definition of 'life'

[nikk]

i am really confused about the perfect definition of 'life'. i was discussing about this with my friend at school i attempted to define it

i said ,'' life is a systematic and specific conglomeration of certain inorganic substances which form a

'system' [an organism] which gets the ability to perform certain processes to maintain the 'system'. ''

 

 

am i right with my definition?

thank you

[granpa]

fire can grow and maintain itself but isnt alive.

 

the ability to evolve is essential

[Mr Skeptic]

I have a degree in biology, but I don't know what life is either. Each definition has its flaws, especially if we aren't restricting it to biological Earth life. At the end of the day, it is just a definition we make up and things fit it or they don't. But it is very educational to try. To start off, decide which things you want to define as alive or whether you want to restrict your definition. Earth life, viruses (which can't reproduce without a living cell), alien organic life, alien inorganic life, mechanical sentient robots, which of these are alive and which of these are not, or which of these do you want to ignore for the sake of simplicity?

 

These concepts might help: life must at some point be able to reproduce (else it would not exist), there needs to be a boundary between the living thing and its environment (for non-virus Earth life, that is a phospholipid bilayer, much the same thing as a soap bubble but more stable), the ability to maintain homeostasis (though again viruses are an exception).

[Andeh]

most definitions (especially ones in a less-fundemental feild like biology) are completely arbitrary--a reflection of the human desire for order, where often there isnt. This is because there are always fringes...we group an evolutionary line into seperate species, but of course there are intermediate stages.

 

Do you consider viruses alive? and even more extreme, do you consider replicating molucules alive, which can "reproduce", compete, and mutute. defining life is like saying that at some point abiotic became biotic--and materialistically, that never happened, since biotic and abiotic are just labels. All that happened was that the "proto-organism" "learned" some new chemical tricks.

 

In short, you can't define life. But like mr skeptic said, it's education to try, since it makes us try to understand it better.

try writing that for the answer to that question on a biology test!!

 

 

[Essay]

i am really confused about the perfect definition of 'life'. i was discussing about this with my friend at school i attempted to define it

i said ,'' life is a systematic and specific conglomeration of certain inorganic substances which form a

'system' [an organism] which gets the ability to perform certain processes to maintain the 'system'. ''

 

 

am i right with my definition?

thank you

 

Life is like pornography; I think I know it when I see it....

 

I like your point about a "system" but even "a system" can be somewhat arbitrarily defined.

...and with such a definition, things like climate, soil, and Gaia might qualify as life, as these "things" are organic/inorganic systems that evolve, or at least co-evolve. Hmmmm, does anything "evolve" or must everything necessarily co-evolve?

 

~

[Ophiolite]

nikk, about five or six years ago at an international conference on exobiology the participants were asked to offer their definition of life. Around one hundred differing definitions were offered. Perhaps once we have encountered other lifeforms and noted their similarities and differences to our own the definition will become easier. (It will never be perfect: this is science.) On the other hand, if some life is so different from us we may not be able to recognise it.

[questionposter]

I don't think there can be a concrete definition of life. What about complex artificial intelligence? It doesn't evolve, it doesn't grow, but it could still process the same information as other living things.

[michel123456]

That's amusing.

Mathematics should be used to answer this question instead of words.

[questionposter]

That's amusing.

Mathematics should be used to answer this question instead of words.

 

Math isn't anything on its own, we assign what it means with words.

[CharonY]

On the other hand, if some life is so different from us we may not be able to recognise it.

This is most certainly true. However, in my mind (as a biologist) "life" is just a convenient classification. In a naive approach I would think that If we discovered a mystery particle, for instance, one might classify it as life when it becomes necessary to invoke biological methodologies or theoretical frameworks to explain some of its properties.

[michel123456]

Math isn't anything on its own, we assign what it means with words.

Oh. I thought mathematics could explain everything (sarcasm).

[Jens]

A NASA panel stated the following:

"Life is a self-sustaining chemical system capable of Darwinian evolution."

from: Joyce, GF (1994) Forward. In Origins of Life: The central concepts. (ed. D.W. Deamer et al.) pp. xi-xii. Jones & Bartlett, Boston.

 

Or in separate statements:

- Life is a chemical system (in contrast to artificial life)

- Life is able to reproduce itself (in contrast to most abiotic chemical systems)

- Life is able to undergo evolution to obtain new features (in contrast to crystals or simple chemical positive feedback loops)

 

From my point of view statement 2 and 3 are essential. Statement 1 is optional and depend on how you want the definition of life to be.

 

This definition includes viruses and excludes artificial computer life.

Prions are excluded, because they cannot undergo evolution to obtain new features (they behave more like crystals).

I think it is o.k. to include viruses because some of the biggest viruses look more like degenerated parasitic cells and it is a bit strange to say they are dead because they lost their ribosomes and are dependent on other life forms for it (but humans are also dependent on other life forms).

Note that having a cell membrane or beeing able to catalyze metabolism is not included in this definition.

 

This definition from my point of view also holds to define the border between abiogenesis and evolution (and not only to identify extraterrestrial life, which was most likely the main reason for this definition).

 

 

 

 

 

[jp255]

In short, you can't define life

 

 

What is wrong with the definition Jens provided then?

[zapatos]

What is wrong with the definition Jens provided then?

Are you limiting your definition of 'life' to life on earth? I would not be surprised to find there is a something on another planet that acts as life on earth does with the exception of Darwinian evolution.

[mellowmorgan]

Are you limiting your definition of 'life' to life on earth? I would not be surprised to find there is a something on another planet that acts as life on earth does with the exception of Darwinian evolution.

I assumed that we were confining the definition of life to the biosphere, as we have no evidence of life existing outside of it.

[jp255]

Are you limiting your definition of 'life' to life on earth?

 

Of course. Why should we make a definition to include other life forms which might or might not exist? I think it is more appropriate to make a definition which can be applied to all life forms we currently are aware of, and if we do come across other life forms then we modify the definition if neccessary. It is a little hard to make a definition for life forms we don't know about and might not exist.

[zapatos]

Of course. Why should we make a definition to include other life forms which might or might not exist? I think it is more appropriate to make a definition which can be applied to all life forms we currently are aware of, and if we do come across other life forms then we modify the definition if neccessary. It is a little hard to make a definition for life forms we don't know about and might not exist.

One of the reasons to make a definition of life which you don't know exists is to aid you in your research. If an astrobiologist wishes to search for life on places other than earth it is helpful to have some idea of the evidence you should be searching for. Developing a hypothesis based on what you already know is not really that hard. It is standard science.

 

Astrobiology makes use of physics, chemistry, astronomy, biology, molecular biology, ecology, planetary science, geography, and geology to investigate the possibility of life on other worlds and help recognize biospheres that might be different from the biosphere on Earth.[4][5] Astrobiology concerns itself with interpretation of existing scientific data; given more detailed and reliable data from other parts of the universe, the roots of astrobiology itself—physics, chemistry and biology—may have their theoretical bases challenged. Although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories.

http://en.wikipedia.org/wiki/Astrobiology

[jp255]

One of the reasons to make a definition of life which you don't know exists is to aid you in your research

How do you propose we do this then?

 

Also, our definition of life does potentially define other lifeforms since the battle of the replicators was an instance in which different fundamental basic building blocks of life existed, all under selection. This is the best definition of life we have.

 

Again, point out the problems with it. Suggest a new definition. Accept the one proposed. Take your pick. This was really aimed at andeh to begin with, but also extends to others claiming there cannot be a definition of life.

[zapatos]

How do you propose we do this then?

No need for me to propose anything. It is already being done.

In the 21st century, astrobiology is a focus of a growing number of NASA and European Space Agency Solar System exploration missions. The first European workshop on astrobiology took place in May 2001 in Italy,[15] and the outcome was the Aurora programme.[16] Currently, NASA hosts the NASA Astrobiology Institute and a growing number of universities in the United States (e.g., University of Arizona, Penn State University, Montana State University, University of Washington, and Arizona State University),[17] Britain (e.g., The University of Glamorgan),[18] Canada, Ireland, and Australia (e.g., The University of New South Wales)[19] now offer graduate degree programs in astrobiology. The International Astronomical Union regularly organizes international conferences through its Bioastronomy Commission.

 

When looking for life on other planets like the earth, some simplifying assumptions are useful to reduce the size of the task of the astrobiologist. One is to assume that the vast majority of life forms in our galaxy are based on carbon chemistries, as are all life forms on Earth.[27] Carbon is well known for the unusually wide variety of molecules that can be formed around it. Carbon is the fourth most abundant element in the universe and the energy required to make or break a bond is just at an appropriate level for building molecules which are not only stable, but also reactive. The fact that carbon atoms bond readily to other carbon atoms allows for the building of arbitrarily long and complex molecules.

 

The presence of liquid water is a useful assumption, as it is a common molecule and provides an excellent environment for the formation of complicated carbon-based molecules that could eventually lead to the emergence of life.[28] Some researchers posit environments of ammonia, or more likely, water-ammonia mixtures.

http://en.wikipedia.org/wiki/Astrobiology

 

Without developing a hypothesis about how life on Mars may have existed, scientists would not know how to design their experiments.

The primary mission of the Viking probes of the mid-1970s was to carry out experiments designed to detect microorganisms in Martian soil because the favorable conditions for the evolution of multicellular organisms ceased some four billion years ago on Mars.[7] The tests were formulated to look for microbial life similar to that found on Earth. Of the four experiments, only the Labeled Release (LR) experiment returned a positive result, showing increased 14CO2 production on first exposure of soil to water and nutrients. All scientists agree on two points from the Viking missions: that radiolabeled 14CO2 was evolved in the Labeled Release experiment, and that the GC-MS detected no organic molecules. However, there are vastly different interpretations of what those results imply.

 

The Phoenix mission landed a robotic spacecraft in the polar region of Mars on May 25, 2008 and it operated until November 10, 2008. One of the mission's two primary objectives was to search for a "habitable zone" in the Martian regolith where microbial life could exist, the other main goal being to study the geological history of water on Mars. The lander has a 2.5 meter robotic arm that was capable of digging shallow trenches in the regolith. There was an electrochemistry experiment which analysed the ions in the regolith and the amount and type of antioxidants on Mars.

http://en.wikipedia.org/wiki/Life_on_Mars

 

Again, point out the problems with it.

Well, as I said, if we are talking about extraterrestrial life, Darwinian evolution may not be a factor. Evolution on earth is possible due to DNA and how it is replicated. It is easy enough to imagine scenarios where that type of replication, and thus Darwinian evolution, would not take place.

[jp255]

Well, as I said, if we are talking about extraterrestrial life, Darwinian evolution may not be a factor

exclusion of natural selection or evolution to the definition of life causes it to breakdown and become much less meaningful for our observable life form, and if removing this aspect of the definition means the definition doesn't work for a life form which we do know exists and is observable, how can you even hope for the definition to work for something that might not even exist in the first place? This doesn't seem terribly logical to me. How do you propose the definition should be altered to appropriately define life without using evolution as inclusion criteria?

 

 

Evolution on earth is possible due to DNA and how it is replicated.

Except evolution is not limited to DNA only. What aspect of replication are you referring to? and which allows for evolution?

 

 

It is easy enough to imagine scenarios where that type of replication, and thus Darwinian evolution, would not take place.

Such as? Also are you explicitply referring to the abiogenesis part of life, where the initial replicators might not be under darwinian evolution? Does this mean that the actual complex Alien (complex life form) might not be subject to evolution?

Edited September 6, 2012 by jp255

[zapatos]

exclusion of natural selection or evolution to the definition of life causes it to breakdown and become much less meaningful for our observable life form, and if removing this aspect of the definition means the definition doesn't work for a life form which we do know exists and is observable, how can you even hope for the definition to work for something that might not even exist in the first place? This doesn't seem terribly logical to me. How do you propose the definition should be altered to appropriately define life without using evolution as inclusion criteria?

I was not trying to develop an appropriate definition of life. You asked if there were any problems with a specific definition and I pointed out a problem that may exist if you were including extraterrestrial life. If you are not including extraterrestrial life I have no problem with that definition. If you are including extraterrestrial life, then I do.

 

Except evolution is not limited to DNA only. What aspect of replication are you referring to? and which allows for evolution?

Without mutations to DNA we would not have evolution. If a life form exists that cannot have a heritable mutation, darwinian evolution would not apply.

 

Such as? Also are you explicitply referring to the abiogenesis part of life, where the initial replicators might not be under darwinian evolution? Does this mean that the actual complex Alien (complex life form) might not be subject to evolution?

Abiogenesis could be part of it. An environment may exist that allows for a simple form of life to form over and over again without reproducing.

 

A life form may exist where the information on how to reproduce is encoded on a portion of the organism critical to life. If that portion of the organism changes, it cannot live.

 

We hadn't been necessarily discussing 'complex' life forms. That will likely be a different issue as that life form needed some method to become complex. Although I suppose it is possible for a life form to originally be subject to evolutioin, but then to have that ability to evolve cease due to environmental or organism functions.

[jp255]

If you are including extraterrestrial life, then I do.

The point is that if we include extraterrestrial life into the equation and you do have a problem with it, then you need to redfine life it so that it does not include natural selection or darwinian evolution and can still accurately define the life forms we observe on Earth. You have not done this, and more importantly it has not been done as of yet (I have not come across such a definition). If you know of one, then I'd definitely be interested in reading it.

 

Abiogenesis could be part of it. An environment may exist that allows for a simple form of life to form over and over again without reproducing.

 

It depends on whether or not you see replicating chemical systems (that are not under selection) as life or not, because without using natural selection then they can count as a life form. If you do see chemical systems as a life form then I have no problem with your statement, but if you don't then redefine life.

 

Do you not think it is more appropriate to redefine life, without using natural selection or evolution, so that the definition is accurate, before you begin saying it is possible for life forms to arise that are not under selection/evolution? Do you not see the logic here? that the definition should be changed and still be accurate before you claim that extraterrestial life could arise without selection or evolution?

 

If you are going to stick to these claims, that extraterrestrial life might not be under selection or evolution, then provide support for it?

 

You asked if there were any problems with a specific definition and I pointed out a problem that may exist if you were including extraterrestrial life

 

The definition of life we have can only hold with inclusion of selection/evolution. Until I see a definition which is accurate without selection/evolution, then there is not a problem. There are many things which may be a problem, but then again, they may not be. In the instance of the latter, you'd be looking a problem that does not exist to your knowledge, and this will be an everlasting problem. There may not even be extraterrestrial life, at least complex life forms.

 

 

I think you are jumping the gun in a way. Is there a reason to think about potential scenarios in which life can arise without evolution/selection if the definition we have cannot hold without it's inclusion? If you do manage to then it's likely that you are wrong.

[zapatos]

The point is that if we include extraterrestrial life into the equation and you do have a problem with it, then you need to redfine life it so that it does not include natural selection or darwinian evolution and can still accurately define the life forms we observe on Earth. You have not done this, and more importantly it has not been done as of yet (I have not come across such a definition). If you know of one, then I'd definitely be interested in reading it.

Since extraterrestrial life is an unknown, I think a definition that is more flexible would be required. Perhaps something that says 'points x, y, and z are required, but indicators of life also include points a, b, and c'. In this case I would include Darwinian evolution as a possible indicator of life, but not a requirement for life.

 

If you are going to stick to these claims, that extraterrestrial life might not be under selection or evolution, then provide support for it?

I'm speculating, and that is why my support has been only imagined scenarios. Is it really that unlikely that the ability to evolve in some form of life might exist at one time and then be lost?

 

The definition of life we have can only hold with inclusion of selection/evolution.

Why?

 

I think you are jumping the gun in a way. Is there a reason to think about potential scenarios in which life can arise without evolution/selection if the definition we have cannot hold without it's inclusion? If you do manage to then it's likely that you are wrong.

Again, I think it is reasonable to create a definition of life that includes 'possible indicators' of life that don't necessarily have to exist in all possible life forms.

[proximity1]

A NASA panel stated the following:

"Life is a self-sustaining chemical system capable of Darwinian evolution."

from: Joyce, GF (1994) Forward. In Origins of Life: The central concepts. (ed. D.W. Deamer et al.) pp. xi-xii. Jones & Bartlett, Boston.

 

Or in separate statements:

- Life is a chemical system (in contrast to artificial life)

- Life is able to reproduce itself (in contrast to most abiotic chemical systems)

- Life is able to undergo evolution to obtain new features (in contrast to crystals or simple chemical positive feedback loops)

 

From my point of view statement 2 and 3 are essential. Statement 1 is optional and depend on how you want the definition of life to be.

 

This definition includes viruses and excludes artificial computer life.

Prions are excluded, because they cannot undergo evolution to obtain new features (they behave more like crystals).

I think it is o.k. to include viruses because some of the biggest viruses look more like degenerated parasitic cells and it is a bit strange to say they are dead because they lost their ribosomes and are dependent on other life forms for it (but humans are also dependent on other life forms).

Note that having a cell membrane or beeing able to catalyze metabolism is not included in this definition.

 

This definition from my point of view also holds to define the border between abiogenesis and evolution (and not only to identify extraterrestrial life, which was most likely the main reason for this definition).

 

 

 

 

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."

[jp255]

The definition of life we have can only hold with inclusion of selection/evolution.

Why?

 

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.