definition of 'life'

[Jens]

I agree to all what has been said by jp555 above.

 

A more detailed biochemical view on it:

Darwinian evolution is not contraint to DNA (by the way: Darwin did not know about DNA). Actually the majority of scientists think it started with RNA. This is nearly the same kind of molecule but with the huge difference that RNA forms stable 3-dimensional shapes with precisely shaped deep surface structures. Why does this matter? This is the exact requirement to act as a catalysator for specific chemical reactions (and this is mainly meant with gaining new properties). To start darwinian evolution chemically you need a small system or one big molecule wich is able to copy itself (directly or indirectly via a cycle) and the same time beeing able to form precisely defined 3-dimensional shapes (to act as a catalysator). However, even this is not enough the starter system or molecule must also be able to change a bit (e.g. via copy errors) and those changes should in principle allow for arbitrary 3-D surface shapes (to have the potential to gain a huge variaty of catalytic features) but without loosing the same time the ability to copy itself. Once this level is reached the system or molecule will evolve into much more complex and perfectly adapted thing, which we could easily identify as life and distinguish from dead chemicals. This is of course not restricted to RNA or DNA based life forms at all.

 

So to my point of view: no life without the principles of Darwinian evolution: copy with mutation and selection. (at least as long as somebody comes up with another principle how a chemical system can gain more an more complexity and perfect adaption to the environment). Darwinian evolution is not something specifc to the Earth.

 

I think that you need an additional qualifier because it seems to me that, by your terms, the Universe itself could qualify as "life". By the Universe itself I mean the whole set of exstant physical reality.

 

So, I would add, at a minimum: "Life is a chemical system (in contrast to artificial life) as distinguished from the Universe as a whole."

 

Hi,

thanks for the note.

 

But I do not understand:

What do you mean: The universe is also a chemical system and reproducing itself?

Since I am not an expert in astronomy a few words on the universe reproducing itself would be helpful.

Edited September 7, 2012 by Jens

[zapatos]

This is to our current knowledge. There could be a better definition yet to be discovered without natural selection/evolution, but currently without it's inclusion there are examples of self-replicating systems such as peptide self replicators which would end up being defined as life.

 

Personally, I think natural selection/evolution is one of the core descriptions which is unique to life. Everything is essentially made up of non-living material, and natural selection/evolution is one of the best descritions which can exclude many natural chemical systems that would be called life if other criteria were used, such as self-organisation, metabolism etc.

 

 

I agree to all what has been said by jp555 above.

 

A more detailed biochemical view on it:

...

Great explanations from both of you, thanks. I understand why natural selection seems a likely component of life now.

 

Do you think it possible that natural selection can ever stop happening in a life form at some point? For example, let's say the DNA (or whatever) is not only the carrier of genetic information, but also serves as a physical structure of the organism that is critical for life. Then if the carrier of genetic information has mutations, it would affect the physical structure, thus resulting in death. And of course then no possibility of passing on the mutation. Or am I getting too far fetched?

[dmaiski]

personally i think it depends on your point of view, all are valid, and all are simulteneously right, and wrong

 

1 from a chemical point of view all life is a reaction capable of self perpetuation, and change(dose not mean evolution, it means change in the literal sense)

2 from the human perspective all life is biological and needs to be capable of evolution

3 from a technological point of view life is a sufficiently complex system that is capable of creating new, more complex systems, or improving upon itself, increasing its complexity

4 from a philosophical point of view "i think, therefore i am" intelligence defines life

5 from a universal point of view, every system feeds into a larger system of infinite complexity (there is no real word to describe it, i guess something between "life" and "existence")

6 from a ??? point of view ??? (hypothetically you can figure this out if you gain omniscience, and its the true meaning of life)

(i guess holistic point of view, the ??? is mercurial in idea and existence, its easier to say its not there and pretend you don’t see it)

 

 

 

 

each system is a less complex definition of life then its successor

so at this point i would say take your pick,

 

a biology defines life somewhere between 1 and 2

a computer specialist will probably accept 3 but not 1, 2, or 5

a philosopher will accept them all, or at least up to 5

a schizophrenic will accept evrything up to 5 or 6 and ask you whether you are stupid? (you should answer "why yes, yes I am")

Edited September 10, 2012 by dmaiski

[Jens]

Do you think it possible that natural selection can ever stop happening in a life form at some point? For example, let's say the DNA (or whatever) is not only the carrier of genetic information, but also serves as a physical structure of the organism that is critical for life. Then if the carrier of genetic information has mutations, it would affect the physical structure, thus resulting in death. And of course then no possibility of passing on the mutation. Or am I getting too far fetched?

 

What you are describing is exactly one part of darwinian evolution (the selection part). Since the mutations are random most of them have a negative impact and some of them might lead to death. This means those highly negative mutations (deadly mutations) are not passed to the next generation in contrast to neutral or benefitial ones.

 

1 from a chemical point of view all life is a reaction capable of self perpetuation, and change(dose not mean evolution, it means change in the literal sense)

2 from the human perspective all life is biological and needs to be capable of evolution

3 from a technological point of view life is a sufficiently complex system that is capable of creating new, more complex systems, or improving upon itself, increasing its complexity

4 from a philosophical point of view "i think, therefore i am" intelligence defines life

5 from a universal point of view, every system feeds into a larger system of infinite complexity (there is no real word to describe it, i guess something between "life" and "existence")

 

Interesting. I agree with respect that you can have different point of views on the definition of life, which are simultaneously rigth and wrong depending on the view. However, I will still try to defend my view

 

1: Agreed. If a system is capable of self perpetuation (or reproduction) and change, evolution will occur automatically.

 

2: Life is biological nearly only means life is life. Since biology is just the science of life, this does not state anything. Or you have the same issue in defining what is biological. So I do not think this definition is helpful.

 

3: Agreed. A computer system which is able to reach more complexity and even intelligence by adaption to some sort of input (most likely from the outside world), is probably considered at least as artificial life. The difference is that the complexity is not reached via physically reproducing the computer system to multiple with selection afterwards. However, there is still the question, if this complexity can only be reached via reproducing, mutation and selection of algorithms. If the answer is yes, than also Darwinian evolution takes place at the level of algorithms.

 

4: O.k. you can have this point of view. But it means that plants, microorganisms, fungi and most animals are not considered as life. So I rather vote for having intelligence and life as two different concepts like in common usage of the terms (outside science). So I also think this definition is not helpful (too far away from common sense usage of the term "life").

 

5: That's too abstract from my personal point of view to be helpful (especially the "infinite" part).

 

I will try to make an update on my definition based on your input within the next week (and post it here).

Edited September 16, 2012 by Jens

[dmaiski]

a bit of input from me:

2. the word "life" existed long before the most fundamental biology was established, biologists hijacked the word for their own use

4. not my words, blame Descartes for Cogito ergo sum, and all those other Christian philosophers(not all of them, and not exclusively, but a lot)

5. an elaboration: basic mechanical forces feed into chemical reactions, which feed into metabolism, which feed into multicellular organisms, which feed into civilisation(an organism made of individuals, not limited to humans(you would think ants were unheard of) which feed into planet wide ecosystems, which feed into gravitational frameworks(on a galactic scale), then universal scale, then if you go into advanced physics, multidimensional systems, probability, then you reach the end of the line you encompass everything as a system that is in a state of flux. (as i said this is the most difficult definition to grasp)

 

 

 

 

 

[Jens]

a bit of input from me:

2. the word "life" existed long before the most fundamental biology was established, biologists hijacked the word for their own use

4. not my words, blame Descartes for Cogito ergo sum, and all those other Christian philosophers(not all of them, and not exclusively, but a lot)

5. an elaboration: basic mechanical forces feed into chemical reactions, which feed into metabolism, which feed into multicellular organisms, which feed into civilisation(an organism made of individuals, not limited to humans(you would think ants were unheard of) which feed into planet wide ecosystems, which feed into gravitational frameworks(on a galactic scale), then universal scale, then if you go into advanced physics, multidimensional systems, probability, then you reach the end of the line you encompass everything as a system that is in a state of flux. (as i said this is the most difficult definition to grasp)

 

2. but then you should take your definition 4, which is actually the very human centric one.

4. same comment. This is the completly human centric one (actually this is a bit thinking that plants are something in the environment like stones.)

5. thanks for the additional input. I see the point. Personally this is not what I am after when defining life. But that's a point of view question (as you have pointed out above.) So what I find as the most helpful definition of life is the one which fits to this forum (probably this is exactly why I am in this forum ): To distinguish between chemistry and the starting point of life in abiogenesis, to identify extraterrestrial life and last and most important: To put this gut feeling into more precise words which says that biological systems are fundamentally different from chemical ones with all their incredible features they have.

 

So again, you are right it is a view point question.

 

...I will try to update my version...

 

Updated version for a definition of life in mutliple statements, which all must be met.

(Biological View)

 

• Life is a chemical system.
  (in contrast to artificial computer life)
• Life is able to reproduce itself
  (in contrast to most abiotic chemical systems)
• Life is able to gain and change properties and pass these changes in reproduction
  (in contrast to: crystals; simple chemical positive feedback loops; fire; prions)
  (including: cellular life forms; viruses; hypothethical early selfreproducing systems without cell membranes or without energy metabolism; extraterrestrial life forms not based on RNA or DNA)
  (comment: "gain" is necessary, because otherwise the complexity can not increase)

These are the same prerequisites to start evolution (in darwinian sense).

To include artificial life you simply leave the first statement out.

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

 

However, this artificial life definition has the disadvantage, that there might be designed artificial life which is actually not reproducing itself or algorythms but somehow is learning and increasing complexity without using darwinian evolution. Personally I think this will limit this artificial "life" into the borders of the design. So also here I assume that the principle of random change and selection is needed to really gain the level of complextiy needed to be clearly stated as life. Since I am biology-minded, I personally could only accept using the word "life" for an artifical system, if it is either able to survive and reproduce in free nature without help of humans and evolve further or if it reaches near human intelligence (so somehow referring to dmaiski number 4 ). So after all for artificial life the number 3 from dmaiski is probably better (since it does not simply apply the biology view to computer science but is really an independent view on the problem):

[3 from a technological point of view life is a sufficiently complex system that is capable of creating new, more complex systems, or improving upon itself, increasing its complexity]

 

 

 

 

 

 

[dmaiski]

the simplest answer to that question is that there is no distinction between basic chemistry and life

its all a mater of scale, that’s what definition 5 is saying

 

when your resolution is 1 meter the things you see alive and evolving are humans, civilisations, ecologies

when your resolution is 1 cm you see individual creatures evolving and developing

when your resolution is 1 um you see bacteria evolving, and gigantic moving cell colonies(humans)

when your resolution is 1 nm you see DNA changing, proteins being made, broken down, doing work, viri, prions, and the minute interactions in cells, ect

when your resolution is 10 angstroms you see a vast and complex array of chemistry, constantly changing, growing, and variable

when your resolution is 1 picometer you see atomic interaction, which is quite interesting in its own right

 

where you want to set your resolution to define "living" is up to you

but the definition that you is the classical definition of life that most people use

 

 

 

 

Murphy law, that is really the definition of life in many ways, no mater the level you are at, unexpected things happen in patterns