Does life have to be Biochemical?

[magma]

Would Aliens require oxygen to generate themselves ATP as energy?

[Doodler]

Atp?

[Swift]

magma

I take those two questions "Does life have to be biochemical" and "Does life need ATP" as two very different questions.

I suspect, though I'm not sure, that there are life forms on Earth that don't use ATP (http://en.wikipedia.org/wiki/Adenosine_triphosphate). Even if not true, it seems easy to imagine life that didn't, there are a lot of biochemical cycles that one might use.

Does life have to be biochemical.... are you talking about an inorganic lifeform (silicon?) or something made of energy? There are previous discussions of both of these around this thread.

[magma]

Atp?

Adenosine triphosphate.

Swift - i would like to know if life elsewhere has to be biochemical to function. Can there possibly be cars/ robots that 'live' if they were synthesised in a way (e.g. we obtained chemical info from non-biochemical species and made a car from it). Forgive me if i'm not making sense.

[Swift]

Swift - i would like to know if life elsewhere has to be biochemical to function. Can there possibly be cars/ robots that 'live' if they were synthesised in a way (e.g. we obtained chemical info from non-biochemical species and made a car from it). Forgive me if i'm not making sense.
I don't know. Carbon-nitrogen-oxygen-hydrogen chemistry is just so darn useful and versatile (and this is coming from an inorganic chemist!). Just look at all the compounds humans have made from them. While one can imagine life based on something like silicon, it just doesn't make as many different kinds of things.

As far as naturally occurring lifeforms that were like robots, it is very hard for me to imagine how such a thing would come about, but maybe that's just my lack of imagination. Could a synthetic lifeform like that exist (think Data or Hal) - yes I can imagine that. But someone (or something) would have to build it.

[BioSci]

Would Aliens require oxygen to generate themselves ATP as energy?

to answer the two questions:

"Does life have to be biochemical?" this is a "circular" question since the definition of Bio-chemical means the chemical reactions that are involved with living organisms. Therefore, if you define something as living - its chemical reactions will be "biochemical".
Perhaps you mean "will alien life have the same biochemical reactions as Earth life?" The answer to that question is unknown since we do not have any evidence for what alien life will be like. But we can guess that alien life is most likely to include some similar "biochemical" reactions but also biochemical reactions that may be quite different. It is highly unlikely that our biochemistries would be "compatible" (that is, we would not be edible nor would they be edible)

"Would Aliens require oxygen to generate themselves ATP as energy?" The answer is no - even on Earth there are organism (anaerobes) that do not require oxygen to generate ATP.
Although all known Earth organisms do use ATP, it is unlikely that an alien organism would actually use ATP. Even if they had a similar biology, the odds are that they would have developed using a different "high-energy" molecule - and even if they used a triphosphate, it is more likely that it would use some other nucleotide as the base molecule - simply on a statistical basis.

[Swift]

Thanks BioSci. So all Earth organisms use ATP, even anerobes (learn something new everyday). I suppose there is speculation on when in the history of life on Earth organisms started to use ATP. Are there ideas about what was used before?

[Van Rijn]

to answer the two questions:

"Does life have to be biochemical?" this is a "circular" question since the definition of Bio-chemical means the chemical reactions that are involved with living organisms. Therefore, if you define something as living - its chemical reactions will be "biochemical".


I suppose it would be difficult to avoid chemical reactions, but we can imagine something along the line of macroscopic machine replicators that might well process materials in bulk similar to today's industrial processes, and where many life functions were mechanical and electrical in nature. Something like that might require a different classification.

[BioSci]

Thanks BioSci. So all Earth organisms use ATP, even anerobes (learn something new everyday). I suppose there is speculation on when in the history of life on Earth organisms started to use ATP. Are there ideas about what was used before?

Any such speculation is just so ... speculative.:)

There are a number of hypotheses regarding how the various biochemical pathways and molecules developed including the use of ATP. The problem is that any hypotheses on abiogenesis are very difficult to test and remain very specualtive.

[BioSci]

I suppose it would be difficult to avoid chemical reactions, but we can imagine something along the line of macroscopic machine replicators that might well process materials in bulk similar to today's industrial processes, and where many life functions were mechanical and electrical in nature. Something like that might require a different classification

Yes, and perhaps the first classification question may be: "Is it living?"

[Van Rijn]

Yes, and perhaps the first classification question may be: "Is it living?"

Assuming a complex system could self replicate in a natural environment, why wouldn't it be considered living?

Edited to add:

Assume this is a complex organized system, it processes energy and materials for itself (metabolism), can respond and adapt to the environment, can repair itself and can replicate based on a detailed stored pattern, but is essentially mechanical/electronic based.

[BioSci]

Assuming a complex system could self replicate in a natural environment, why wouldn't it be considered living?

Edited to add:

Assume this is a complex organized system, it processes energy and materials for itself (metabolism), can respond and adapt to the environment, can repair itself and can replicate based on a detailed stored pattern, but is essentially mechanical/electronic based.

Good question and one that is not simply answered.

From Wiki:

A conventional definition

Although there is no universal agreement on the definition of life, scientists generally accept that the biological manifestation of life exhibits the following phenomena:
1. Homeostasis: ...
2. Organization: ...
3. Metabolism: ...
4. Growth: ...
5. Adaptation: ....
6. Response to stimuli: ...
7. Reproduction: ...

Note that this definition includes the disclaimer that it is limited to a "biological manifestation". Most likely to differentiate from things like storms, flames, software, etc.

If some sort of "organism" met your criteria - it might be considered "alive" - or "just a robot?"

[Ronald Brak]

I'll mention that people have made simulated life inside computers. They anabolize and catabolize (make stuff and break stuff) in their electronic worlds and I'm tempted to count them as alive, if only to score brownie points with our future electronic masters.

[Van Rijn]

Heh. I was thinking about software, though I think current examples are too simple. In his "Uplift" universe, David Brin had these "physical" designations for known life forms: Oxygen-breathing, hydrogen breathing, mechanical, memetic, and quantum. (For civilization types, there was also "retired" and "transcendent.")

Of course that's fiction, but I have this image of a mechanical species debating whether greasy waterbags full of complex chemicals could be considered alive. However things turn out, the question of a careful definition of life is indeed a hard one to answer, and is likely to get much harder.

[weatherc]

Of course that's fiction, but I have this image of a mechanical species debating whether greasy waterbags full of complex chemicals could be considered alive.Which brings us to Bisson's short story, "They're Made out of Meat" (http://www.eastoftheweb.com/short-stories/UBooks/TheyMade.shtml) which has been discussed on this board before.

[eburacum45]

I have only read one Uplift novel; how do 'memetic' lifeforms operate? Are they, as I suspect, the equivalent of computer viruses? That is quite a nice distinction.
In some ways memes (as defined by Dawkins (http://en.wikipedia.org/wiki/Meme)) themselves could be considered self-replicating entities in a culture; perhaps memes could evolve to such a level of sophistication (by 'survival of the most believable') until they are self-evidently separate entities.

[Dr Nigel]

Good Note that this definition includes the disclaimer that it is limited to a "biological manifestation". Most likely to differentiate from things like storms, flames, software, etc.

If some sort of "organism" met your criteria - it might be considered "alive" - or "just a robot?"

Additionally, many biologists include that something alive would have a boundary of some kind, which mainly serves to distinguish presently-known organisms from other natural phenomena that could meet the other aspects of the definition. Or, in other words, a clarification of the "biological manifestation" aspect of Wiki's definition of life.

Although, if you think about it, the term "biological manifestation of life" is a tautology and does not actually help to clarify anything.

[Van Rijn]

I have only read one Uplift novel; how do 'memetic' lifeforms operate? Are they, as I suspect, the equivalent of computer viruses? That is quite a nice distinction.


A bit more than that. In his stories, they are "life" that can operate in logic or minds of other lifeforms, or in "E space" (some dimension or hyperspace).

I found a sample on his website that has one scene with a sentry operating in E space:

http://www.davidbrin.com/heavensreachsample5.html

[springa]

I think that mechanical and "memetic" life is quite possible, but wouldn't any such life have to be originally created by a chemical/biological lifeform? I can't imagine how such a life form could arise on its own without lots of manipulation by an already-existing intelligent lifeform. Then again, as a previous poster commented, this may just show the limits of my imagination!

The biggest problem with all speculations about life beyond the earth is that the only life that we have solid facts about is earth life. We're trying to guess the nature of life in many different parts of the universe based on a representative sample of 1 planet. Any statistician will tell you that a representative sample of 1 is not something that you can use to draw reliable conclusions about a large group. :think:

[Amber Robot]

Carbon-nitrogen-oxygen-hydrogen chemistry is just so darn useful and versatile (and this is coming from an inorganic chemist!).

Not to mention that they are the most common elements in the universe!

[eburacum45]

Mechanical-synthetic lifeforms might need to be created by biological creatures like ourselves, but once created they might prove to be more versatile and capable of replicating in more diverse environments than biological life. It may easily be the case that the most common form of self-replicating complexity in the universe is mechanical in nature, robots and Von Neumann machines created by a billion dead civilisations.

Can such a form of self-replicating complexity arise spontaneously? Geological processes create some remarkably complex structures under different circumstances; could it be that occasionally a simple mineralogical self replicating entity arises, one with the capacity to evolve into more complex forms? It seems very unlikely, but it would be interesting to be proved wrong.

[Thanatos]

Can't help but wonder if somewhere 'out there' are sentient machines who speculate they are ancestors of primitive nanites produced by chance collisions between nickel iron meteors and mineral deposits. Of course, they would reciprocate their EM sensors in derision at the notion the chemically engineered molecular systems they create to produce fuels and building materials could arise spontaneously.

[Dr Nigel]

Not to mention that they are the most common elements in the universe!

Er ... yes, also Helium, which is the second-most common element after hydrogen, but is not so useful to life, being chemically inert.

[Dr Nigel]

I don't know. Carbon-nitrogen-oxygen-hydrogen chemistry is just so darn useful and versatile (and this is coming from an inorganic chemist!). ...

Yes, but life as we know it could not get by without also having access to sulphur and phosphorous.

[bindegal]

Can't help but wonder if somewhere 'out there' are sentient machines who speculate they are ancestors of primitive nanites produced by chance collisions between nickel iron meteors and mineral deposits. Of course, they would reciprocate their EM sensors in derision at the notion the chemically engineered molecular systems they create to produce fuels and building materials could arise spontaneously.

LOL :clap:

I really like that thought, and why not?

[Romanus]

<<Would Aliens require oxygen to generate themselves ATP as energy?>>

Probably not; then again, anaerobic ETs would probably live an extremely lethargic existence compared to us crazy oxygen-burners.

[greenfeather]

Would Aliens require oxygen to generate themselves ATP as energy?

As to whether life has to be biochemical...I'm reading the coolest book, THE SUNBORN by Gregory Benford. It portrays these plasma intelligences that live in the space beyond the solar system (and thru most of the galaxy). "They live through the adroit weaving of electrical currents.They feed on the electrical potentials that trickle through the comet clouds. Their interiors are highly ionized plasmas...."

These Beings tell strange tales of "the little rocky worlds that have been lately rotting into life." "A low obscene hot life. SOLIDS. Not powered by the clean transformations of electromotive force, but by the clumsy building up and tearing down of molecules."

This is some great prose!! And it makes you think: there is so much energy, heat and power in space, could something have evolved that can use that power? Also, when you think about molecular processes like the breaking down of the glucose molecule and the creation of ATP... those are some awfully clumsy processes. Who knows, maybe there are other processes going on.

[Dr Nigel]

Who knows, indeed?

I have to say, it would be extraordinary indeed if lifeforms existed that were composed of rarefied plasma. How would they maintain structural integrity?

[Amber Robot]

I have to say, it would be extraordinary indeed if lifeforms existed that were composed of rarefied plasma. How would they maintain structural integrity?

Magnetic fields?

[greenfeather]

Who knows, indeed?

I have to say, it would be extraordinary indeed if lifeforms existed that were composed of rarefied plasma. How would they maintain structural integrity?

Um...electromagnetics? Gravitational flows? (we're talking large beings here. Larger than planets.

We'd better ask Greg Benford, the author. Matter of fact there was a thread here about plasma life. Apparently plasma can form membrane-like bubbles.

[A.DIM]

Any such speculation is just so ... speculative.:)

There are a number of hypotheses regarding how the various biochemical pathways and molecules developed including the use of ATP. The problem is that any hypotheses on abiogenesis are very difficult to test and remain very specualtive.

my bold


Hi BioSci.

I just wanted to say I'm glad to hear such a thing from yet another person who actually knows what they're talking about.

Thanks.

[Dr Nigel]

.... Apparently plasma can form membrane-like bubbles.

I sincerely doubt that. Membrane formation is dependent on the interaction of hydrophilic versus hydrophobic components. There is no parallel in plasma physics.

[greenfeather]

I sincerely doubt that. Membrane formation is dependent on the interaction of hydrophilic versus hydrophobic components. There is no parallel in plasma physics.

Hi!

Disclaimer: I'm a lateblooming science freak who just learned about hydrophilic membranes LAST MONTH. So I don't know anything. However, I did read this entry in the 'Encyclopedia of Astrobiology' about plasma-based life.

http://www.daviddarling.info/encyclopedia/P/plasma-based_life.html

It's about an experiment with electrodes, where concentrations of positively & negatively charged ions formed a sphere that could replicate. I'd paste the whole few paragraphs, but it's probably against the rules of this forum. Nevertheless, check it out, it's fascinating!!

[Dr Nigel]

Thanks for the link, greenfeather, it makes for interesting reading.

The penultimate paragraph mentions lifespan of the globules without mentioning any figures.

The properties of the globules were, in general, discussed too vaguely to be satisfying. The text claimed that the globules could "replicate", "grow" and "communicate", but did not mention how these things occurred, or if any intervention was required to stimulate these events.

However, one thing that was glaringly absent was any comment about energy use. The electrical arc cuased these things to form, and they seem to have a finite lifespan (though whether hours, minutes, seconds or microseconds was not even hinted at). There was no comment at all about whether these globules required energy once they had formed.

There also was no comment about whether the conditions used might actually be found anywhere in nature. A "low-temperature" plasma of argon could, I guess, form in a gas cloud in space, if the cloud had a sufficiently high argon content. But, in a rarefied gas or plasma, where does the electrical arc come from?

[BioSci]

my bold


Hi BioSci.

I just wanted to say I'm glad to hear such a thing from yet another person who actually knows what they're talking about.

Thanks.

It is not that abiogenesis is speculative - only that the exact path taken is difficult to determine. It is much like trying to map someone's walk from New York to Los Angeles - one may know the start and the end but without a record of the trip, the path taken for the walk is difficult to determine because many paths are possible. But one could map the more likely and unlikely paths.
The actual process of abiogenesis on earth is similar in nature. We know the general starting conditions and we know how life functions today. There are many ways to connect the two but little direct evidence to say which is actually the most likely.

[A.DIM]

Oh?

:think:

If that were the case would we not by now have multiple experiments showing the many life from non life possibilities?
I mean, if abiogenesis is so likely and it's simply a matter of "which way was it?" then why haven't there been a plethora of experiments showing life from non life and all that is left is to choose which one got us here?

[BioSci]

Oh?

:think:

If that were the case would we not by now have multiple experiments showing the many life from non life possibilities?
I mean, if abiogenesis is so likely and it's simply a matter of "which way was it?" then why haven't there been a plethora of experiments showing life from non life and all that is left is to choose which one got us here?

Nope... - unless you have a few million years to run your experiments.

[mike alexander]

There's also Fred Hoyle's Black Cloud.

I remember a quote from Freeman Dyson to the effect that that the main requisite for life is a good signal to noise ratio.

[Van Rijn]

Nope... - unless you have a few million years to run your experiments.

. . . and run a large number of parallel experiments.

[Dr Nigel]

Nope... - unless you have a few million years to run your experiments.

And wipe out all potential competitors before you start...

Edited to add : And this would not get past the University's ethics committee. Trust me on this one.

One of the key conditions for abiogenesis is no pre-existing life in the same place. This is because the first self-replicators that might arise through abiogenesis would not be very efficient, and any pre-existing life would out-compete it for resources (and quite probably consume the primitive self-replicator).

[A.DIM]

Nope... - unless you have a few million years to run your experiments.

Ah yes, the approximate, and rather brief, window within which it is thought Life arose.

Is there a single experiment you'd point to which shows one of these potential Life from nonLife (just add time) chemical reactions?

It seems everything I've read implies abiogenesis is further away than it ever has been?

[BioSci]

Ah yes, the approximate, and rather brief, window within which it is thought Life arose.

Is there a single experiment you'd point to which shows one of these potential Life from nonLife (just add time) chemical reactions?

It seems everything I've read implies abiogenesis is further away than it ever has been?

You may need to update your reading of the literature (and stay away from creationist inspired screeds - there is a lot of that on the web). A very readable account and one that is backed by current research can be found here:
http://www.talkorigins.org/faqs/abioprob/originoflife.html
- with an extensive list of primary research papers if you want to explore further.

Abiogenesis is the leading candidate for how life started on Earth because it has a great deal of actual research which supports the basic concept. It is a very complex, difficult question to try and determine specifically how it actually progressed. It is an area of active research and some scientists are optimistic that they have approaches that could demonstrate or even produce "proto-life" reactions: http://www.carlzimmer.com/articles/2004/articles_2004_Before_DNA.html

As an alternative hypothesis, the evidence for Panspermia consists essentially of "we are not able to prove that it could not happen". Weak indeed!

[Dr Nigel]

Also, all Panspermia succeeds in achieving is moving the arena for the origin of life from Earth to some other place of which we wot not. (Forgive the archaic English, but it seemed rather poetic).

If life began on Earth, then its origin is accessible in one form or another to experimentation. This is because we know many of the conditions that prevailed upon the early Earth, and these conditions can be simulated.

However, if life began elsewhere, we have no clue about the conditions in which it arose, so all we can do is speculate.

[Thanatos]

Our ignorance of chemistry is the main culprit. Just because we can't reproduce the key processes does not mean they are ruled out. That is pure arrogance. I'm not willing to hand the keys back to the temple priests on those grounds. It is bad science to propose life on earth was 'seeded'. The logistics required are absurd. Get used to the idea life originated here. The evidence is overwhelming.

[Dr Nigel]

Thanatos, I'm not so sure that it is our ignorance of chemistry. Our knowledge of chemistry as a physical science is detailed and extensive. However, what I think you meant is that we are ignorant of the details of the chemistry that was occurring on the early Earth. While we know the general conditions that prevailed, there would have been small variations from place to place, and we do not know the precise details of the conditions that obtained at the site where chemistry first became biochemistry.

[torque of the town]

I have just started reading STARDUST by John Gribbin, and just wanted to say how much I am enjoying this thread.

[parallaxicality]

As regards silicon biochemistry, I can only say that the Earth is composed of 15 percent silicon vs. something like 0.0001 percent carbon. And yet it is carbon, not silicon, out of which life formed. It seems unlikely to me that any other chemical pathway is ultimately feasable in creating life.

Regarding life beyond chemistry, since we don't really have a clue what life is, it's hard to say what characteristics an object must have to be considered alive.

[Dr Nigel]

Parallaxicality, I agree. Silicon is a useless element compared to carbon. It does not readily form bonds with itself, and its oxides are far too stable to break with a couple of photons of sunlight.

One of my organic-chemistry lecturers was fond of stating that, of 11 million known chemical compounds, 10 million were organic (i.e. carbon-based).

[Thanatos]

And carbon is the perfect element to create the diversity of compounds necessary for life as we know it. I expect this mold will recurr as the dominant template for life throughout the universe. Aside from some fine points, Dr. Nigel, we are in agreement. It is illogical to conclude this planet is a special case of fortuitous coincidence. I am much more concerned about our ability to self destruct. We need to raise the standard of life here above the "us against them" mentality. Interdependence is a good idea. And perhaps the internet will save us all from the folly of self importance. Starting a dialogue here might curb our leaders appetite for power. A world democracy? Perhaps. Civility starts one person at a time.

[Dr Nigel]

And carbon is the perfect element to create the diversity of compounds necessary for life as we know it. I expect this mold will recurr as the dominant template for life throughout the universe. Aside from some fine points, Dr. Nigel, we are in agreement.

I'm inclined to agree with you. :)

[Infinite]

Life as we know it and define it exists no doubt. But how would we know what other life forms look like if we do not have the ability to recognize it with our current technology? Take for example microbes. Before the microscope microbes did not exist. So my answer to the question "does life have to be biochemical" would be - Life could be anything ----Life = thought = energy = conscience = infinite.

[Dr Nigel]

Life as we know it and define it exists no doubt. But how would we know what other life forms look like if we do not have the ability to recognize it with our current technology? Take for example microbes. Before the microscope microbes did not exist. So my answer to the question "does life have to be biochemical" would be - Life could be anything ----Life = thought = energy = conscience = infinite.

Welcome to BAUT forum, Infinite.

We (humankind, I mean) are indeed struggling to arrive at a universal definition of life. However, answering the question in the OP (Does life have to be biochemical?) can largely be done with knowledge that we already have.

The laws of chemistry are universal, and we know a great deal about chemistry. Any life form will need (a) molecules to store information; (b) molecules to catalyse chemical reactions, (c) molecules to store and transfer energy, and (d) molecules to build and maintain the structure of the organism.

Carbon is the only element that permits sufficiently varied chemistry to fulfil these roles. Therefore, it is a reasonable assumption that any life we may discover elsewhere will contain carbon-based chemistry. From this starting point, we cannot be definite about what kind of chemistry will take place, but we can be fairly confident about some of the things it will be doing.

[RBG]

My idle philosophic ponder for today:

Here we have Earth, which appears to be the ideal general environment for the arising of life, yet there is only one specific carbon-based form of it anywhere here to be found.

Can it be that the physics of life as we know it is a universal standard like the physics of gravity or light or black holes, etc. meaning it is what it is and must be, in the same way you don't expect to find other "competing" or different forms of gravity or light or black holes?

RBG

[parallaxicality]

Some people think so, most notably Simon Conway Morris. Life and evolution have rules, or as he put it, "follow an invisible landscape" that result in similar forms and uses for the same structures (such as the camera eye) and chemicals (such as haemoglobin, which also occurs in legumes). There is absolutely no way to prove this as yet, but I wouldn't be surprised if it were mostly correct.

Carbon is the fourth most common element in the universe. Amino acids have been found in interstellar space. Therefore it seems likely that the same biological precursors that rained down on our world during its formation also rained down on other suitable worlds. It's looking more and more like life is a "cosmic imperative" (who came up with that phrase anyway?)

Anthropic principle, anyone?

[Dr Nigel]

Carbon is the fourth most common element in the universe. ...

After, I assume, hydrogen, helium and ...? (one of Li, Be or B, I guess)

[parallaxicality]

Oxygen.

Strange. You'd think there'd be more lithium or beryllium, but apparently not.

[Swift]

Some people think so, most notably Simon Conway Morris. Life and evolution have rules, or as he put it, "follow an invisible landscape" that result in similar forms and uses for the same structures (such as the camera eye) and chemicals (such as haemoglobin, which also occurs in legumes). There is absolutely no way to prove this as yet, but I wouldn't be surprised if it were mostly correct.

Carbon is the fourth most common element in the universe. Amino acids have been found in interstellar space. Therefore it seems likely that the same biological precursors that rained down on our world during its formation also rained down on other suitable worlds. It's looking more and more like life is a "cosmic imperative" (who came up with that phrase anyway?)

Anthropic principle, anyone?
I'm don't disagree with any of that, but if there are such "rules", how far do they extend?

Is all (or most) life based on carbon-oxygen-hydrogen-nitrogen?
Is it all based on water as a solvent?
Is it all based on long-chain molecules for whom their folded structure is critical?
Is it all based on amino acids and proteins?
Is it all based on DNA/RNA?

Even if the answers to all of that is yes (and I wouldn't bet on that), there still could be an immense variety to life.

[BioSci]

I'm don't disagree with any of that, but if there are such "rules", how far do they extend?

Is all (or most) life based on carbon-oxygen-hydrogen-nitrogen?
Is it all based on water as a solvent?
Is it all based on long-chain molecules for whom their folded structure is critical?
Is it all based on amino acids and proteins?
Is it all based on DNA/RNA?

Even if the answers to all of that is yes (and I wouldn't bet on that), there still could be an immense variety to life.

Good questions:

I would guess (based on our current knowledge of biochemistry & etc. that assumptions 1-3 may well be universally true (~99% probable)

#4 may be less certain (~50% - but with different groups of amino acids)

and #5 is most likely not a universal biochemical solution (although some type of nucleotide polymer may be common, DNA/RNA as found on earth is highly unlikely to be found everywhere that life arises).

[Dr Nigel]

...
Is all (or most) life based on carbon-oxygen-hydrogen-nitrogen?

Yes, all life that is known to humankind relies on C, O, N and H (also uses phosphorus and sulphur).


Is it all based on water as a solvent?

Yes. Water is the most versatile solvent.


Is it all based on long-chain molecules for whom their folded structure is critical?

Yes, even viruses require their proteins to be correctly-folded. It could even be argued that prions are a form of life (proteins that catalyse the conversion of an innocuous protein into a prion; prions are the agents of transmissible spongiform encephalopathies), but this is a bit off the wall.


Is it all based on amino acids and proteins?

Yes, and also DNA and RNA unless you consider prions to be alive. (Note: I know that some viruses do not contain either RNA or DNA*, but their replicative life cycle pretty much has to involve both).


Is it all based on DNA/RNA?

Yes, unless you consider prions to be alive.


Even if the answers to all of that is yes (and I wouldn't bet on that), there still could be an immense variety to life.

Quite right. My answers all depend on "life as we know it". And life as we know it comes from only one planet. Of course, this does rather lead into the question of how to define life - if we were to encounter life that is other than as we know it, would we recognise it as life?

There are some chemical constraints to life. The versatility of carbon as a backbone for complex molecules has already been mentioned.

DNA and / or RNA are extremely effective at encoding information for life, but (in mammals) they do not work alone. There is growing body of evidence hinting that information can be passed from one generation to the next in the chemical composition of the cytoplasm of the egg cell.

Proteins may or may not be essential. It is true that RNA can catalyse some chemical reactions, but it is not clear if RNA would ever be capabale of the vast range of chemistry that can be catalysed by proteins. For example, the mechanism of the enzyme ribonucleotide reductase (which converts a ribonucleotide into a deoxyribonucleotide) involves free-radical chemistry that is only quenched by the formation of a thiyl radical in a cysteine residue; without this quenching mechanism, the peptide backbone itself is cleaved and the enzyme activity is destroyed. This could not be done by RNA (because nucleic acids contain no sulphur).

So it seems likely that life yet-to-be-discovered will use carbon chemistry, and it will use molecules that are at least similar to DNA, RNA and proteins.


*ETA: I realise this may not be clear. Most viruses contain RNA or DNA, but few or none contain both. None contain neither DNA nor RNA. What I meant here was that many viruses contain only one type of nucleic acid.

[Van Rijn]

Quite right. My answers all depend on "life as we know it". And life as we know it comes from only one planet. Of course, this does rather lead into the question of how to define life - if we were to encounter life that is other than as we know it, would we recognise it as life?


That depends on both the definition and how we search for it. For instance, if we use a definition that includes the statement that "life must be carbon based" than even if we did manage to find something that has organization, metabolism, grows, adapts, reproduces, and responds to stimuli but was made of silicon compounds, then by the definition, it wouldn't be life. And given such different chemistry, search methods tailored for earth life could miss it.

I'd go for a definition based on function. For instance, if we do manage to build self replicating machines (macro or otherwise) that can do all the regular "life" functions in a natural environment, I think it should be classified as life, even if it is made of metal and silicon.

[eburacum45]

Conway Morris' idea of an 'invisible landscape' which dictates the form and biocheistry of life may be correct, but I think it is very wrong to think that life on Earth has explored even a tiny fraction of that landcaspe.

I suspect as well that the landscape of life is only similar on worlds which have roughly comparable physical characteristics; a planet with half the gravity of Earth, or twice as much, would have an entirely different 'phase space' of biological possibilities (if I can venture to use a concept from physics).

On a low gee or high gee world the atmospheric composition and pressure would be quite different in all probablity, and perhaps also the geological behaviour of the crust, the intensity of available insolation, so on and so forth; also the simple act of locomotion on land would require different mechanisms on worlds with different gravity regimes. So even with an invisible landscape concept, I don't think it is possible to make any reliable predictions about extraterrestrial life.

[Ronald Brak]

Gravity will certainly affect life on other planets but for life in the ocean it won't be such a big deal (of course pressure would increase more rapidly with depth). Also earth type modes of locomotion would work fine on a high gravity world, provided you are less massive than what you would be on earth. That is, a creature the size of a cow might need the rough body shape of a rhino. A mouse might need to be built like a hamster and so on. So nature on a high gravity planet might not come up with anything too spectacular. But note that I'm not saying it couldn't, just that it might not.

[Thanatos]

Self replication is probably the most fundamental trait required to contend to for the title 'life form'. That, of course, would include a number of processes most of us would resist characterizing as 'living'. Pinning the tail on the life donkey is a slippery affair. Self replicating robots - a common SF motif - could conceivably interact with the environment [and ourselves] in ways not easily distinguished from behaviors exhibited by biological life forms. It appears highly unlikely such entities could arise via naturally occurring processess, but I'm not sure that is relevant. I suspect the point would be moot if we someday found ourselves competing with them for resources. Herein, I think is the real issue. Life is whatever competes for resources with other like minded entities. In that sense, an entity is alive if it takes steps to avoid being eaten.

[Chip]

...we can imagine something along the line of macroscopic machine replicators that might well process materials in bulk similar to today's industrial processes, and where many life functions were mechanical and electrical in nature....

Which kind of reminds me of that oddball B movie from the 1950s, "Kronos".
http://www.imdb.com/title/tt0050610/

[Van Rijn]

You managed to come up with a '50s science fiction movie title I don't recall at all. I put it in my netflix queue so I can correct ths.

[parallaxicality]

I have only read one Uplift novel; how do 'memetic' lifeforms operate? Are they, as I suspect, the equivalent of computer viruses? That is quite a nice distinction.
In some ways memes (as defined by Dawkins (http://en.wikipedia.org/wiki/Meme)) themselves could be considered self-replicating entities in a culture; perhaps memes could evolve to such a level of sophistication (by 'survival of the most believable') until they are self-evidently separate entities.

It's sort of ironic that Dawkins invented the word meme mainly to trash religion, and yet it has evolved out of its original concept and taken on a far broader definition. I like the idea of memes, but sentient memes seems unlikely to me, since no one is calling genes sentient. If we were to take the analogy further, we might see the "memesphere" as a kind of virtual ideal overlay of our world, akin to Plato's world of ideas, in which memes organise and interact. Collections of memes merge and form entities (The Crazy Frog is an example of a collection of memes which merged and became a separate entity). I'm not entirely sure, however, that meme-as-gene analogy holds in the long term; the Crazy Frog is a surprisingly long lasting meme entity, but you can't argue that it's doing all it can to ensure its longterm preservation. Pet rocks, mood rings and 70s hairdos are all memes that have shown little longevity.

[HenrikOlsen]

And that's where the concept of meme-plexes come in, collections of memes which are transfered together.
Typical constituents of the historically very successful ones are "This set of ideas is a whole and may not be adopted in parts or modified in any way", "Pass this on" and "It's ok to kill those who aren't convinced".