What is the definition of life?

[RAINCRAFTER]

We all know by instinct what is "lively" here on Earth and what isn't.

Humans? Alive. Fire? No.

However, we need to discover the best definition for life if we want to go anywhere with the hunt for ET. Wikipedia and the majority of the world says it's 7 processes called "biological processes".

However (again ), NASA thought this would get in the way of searching for aliens (one example is that one process is re-production. Mules don't re-produce, so they would be dead even though our instincts would say otherwise.), so they made their definition anything that metabolizes. This includes mules, fire, hurricanes, etc. (This is flawed because FIRE IS ALIVE!?!?!?caps), so yeah. Writing a definition for life is sadly extremely hard. (You have to put a gut felling into words and have it be scientifically accurate.)

So good luck to anyone here or anywhere else trying to define life; this is RAINCRAFTER, signing out.

[Bill Phil]

Actually, the individual cells in a mule reproduce, thus the mule is alive.

The 7 biological processes work, although they may need some editing.

- - - Updated - - -

Also, NASA isn't actively searching for ET life.

[KerikBalm]

Well, it was with the viking probes.

Like many things... there is no clear-cut line.

Its a spectrum.

When you get down to biology, its just chemistry, chemistry is just physics...

Even in biology... the study of life... we don't define life.

People may debate whether or not viruses are alive.

Molecular biologists/biologists that study viruses or host responses to viruses... don't even address the issue.

We can characterize what the virus does, how it acts, what its properties are... and so on.

If you want to call that life, then do so.

If you don't, then don't.

There are a number of virus-like bacteria, and cell-like viruses.

Then there are virus like RNAs and DNAs....

Then there are life like RNAs.... and the RNA world hypothesis.

RNA can copy RNA sequences.... it can catalyze reactions... it can store hereditary information.

We've made self replicating RNA sequences

Could the right sequence of RNA, by itself, be alive? A lot of evidence points to that being how life started, after all...

At what point during abiogenesis, did the stuff stop being a complex chemical system, and start being "alive"?

This discussion is sort of pointless... its a completely arbitrary distinction that lacks any consensus

[Technical Ben]

What is the definition of blue.

Point to something blue, and point to something else the same. We have our requirement to match and communicate, without actually needing to provide a definition.

If we find something like life somewhere else (in space), we can then check to see if it matches life. In the instance of a Virus, does it match a Cat or a bacteria? No it does not. So we can say it's not "alive", though it does match parts of a cat or bacteria, so we can say "it's a part of a living thing" (possibly bacteria or other cellular machinery as it's originator). Or we look at the whole system, a lone "virus" is not alive, but in it's environment is. Likewise a cell may only be able to reproduce in the body of a living creature, so we define the whole as alive, and the singular as a part.

There is little to zero chance of finding a virus alone in space or on Mars. As it does not have everything it needs to be "alive". However, we could find a tree anywhere, of any type or construction, and know it's "alive" just by looking at it.

[xenomorph555]

I guess you could just sum it up as anything with movement, respiration, sensitivity, growth, reproduction, excretion and nutrition although that is the simplest definition.

[Shpaget]

Life is just an arbitrarily complex chemical process.

You say fire is not alive. I say it meets some important the criteria. It comes into existence, it lives, multiples and eventually dies. It's a chemical process and depending on what is burning and in what kind of environment can be rather complex.

I guess you could just sum it up as anything with movement, respiration, sensitivity, growth, reproduction, excretion and nutrition although that is the simplest definition.

So... fire.

[Technical Ben]

Life is just an arbitrarily complex chemical process.

You say fire is not alive. I say it meets some important the criteria. It comes into existence, it lives, multiples and eventually dies. It's a chemical process and depending on what is burning and in what kind of environment can be rather complex.

So... fire.

Nope. Sorry. Look into the details. Life can exist with zero chemical processes. In fact, current life is not a chemical process, it's a mechanical one. Proteins do not fold via chemical interactions, but mechanical (atomic scale mind) ones.

Mixing the chemicals, providing the chemicals and adding energy is not enough. We have to, or life has to, physically put the building blocks into the specific place/chain/order.

Fire does not carry any information, or "multiply" (copy it's self). It spreads, but that is not a copy of it's self. Fire is like running water. Life is like a car or a house.

[LordFerret]

42 ... of course.

[spudman2]

The seven currently accepted criteria for something to be considered "alive" are:

-Boundary between "inside" and "outside"

-An ordered structure

-Utilizes an energy gradient

-Reproduces

-Change in population traits over time (it can evolve)

-Reacts to stimuli

-Grows. And not just one part of it, either. All parts of it.

So I suppose that, yes, a mule is not alive, as it cannot reproduce. This definition of life is fuzzy, though, and user discretion is a part of it as well. Also, the mule is composed of cells that do meet the criteria for life.

[Technical Ben]

The set "mules" reproduces though. As in, they are born. They then stop reproducing. Perhaps "is part of a reproduction cycle" would be more fitting. As many organisms continue to live past their reproductive cycles.

[Tex_NL]

The worlds most renowned scientists and philosophers have been trying to answer this question for decades and have not come up with a solid answer. Don't expect a group of random gamer nerds to solve it over night.

[Shpaget]

Life can exist with zero chemical processes.

Where does it get energy to multiply from if not a chemical process?

Fire does not carry any information, or "multiply" (copy it's self). It spreads, but that is not a copy of it's self.

Sure it does. The smoke from the fire carries the traces of the material burning and not all fires are the same.

You're not a copy of your parents, are you not alive?

[KerikBalm]

There is little to zero chance of finding a virus alone in space or on Mars. As it does not have everything it needs to be "alive". However, we could find a tree anywhere, of any type or construction, and know it's "alive" just by looking at it.

What about most mycobacterium? What about other parasites?

What about humans? we can't even make 8 of the amino acids, or various vitamins.

We can't capture naturally occuring energy on our own (not autotrophs)... a human... or group of humans... without other life nearby (ie, plants and bacteria).. cannot survive or reproduce.

So we need some complex chemical/molecular compounds... viruses need some even more complex ones (ribosomes for instance)

We can't exactly used the "has ribosome Yes/No" condition for life.... because odds are that any alien life won't have any.

[spudman2]

Perhaps "is part of a reproduction cycle" would be more fitting.

Perhaps, yes. Like I said, it's fuzzy.

[lajoswinkler]

Life is just an arbitrarily complex chemical process.

You say fire is not alive. I say it meets some important the criteria. It comes into existence, it lives, multiples and eventually dies. It's a chemical process and depending on what is burning and in what kind of environment can be rather complex.

So... fire.

Fire does not apply, neither do saturated solutions which evaporate and deposit crystals. The only people I've ever heard considering such things living are philosophers and people studying the humanities. When one doesn't know natural science, one can not present explanations pertaining to the universe. It's like those monks debating about the number of teeth in horse's mouth. One needs to open the horse's mouth and count the bastards.

The most important property of a living organism is that it's an ordered pile of matter that is resisting dispersion into disorder by creating localized order at the expense of increasing much more environmental disorder. Fire does not do that, and neither do crystalizing solutions.

Watch this and things will be a lot more clear.

Viruses don't do it, either. They're not alive by the basic definition.

Also, life and living organism are not synonims. Life is a much broader term.

Edited July 13, 2015 by lajoswinkler

[Technical Ben]

Where does it get energy to multiply from if not a chemical process?

Theoretically I could build a form of life out of robots. I could power those robots in any way. Though, most energy methods would be defined as "chemical". Point still stands. Life is more than just "chemicals". Much much more.

Sure it does. The smoke from the fire carries the traces of the material burning and not all fires are the same.

You're not a copy of your parents, are you not alive?

Fire the process is copied. Fire the data is not. Is there any data within fire? Yes. Is there any information? Yes. Is there any data holding a record of specific mechanical operation for copying the previous action and "thing" we call life? No. Where as DNA does have this, and life as we know it, observe it and define it has DNA.

A printing press copies data. It copies books. However, it does not have the ability to build it's self. A river grows, but does not "copy" it's self or it's data.

Which does fire match? A printing press, a river or life?

Why should my parents need to be the same? We do not. Fire follows the natural forces in physics. Life specifically can decide differently, and is neutral to the physical interactions, but instead applies logical or computational interaction. An example of this is your PC. The game KSP is specifically neutral to (does not care about) the chemical processes on you PC. It decides on your vessel velocity based on logical processes, and decides if you reach orbit. Make the HDD out of copper or plastic (DVD), magnetic or optical, it still functions. DNA likewise is a chemically neutral ordering, that gives a specific function.

Fire is specific chemical interactions, life is the logical operation of a set of any mechanical interaction fitting the similarity with what we currently know to be life. Chemical or not. Life is not limited by it's chemicals, nor is it limited to any specific chemicals (as far as we know, it just performs best with the current set).

Thanks KerikBalm. A Virus performs no function without a cell. Humans continue to live despite not being able to provide all the vitamins all the time. But we can if you wish define a person as only "living" when in the ecosystem. As in reality, we would quickly die anyhow without it. Thus "A person in space with no vitamins" is, as far as factually correct, sadly dead (mainly due to the lack of oxygen, not vitamins. ).

Edited July 13, 2015 by Technical Ben

[KerikBalm]

Viruses don't do it, either. They're not alive by the basic definition.

They do it once they've infected a cell...

A dormant bacterial spore doesn't do it... is it alive?

Is a tardigrade not alive because it doesn't do it during dessication?

Is cryptobiosis life? is a tardigrade still alive when it is dessicated. Is a seed alive before it germinates?

Is a virus alive when it is infecting a host?

Is a mitochondria alive?

Is a chloroplast alive?

Its not so simple

[Technical Ben]

Is a frozen man or kerbal "alive" while frozen? Answer? Yes or No.

Likewise, is a Virus "alive" while dormant? Same answer as above.

Is it "living" over it's entire lifespan? Both answer Yes. If we wish to be pedantic, we can define any sense and understanding away from every topic and conversation.

The difference between my arm, and me is only in description. I am alive, so is my arm. If I kept my arm on life support in a box, it's still alive. If I froze it, it is no longer "alive", but has the ability to live (be thawed out and put back I hope!).

Likewise, a Virus while dormant, is by definition dormant. Why try to say that 4 apples is anything other than 4 apples? Dormant being the opposite definition to "alive". Don't ask "is this thing I decided is dormant and not alive.... Alive?" well no, you said it's not already.

Is it alive while infecting? Yes. It's now doing something. Is it doing those things we call "living"?

We do learn something amazing from it though. If applying science, and observation. We can conclude, a thing we defined as living, then as dead, we then observed to be living again. Now that's something amazing!

[lajoswinkler]

They do it once they've infected a cell...

A dormant bacterial spore doesn't do it... is it alive?

Is a tardigrade not alive because it doesn't do it during dessication?

Is cryptobiosis life? is a tardigrade still alive when it is dessicated. Is a seed alive before it germinates?

Is a virus alive when it is infecting a host?

Is a mitochondria alive?

Is a chloroplast alive?

Its not so simple

Yes, it's not simple and certainly not yes or no, but by the basic definition of a system resisting dispersal, viruses are not. The only thing that resists (total) dispersion is the data they carry. That's completely different thing and can't be lumped into the same category.

[KerikBalm]

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?

[Shpaget]

I've never heard of this dispersal definition.

Does a rock not resist dispersal?

[Technical Ben]

That's like saying some life is green, some rocks are green, thus a rock is alive. Just because a rock does one or two things, does not mean it does them all.

For those stuck on a virus. I ask. If I cut off a branch to a tree. Is it alive? If I freeze some peas, are they alive?

[sndrtj]

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 .

[KerikBalm]

< 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.

Edited July 14, 2015 by KerikBalm

[Technical Ben]

"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.

But is the better biological definition:

"4. Being in a condition of biological rest or inactivity characterized by cessation of growth or development and the suspension of many metabolic processes: a dormant bud; a dormant bacterium."

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.

This will also help you find the answer to "is Mitochondria alive" (by our definition of alive).

Edited July 14, 2015 by Technical Ben