form of DNA, entangled quantum matter, hydrogen plasma, source code or whatever. As long as this information is, in principle, transferable from one generation to the next (again, the mechanism of transfer is irrelevant), it adheres.

So most definitely, if something evolves (and then it transfers heritable information from one generation to the next by definition), and it somehow maintains some form of homeostasis (again, type and mechanism are not specified), I would consider it alive.

Even artificially created things can be entirely alive by these three items.

Ah ok, if that is the case, then I am agree, it seems a good start point definition for life.

But genetic is the study of genes, and genes are small pieces of information base on ADN.

So maybe a new word should be used as remplacement.

[BigFatStupidHead]

Ah ok, if that is the case, then I am agree, it seems a good start point definition for life.

But genetic is the study of genes, and genes are small pieces of information base on ADN.

So maybe a new word should be used as remplacement.

I often refer to it as XNA.

(also an actual biologist HIGH FIVES!)

[Aanker]

I think, roughly speaking, that we can look at life in three different - increasingly discriminating - ways.

1) At the most basic level, we regard life as a mere continuation or extension of organic chemistry and chemistry in general. Obviously this is of interest in astrobiology when we investigate the possibilities of life based on other elements than carbon. In the attempt to understand the elementary processes that underlie the formation of primitive cells and a genetic code, we need to accept that life somehow transitioned into being from 'non-living' chemistry.

2) life as a phenomenon or system of its own. Organized chemistry in a chaotic environment. Looking at it as a whole system of organisms in an environment (or on a planet), I think we have to include viruses in what we regard as life. Suppose that we ran into Martian 'viruses'. As an organism on their own, they would not be classifiable as life, but they would be part of the greater phenomenon of life on Mars. We would hardly classify them as 'Martian sort of living molecules'.

3) on the level of the individual organism, we can recognize a few traits that typically accompany life. Those have already been listed in this thread, and may be used to assess the 'liveliness' of a studied sample.

This was mostly a comment on the whole virus situation, where it may also be prudent to include infectuous proteins as a curiosity or side note.

[KerikBalm]

But genetic is the study of genes, and genes are small pieces of information base on ADN.

So maybe a new word should be used as remplacement.

Genetics is the study of inheritence.... genes are units of inheritence.

Genes are not neccessarily based on DNA, there are many many many RNA genomes.

Indeed, the best theory right now, is that when life started, it was all RNA genomes, and DNA came later.

As to non-technical, TechnicalBen

Once we set the arbitrary definition

but we haven't set such a definition....or perhaps we have set many many definitions... science can't answer which definition to use.

We have dormant life, where it has everything required but the energy/food. It's suspended, but able to live given 1 addition

You seem to have ignored my earlier point that there is a whole spectrum of dormancy...

We could have a third group I suppose, those that can only live with both additional life and additional metabolism? Would that be true?

No... because the amount of additional metabolism needed is also on a spectrum.

A frozen bacterium has all the parts required for life

Not if it is a mycobacterium(well, most mycobacteria)... then it needs a host cell, like a virus.

but the machinery and information they do not possessed themselves to "live"

And you and i require the machinery to synthesize 8 amino acids, multiple vitamins, etc. We take such things from our environment... viruses do too, their environment is the inside of a cell.

Mitochondria have smaller genomes than a great variety of viruses. They *are* a symbiotic bacteria... but we can't even say if they are alive. Of course... they are pretty much the most extreme example of symbiosis that one can imagine.

Perhaps at that level of symbiosis, they shouldn't be considered seperately from their host cells. They are certainly considered a component of a living thing, if not a life form by themselves.

But you can seperate them. You can "infect" one cell with the mitochondria of another.

Viruses are also symbiotic (parasitism is a form of symbiosis). The infected cell, is by most people's judement... still alive... does that make a virus particle a component of a living system?

However, they are very much a form of "life". Would you agree?

No, I would neither agree nor disagree... most people disagree though.

Most biologists who study them or their hosts don't touch the subject in a formal setting.

They are what they are... it doesn't matter to the research one bit if you label what they are a form of life or not.

I suppose we could say our DNA is "life"

I wouldn't, I think most biologists would say its not living either.

Its a component of a living system.... like a protein or a lipid membrane, but its not alive on its own.

Some RNAs on the other hand...start to show many properties that start to look like life - although, so far, not all the properties at the same time.... but the right population of RNAs just might be...hence the RNA world hypothesis.

Pretty much, if its not a cell, we don't say its alive, but many theories of abiogeneis have a stage of pre-cellular life, or at least a very complex pre-biotic chemisty of multiple replicating elements, in a synergistic feedback loop... even if they don't want to call such complex chemistry life at that point

[amorymeltzer]

For what it's worth I largely answered this same question on the Biology StackExchange site, with a little bit more here.

tl;dr

- It's complicated (THANKS viruses)

- Basically, life has ribosomes, non-life doesn't

- It doesn't really matter

Source - I'm a geneticist/virologist.

[smjjames]

- Basically, life has ribosomes, non-life doesn't

That's TERRAN life anyway. Alien life forms, even if carbon based, could vary from using something that looks and behaves similar to our ribosomes (that is, an analogue), to using something different or even a different system alltogether.

[KerikBalm]

- Basically, life has ribosomes, non-life doesn't

I believe I basically said this earler (or maybe I just thought about saying it).... but that won't apply to life of an independent origin... and on the "has ribosomes" criteria... mitochondria and chloroplasts should be regarded as alive... when they generally are not.

They don't even encode a polymerase to copy their genetic material (at least none, that I know of... mitochondria are very diverse, some have relatively large genomes, some chloroplasts have comparatively huge genomes, and some mitchondria have no genome at all).... many viruses at least do this.

[amorymeltzer]

on the "has ribosomes" criteria... mitochondria and chloroplasts should be regarded as alive... when they generally are not.

As I said in my linked answer, mitochondria do indeed encode rRNA but the ribosomal proteins are made from nuclear genes.

[KerikBalm]

Well, thats all well, and good, but the catalytic core of a ribosome is just RNA. Ribosomes are RNA-Protein complexes

Its believed the RNA core came first, the peripheral proteins later.

Mitochondria encode all the RNA components of theire ribosomes...

If we found a cell with a pure RNA ribosome, it wouldn't be alive because it had no ribosomal proteins, just a catalytic ribosomal RNA?

Now we're basing it on what they encode rather than what they have?

What about an RNA polymerase ribozyme? like you might find if the RNA world hypothesis is valid. The first life may have had only catalytic nuclic acids... no proteins... not ribosomal protein anyway.

Viruses may encode for the polymerase neccessary to duplicate theire genome, even if they don't have it in the virus particle... do they get points for that?

This definition doesn't even work for early life on Earth, let alone alien life.

*edit*

Are you sure that no mitochondria encodes ribosomal proteins? I know human/mammal/metazoan ones do not... but there are some mitochondria with genomes much bigger than what animals have... off the top of my head I'm not sure what they encode...

and for our purposes, we might as well consider chloroplasts as well... they have some pretty big genomes too... tomorrow maybe I'll check to see if any of them encode ribosomal proteins.

Maybe under this definition... some chloroplasts will be alive, and some won't

Edited July 14, 2015 by KerikBalm

[AngelLestat]

I often refer to it as XNA.

(also an actual biologist HIGH FIVES!)

Cool, the first time I read this news:

http://io9.com/5903221/meet-xna-the-first-synthetic-dna-that-evolves-like-the-real-thing

Genetics is the study of inheritence.... genes are units of inheritence.

Genes are not neccessarily based on DNA, there are many many many RNA genomes.

Indeed, the best theory right now, is that when life started, it was all RNA genomes, and DNA came later.

Yeah but not so sure about how much frame has this concept, it seems that the frame is always organic chemistry.

-------------------------------------------------------------

I love how something as simple like the evolution process and heritage can create such complex machines which complexity still eludes our understanding, like conscience.

But I also love to see this same mechanism applied in very different cases as memetics.

Wiki: The meme, analogous to a gene, was conceived as a "unit of culture" (an idea, belief, pattern of behaviour, etc.) which is "hosted" in the minds of one or more individuals, and which can reproduce itself, thereby jumping from mind to mind. Thus what would otherwise be regarded as one individual influencing another to adopt a belief is seen as an idea-replicator reproducing itself in a new host. As with genetics, particularly under a Dawkinsian interpretation, a meme's success may be due to its contribution to the effectiveness of its host.

Memetics is also notable for sidestepping the traditional concern with the truth of ideas and beliefs. Instead, it is interested in their success.

As Richard Dawkins prove, the evolution process is better understood from the gene perspective, so we are just survival machines – robot vehicles blindly programmed to preserve the selfish molecules known as genes.

"A chicken is just an egg's way of making more eggs."

Is not so easy to understand, but if we do, we can find similarities of memes with genes.

If somebody wants, I can explain this with more detail.

[Xannari Ferrows]

Whatever you want it to be. Simple as that.

[PB666]

Interesting. I don't think what you are saying is at all mutually exclusive with my earlier three points. In fact, you seem to mostly describe (which I would call) homeostasis, i.e. the maintenance of catabolic processes - order, structural genesis - which will be utilized for metabolic processes - destruction of order, energy transfer.

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You're missing the point that there are always going to be things skimming the border. This is inevitable.

As for dormancy. Dormant cells or individuals do not cease catabolic and metabolic processes (i.e, homeostasis). They slow it down, sure, but do not cease it. So there is no contradiction here. When we take frozen specimens, you could say that they are currently no longer alive. That said, alive <--> dead does not necessarily have to imply a purely one directional process. Frozen bacteria could be considered as currently-dead bacteria which haven't yet lost the complete capability of being resuscitated (although it will only work in a subset), to give an example.

I think its best to deal with these together. Note I was poking fun at the biologist, afterall it is they who define species, and it largely species that lack an objective definition.

The extremes of the universe are great, from the infinite energy densities of the early universe, to evacuated coldness of intergalactic space. To the massive compressive forces of black holes, to the atomizing energies that expand from super nova. What we consider to be extreme are nothing. Life exists between these extremes and a very narrow band intermediate between boson forming pressures in a black hole complete vacuum of space. life tends to exist at the lower end of pressure scale and the lower end of the tempertature scale, where the covalent bond forming chemicals at temperatures were covelant bond forming chemicals can exist in the liquid, solid or gaseous state. Life on earth exists at the fringe of energy density with the rather large energies being at the center of the planet (mass and temperature), and yet it avoids the cold gasless environment of the upper atmosphere. The zone of life is a border, it is a small band, at the surface on a temperature from about -20 to 100'C and at great pressures up to 300'C. This zone is a convergence of energy from a variety of sources that juxtaposed to each other create chemical energy potentials, added to this there is a constant feed of EM from the sun. There is nothing about this zone that says life must have homeostasis at all times, and the dormancy of some seeds indicate that certain forms of properly preserved could germinate their kind after 1000s to millions of years of all but complete dormancy. You could argue that homeostasis is still present, and I would counter that autodegradation as a consequence of environmental effects (Ozone, Cosmic radiation, radioactive decay) are more likely the explanation that a necessity of homeostasis. The only property of a cell that is not conducive to preserving dehydration are the plasma membranes, you can take many proteins and carefully lypohilize them of all water, and they will happily rehydrate and become active. Of course cells are damaged by long dormancies, but most seeds carry an excess of energy reserves by which they can repair themselves and quickly begin the process of growth.

The bacteria you mention are the subset that have evolved to survive long episodes of freezing in arctic ice, not all bacteria survived, but even at -10 life can operate, however below -20' and certainly by -50' life stops. This is the reason we use -80'C freezers to store cells. The largest barrier to cell survival at low temperatures is the stability of the plasma membrane, transitions of temperature close to the melting point of the membrane can cause it to fracture, so that it is best to freeze below that temperature quickly once water has frozen, and avoid getting close to that temperature until one wishes to revive the cells.

[baggers]

Is a virus alive? In most cases no. It however is quite obviously a biological entity, in the sense that it most definitely carries genetic information and evolves. There are some viruses that skim the border, and seem to have some form of sustained homeostasis.

Let's say a nano-bot isn't alive, like a Virus isn't alive because it can't reproduce itself without paraziting a "real" living cell.

What about a self-replicating nanobot?

[Fel]

haha, you are just trying to define us "humans", and anything that is not like us (with our mechanism) then is not alive..

but let me see this from another perspective...

What if something naturally evolve with a different mechanism (this mean "not genes") and is intelligent..

Then you comunicate with this being, is that thing alive?

It would not enter in the "artificial life" definition because it was not created.

So by your definition, a coral would be more alive than this thing?

The definition of life is simple: "Any definition that indicates humans are alive and suits our current moral stance, whatever it may be."

I think it would be most interesting if creatures of "living fire" came to visit us, and declared we were not alive because we didn't follow their rules of life. What might the conversation be like, as both sides explain the other's existence is impossible?

Remember, what we know about the universe, on matter, on life, is enough to fill one whole paragraph in a 32 volume encyclopaedia set on the universe. We are ignorant of what is possible outside our sheltered lives, whether other subatomic particles exist, whether there are whole new concepts of matter, physics, life even. We, like a thousand years ago, think we're at the pinnacle of our achievements, that there is are only a few questions that remain unanswered; do we not learn from our past mistakes, or are we doomed forever to live in arrogance?

[Technical Ben]

I'll mention it again, as it seems to have been missed. All those using "chemistry" in their definitions. Life as we know it, is not a chemical process alone. It requires mechanical and logical processes.

DNA is our logical operator, that stores information, and mechanical processes to read/transcribe and action it. All life has these processes, no life as we know it relies on chemical processes. Viruses included.

KerikBalm, I am not sure what your argument is. Yes there is at times a "spectrum". There also is in colour. While Science cannot decide what to define or call each colour, we can, and we can stick to it. While cultures use different meanings and understandings, we can still have one that scientifically is accepted. IE, wavelength.

So, if I can define the wavelength as a green colour, why can I not define a process as being, or not being "life" or other processes as being/not being "living"?

Not if it is a mycobacterium(well, most mycobacteria)... then it needs a host cell, like a virus.

Yes. And a Cat is not a Dog. But it still means a definition of "feline" is correct when talking about my cat, even if you point out that bacterium are sometimes parasitic. In which case, we define those parasitic ones as.... parasitic, and not individually operating. Where did I say we cannot do so???

No, I would neither agree nor disagree... most people disagree though.

Most biologists who study them or their hosts don't touch the subject in a formal setting.

They are what they are... it doesn't matter to the research one bit if you label what they are a form of life or not.

Fine. The question was for the thread "what is a virus/bacterium/parasite/dormant thing, is it alive [that is, what is it doing]?" or visa versa, "is a living thing classed as..."

We either use an agreed definition, of living. Or we answer specifically each thing we discuss, as you are offering. That's also great.

Edited July 15, 2015 by Technical Ben

[KerikBalm]

KerikBalm, I am not sure what your argument is. Yes there is at times a "spectrum". There also is in colour. While Science cannot decide what to define or call each colour, we can, and we can stick to it. While cultures use different meanings and understandings, we can still have one that scientifically is accepted. IE, wavelength.

You're assuming the question is answered, to answer the question.

We *can* make a definition and stick to it.... we *haven't* made such a definition, and the whole point of this thread as far as I can tell, is to reach a consesus on what that definition is

Yes. And a Cat is not a Dog. But it still means a definition of "feline" is correct when talking about my cat,

Ummm.... ok.... so technical...

even if you point out that bacterium are sometimes parasitic. In which case, we define those parasitic ones as.... parasitic, and not individually operating. Where did I say we cannot do so???

You said:

has all the parts required for life

When many many *obligate* intracellular bacteria do not. You can't just feed them nutrient rich media, and have them grow....

https://www.ncbi.nlm.nih.gov/pubmed/17038615

"but numerous genes considered essential for life are missing, suggesting that Carsonella may have achieved organelle-like status."

Fine. The question was for the thread "what is a virus/bacterium/parasite/dormant thing, is it alive [that is, what is it doing]?" or visa versa, "is a living thing classed as..."

We either use an agreed definition, of living. Or we answer specifically each thing we discuss, as you are offering. That's also great.

THERE IS NO AGREED DEFINITION.

That is the whole point.

[lajoswinkler]

but viruses do do it at one point in their replication cycle.

They cause simple molecules like ATP and amino acids to form complex ordered structures, at the expense of order in the cell.

Bacteria do this in an environment that consists of most of the surface of the Earth... outside this environment, they are (mostly) dormant.

Viruses do this in an environment that consists of the interior of cells... outside of this environment, they are (mostly) dormant.

Then we have the bacteria that only really do it in an environment that consists of the interior of Eukaryotic cells... are obligate intracellular parasites alive? they can plausibly evolve even further to become what one would truly consider a virus... would you then say they evolved into non-life?

What about mitochondria? they have a genome, synthesize proteins, make a whole lot of ordered molecules (ATP) and release a whole lot of disordered molecules as well (CO2).

yet, outside of a cell, they quickly lose the ability to do this, and they can't replicate.... but they evolved from something that could...

Are they alive? can something alive evolve into non-life without becoming extinct?

Of course, if you look at the complexity of the molecules that a human needs to survive... compared to the very complex cellular machinery that a virus needs to reproduce... you'll see orders of magntitude in differences.

Thus you could draw an arbitrary line to exclude them... similar to how one can draw an arbitrary line to say Mars and Mercury are planets, whereas Pluto and Ceres are not...

Hypothetically, we could have bodies with a stern levison parameter everywhere inbetween that of mercury and pluto...

or you could apply the parameter to moons, and the only difference is the body it orbits... the only difference between viruses and life are the environments they inhabit.

If: orbit star -> Then planet --- excet If: orbit planet -> Then Moon. If: (life parameter in any environment) -> Then life --- except If: (life parameter in environment that is itself living) -> then Virus.

But when does a Moon stop being a moon, and star being part of a binary planet?

When does a parasite become a symbiote?

If a non-living virus has a beneficial effect on its host (there are rare examples of viruses that are beneficial in certain circumstances)... when does it become a symbiote/part of a living system?

You could say for practical purposes, its easy to draw a line to exclude viruses...

But how practical? From a molecular biology standpoint... its not so practical... the things we study... gene regulation, phylogentic trees, evolution, protein function, signaling pathways (some overlap with gene regulation here), etc... its just like the stuff people call living... so for practical purposes in my field... why exclude them?

That's why there are several levels of defining life. Would artificial intelligence be alive? Yes, but not according to the basic physiology. It's important to include conditions when definitions are made.

I've never heard of this dispersal definition.

Does a rock not resist dispersal?

Rock does not resist dispersal. It is absolutely passive. Ever heard of erosion? Dissolved acidic gases in rain such as carbon(IV) oxide will dissolve limestone. Result: karst.

If you put a living cell into an environment it does not find optimal, like for example a hypertonic solution, it will not passively let itself into equilibrium. It will use its membrane motors to resist, shifting ions across the membrane to try to attenuate the loss of water.

When it spends all of its energy storage (ATP), it will stop resisting and soon equlibirium will occur. Death.

Of course, there isn't a special "force" that makes the cell behave like that. It does not go against the laws of physics. It's just a special system of coupled feedback loop reactions.

Living things is resisting total equilibrium, and that's the key of biological life.

Edited July 15, 2015 by lajoswinkler

[Technical Ben]

You're assuming the question is answered, to answer the question.

We *can* make a definition and stick to it.... we *haven't* made such a definition, and the whole point of this thread as far as I can tell, is to reach a consesus on what that definition is

Ummm.... ok.... so technical...

You said:

When many many *obligate* intracellular bacteria do not. You can't just feed them nutrient rich media, and have them grow....

https://www.ncbi.nlm.nih.gov/pubmed/17038615

"but numerous genes considered essential for life are missing, suggesting that Carsonella may have achieved organelle-like status."

THERE IS NO AGREED DEFINITION.

That is the whole point.

Again. I don't know what the confusion is. Sorry. Yes, there is no consensus amongst the entire world, but that's true for the colour green. Or the planet Pluto.

Both those concepts, colours and planets, are arbitrary and fuzzy as you say. I agree. But both concepts can be stated scientifically. A wavelength, and an object orbiting a sun. Yes both have edge cases, so we can say "this specific wavelength" or "this specific sized planetoid is orbiting a star with fusion" and rule out as many edge cases as possible.

There are always edge cases. That's reality for us. But it does not stop astrophysicists, particle physicists or anyone else. We just learn what the edge cases are, and how we need to apply the information we learn from them. To either improve understanding, or change our definitions (such as with Pluto ).

Or we can state parasitic life is parasitic, life within an ecosystem is living within an ecosystem, and rocks are rocks.

So, can we not ask, is a virus alive in the same way a cat is alive? Is a virus alive in the same way the cells of a cat are alive? Or is a virus only one part of life, in the same way DNA is only one part of life?

Edited July 15, 2015 by Technical Ben