# life on earth **URL:** \ **Category:** Factual Questions\ **Created:** [March 25, 2013, 5:12pm UTC](https://boards.straightdope.com/t/life-on-earth/653932 "2013-03-25T17:12:42Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [March 26, 2013, 5:03am UTC](https://boards.straightdope.com/t/life-on-earth/653932/21 "2013-03-26T05:03:18Z")
> [@Chronos](#): > > You could probably make a molecule equally as complex as organic molecules, without any carbon in it, by using silicon for long backbones and nitrogen, phosphorus, etc. for smaller complicated structures attached to that backbone. But then you have to ask, why? Carbon is significantly more common in the Universe than silicon, nitrogen, or phosphorus, and it’s a lot simpler to make most of the molecule out of the same stuff rather than using three or four different elements to substitute for it. Good point. If you’re going with a helix-like structure, even if it’s a triple or quad-helix, meh, I guess we don’t really care too much about the architecture. My point is that it will subtly influence a host of things most of which we - and least of all me can imagine. For example, what will the epigenitic characteristics be? Genes are silenced or amplified by adding tags either to the transcription regions/factors (???) themselves or the areas on the backbone I think (would need to double check this) Adding such tags is going to have a different dynamic I imagine. Different binding properties for example, stability, different enzymes might have different affinities. And that’s just one example. > [@Leo\_Bloom](#): > > In “how it coils and uncoils … would be different here” do you mean that the energy expenditure in the physical bending and twisting of molecules as they go through their paces depends intimately on local gravity, or whatever Brownian motion going on at those levels (although I think Brownian motion on Jupiter would be the same as here, but I’m sure I’m wrong). > > If that’s not what you mean that’s what I thought of :). Is there any truth in the previous paragraph? Probably not although flexibility might be an issue and therefore the size of the nucleus required for a certain number of base pairs might be an issue. DNA is tightly wound around [histone](http://en.wikipedia.org/wiki/Histone) molecules in the nucleus when it’s not actively being used for transcription. And the wrapping of the helix around histone is a way of silencing a particular section - which can be controlled via various epigenetic means. So this very simple aspect of the structure can even have unforeseeable consequenses. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [March 26, 2013, 8:19am UTC](https://boards.straightdope.com/t/life-on-earth/653932/22 "2013-03-26T08:19:25Z")
If it’s not to be carbon, hydrogen, oxygen, then the question isn’t really ‘what elements instead?’, its a series of higher-level questions, such as: Are liquids/solvents necessary? Is complexity necessary? Are complex molecules necessary? (or can the necessary complexity arise from the complex arrangement of simple building blocks? - Lego style) If carbon, hydrogen and oxygen aren’t implicated, are they a hindrance? (given that they’re really common) Are other temperature ranges workable? (liquid methane, or molten NaCl, for example) ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [March 26, 2013, 11:49am UTC](https://boards.straightdope.com/t/life-on-earth/653932/23 "2013-03-26T11:49:19Z")
Let me point out that silicon based life forms already exist; I am using one to make this post. But they are all based on (more-or-less) intelligent design. The real question is, is there any possibility of them being created by evolution? ---
**Author:** ![Si\_Amigo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/si_amigo/32/2877_2.png) [@Si\_Amigo](https://boards.straightdope.com/u/Si_Amigo)\ **Post date:** [March 26, 2013, 12:11pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/24 "2013-03-26T12:11:02Z")
> [@deltasigma](#): > > OT, but is it just me that looks at the OP’s profile before responding to threads like this? Aside from the fact that it seems like a poorly thought out question, the person has one other, IMHO fairly [doubtful post](http://boards.straightdope.com/sdmb/showthread.php?t=686250) to make up their grand total of 2 since their incept date. > > Just sayin. Carry on. The fact that his user name is in all CAPS raised a flag for me; but that doesn’t take away from the question. He/she doesn’t appear to be trolling since there is no back and forth questioning from the OP on our response. Just a new kid trying out the board IMHO. ---
**Author:** ![Si\_Amigo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/si_amigo/32/2877_2.png) [@Si\_Amigo](https://boards.straightdope.com/u/Si_Amigo)\ **Post date:** [March 26, 2013, 12:13pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/25 "2013-03-26T12:13:09Z")
> [@Hari\_Seldon](#): > > Let me point out that silicon based life forms already exist; I am using one to make this post. But they are all based on (more-or-less) intelligent design. The real question is, is there any possibility of them being created by evolution? Your computer is alive? It is self aware and can reproduce? Where do I get one of these? :dubious: ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [March 26, 2013, 1:18pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/26 "2013-03-26T13:18:33Z")
> [@deltasigma](#): > > I’m not sure I see your point. Certain chemicals have certain valence characteristics that make them uniquely suitable for combining with a range of other elements. Carbon happens to be one of them, but there are others as well, Nitrogen for example can replace a carbon in a ring-like structure. No, obviously you don’t see my point. Perhaps you should go back and read my original post again, for comprehension, because “xeno nucleic acids” are in no way a counter example to anything I said there. I said any life would have to be based on the chemistry of carbon and (almost certainly) of water. I did not say, indeed I specifically denied, that it would have to be based on exactly the same sorts of organic (i.e., carbon based) molecules that “life as we know it” uses, i.e., DNA, RNA and proteins.\* Your “xeno nucleic acids” are simply an example of another type of organic macromolecule that might, conceivably, be able to play a role in some alien type of life similar to that which DNA or RNA play in life as we know it. (Whether they would really be up to job, we just don’t know, but they might.) Actually, they are really fairly minor variants on teh natural Earth nucleic acids, and I would not be at all surprised if alien life, if we find it, turns out to be based on types of macromolecules that are a lot more different than that. What I would be very surprised to find, however, would be alien life that is not based on the chemistry of carbon-based macromolecules in an aqueous environment. There is no other chemistry that will support the necessary levels of complexity. Carbon does not “happen to be one of them”. Carbon, to all intents and purposes, is THE one. Just because nitrogen can substitute for carbon in the structure of certain ring molecules (and, indeed, does so in many of the key molecules of Earth life) it does not follow that you that you can build large macromolecules out of nitrogen. You can’t. Nitrogen is actually a key element in “life as we know it”, but I think that it is conceivable that its role might be taken over by some other element in alien life forms. It is not plausible that the role of carbon could be taken over by some other element, certainly not by nitrogen. The only remotely plausible alternative is silicon, and there are good reasons to think that even silicon is not really up to the job. Silicon will form large polymers like carbon does,\*\* but there is no evidence that it is capable of supporting anything like the degree of structural complexity that organic, carbon based molecules can and do, and must be able able to do to sustain life. ¬¬¬¬¬¬¬¬ \*Actually, of these three types of macromolecule, proteins are by far the most complex and by far the most intimately constitutive of life. DNA, despite its fame in the public mind, is far less complex and far less important (with RNA somewhere in between). It is really mostly what your proteins do that makes you alive. The DNA is only there to make sure that the right proteins, with the right structures, get made. \*\* They are not structurally the same, however, despite the fact that both silicon and carbon are quadrivalent. Silicon polymers are _silicones_, consisting of alternating silicon and oxygen atoms. Carbon polymers largely consist of carbon atoms directly linked to one another. I do not know for sure if this difference is directly linked to silicon’s likely inability to be able to form really complex macromolecules, but I suspect it may be. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [March 26, 2013, 1:22pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/27 "2013-03-26T13:22:58Z")
> [@Hari\_Seldon](#): > > Let me point out that silicon based life forms already exist; I am using one to make this post. But they are all based on (more-or-less) intelligent design. The real question is, is there any possibility of them being created by evolution? If you think you rcompute is alive, you do not know the meaning of “life”. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [March 26, 2013, 2:42pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/28 "2013-03-26T14:42:40Z")
Which is? It was never made explicitly clear. ---
**Author:** ![Si\_Amigo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/si_amigo/32/2877_2.png) [@Si\_Amigo](https://boards.straightdope.com/u/Si_Amigo)\ **Post date:** [March 26, 2013, 2:49pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/29 "2013-03-26T14:49:06Z")
> [@Leo\_Bloom](#): > > Which is? It was never made explicitly clear. Since the OP is asking about life on other planets I assumed we would be using [NASA’s definitions of life](http://www.nasa.gov/vision/universe/starsgalaxies/life%27s_working_definition.html). ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [March 26, 2013, 3:04pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/30 "2013-03-26T15:04:43Z")
> [@Si\_Amigo](#): > > Since the OP is asking about life on other planets I assumed we would be using [NASA’s definitions of life](http://www.nasa.gov/vision/universe/starsgalaxies/life%27s_working_definition.html). Great cite. Thanks. Also, as you suggest, a handy way to settle things which everyone respects (NASA oughtta know). Sort of like the back of the box. Although,and obviously I’m a piker, I didn’t get why a solvent is a sine-qua-non. Without a solvent carbon is just a hunk of coal? ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [March 26, 2013, 4:10pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/31 "2013-03-26T16:10:04Z")
The solvent is medium for the necessary chemical reactions to take place. Of course, that doesn’t mean that a solvent is the only possible solution, just the most likely (given what we know from our one example). That Nasa link is a good one. My definition for life is “that which evolves itself.” Admittedly, that leaves out a lot of important characteristics, but I don’t know of any examples that meet my definition but that we wouldn’t call life. If there are any, please let me know! Also, yes, I realize I’m using an intransitive verb transitively. I’m coining a new usage. 🙂 No doubt there are other objections or it would be more common (it’s too simple/obvious to be original). BTW, some posts seem to assume that life has to have chemical origins. Chemistry is the only thing we know of that has the necessary properties, but there could be other forms. I used to toy with the idea that there could be intelligent weather patterns on Jupiter. The red spot is really just a big community (or big organism). Idle, sci-fi musings, of course. And mentioned above was the possibilty that there’s life inside stars. In either case, if there were and we found a way to observe it, we’d certainly learn a lot about the dynamics of and requirements for evolving systems. But I wonder whether we’d manage to observe it in the first place. Life could also occur on spatial scales or timescales that would make it nearly impossible for us to notice. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [March 26, 2013, 4:12pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/32 "2013-03-26T16:12:18Z")
> [@njtt](#): > > Also, carbon and silicon are the only elements that come close to be able form the sort of large complex macromolecules (especially nucleic aids and proteins) that are essential for life, and frankly, silicon won’t get you really very big complex molecules either, the way that carbon can and does. I am not sure that we can rule out the possibility of silicon based life altogether, but it really does not look very likely. It may well be that life does not have to rely on nucleic acids and proteins, that some other sort of carbon based macromolecules could play similar roles somewhere else in the universe, but it is near to a certainty that they would be carbon based molecules: carbon combined with other light, reactive, covalently bonding elements, which pretty much means the same set of elements that Earth life depends upon. It is also near enough to a certainty that in order to function so as to sustain life, those molecules would need to be in an aqueous (watery) environment. > [@](#): > > Carbon does not “happen to be one of them”. Carbon, to all intents and purposes, is THE one. Just because nitrogen can substitute for carbon in the structure of certain ring molecules (and, indeed, does so in many of the key molecules of Earth life) it does not follow that you that you can build large macromolecules out of nitrogen. You can’t. Nitrogen is actually a key element in “life as we know it”, but I think that it is conceivable that its role might be taken over by some other element in alien life forms. It is not plausible that the role of carbon could be taken over by some other element, certainly not by nitrogen. The only remotely plausible alternative is silicon, and there are good reasons to think that even silicon is not really up to the job. Silicon will form large polymers like carbon does,\*\* but there is no evidence that it is capable of supporting anything like the degree of structural complexity that organic, carbon based molecules can and do, and must be able able to do to sustain life. Carbon is not the only atom capable of forming complex molecules. You don’t need the ability to form four bonds, only three. Boron and Nitrogen can both do this. Indeed, look at the [Wikipedia Boron page](http://en.wikipedia.org/wiki/Boron#Chemistry_of_the_element): There’s a nice picture of a fairly complex molecule formed with Boron, Oxygen, Hydrogen, and no Carbon. Further, here’s a [PDF of a paper](http://pac.iupac.org/publications/pac/pdf/1991/pdf/6303x0345.pdf) from 1991 examining boron-nitrogen chemistry. Look at Figure 2. You’ve got a benzene-like ring, and variants with four or eight atoms forming the ring. Note that those are not complete molecules, but structures that can form parts of more complex molecules. Also, in the top middle is a Boron-Nitrogen unit that can form an arbitrarily long chain, with two free bonds per unit. It’s hard to imagine non-Carbon life ever forming, because Carbon is so prevalent, but that doesn’t mean it’s impossible. ---
**Author:** ![eltro102](https://avatars.discourse-cdn.com/v4/letter/e/7cd45c/32.png) [@eltro102](https://boards.straightdope.com/u/eltro102)\ **Post date:** [March 26, 2013, 7:18pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/33 "2013-03-26T19:18:22Z")
> [@Mangetout](#): > > If it’s not to be carbon, hydrogen, oxygen, then the question isn’t really ‘what elements instead?’, its a series of higher-level questions, such as: > > Are liquids/solvents necessary? > Is complexity necessary? > Are complex molecules necessary? (or can the necessary complexity arise from the complex arrangement of simple building blocks? - Lego style) > If carbon, hydrogen and oxygen aren’t implicated, are they a hindrance? (given that they’re really common) > Are other temperature ranges workable? (liquid methane, or molten NaCl, for example) all of these may be necessary or may not, we know they are for _life as we know it_ because if life arose without those requirements, it would be \*life as we don’t know it \* and very well unrecognizable to us - anything can be defined as life if you extend time scales, allow non-organic materials, generic non-biological processes etc, (eg cellular automatons) ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [March 26, 2013, 7:26pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/34 "2013-03-26T19:26:49Z")
> [@ZenBeam](#): > > Carbon is not the only atom capable of forming complex molecules. You don’t need the ability to form four bonds, only three. Boron and Nitrogen can both do this. Indeed, look at the [Wikipedia Boron page](http://en.wikipedia.org/wiki/Boron#Chemistry_of_the_element): There’s a nice picture of a fairly complex molecule formed with Boron, Oxygen, Hydrogen, and no Carbon. > > Further, here’s a [PDF of a paper](http://pac.iupac.org/publications/pac/pdf/1991/pdf/6303x0345.pdf) from 1991 examining boron-nitrogen chemistry. Look at Figure 2. You’ve got a benzene-like ring, and variants with four or eight atoms forming the ring. Note that those are not complete molecules, but structures that can form parts of more complex molecules. Also, in the top middle is a Boron-Nitrogen unit that can form an arbitrarily long chain, with two free bonds per unit. > > It’s hard to imagine non-Carbon life ever forming, because Carbon is so prevalent, but that doesn’t mean it’s impossible. “Fairly complex” molecules, with numbers of atoms in the teens like the ones shown there, are a _very_ long way from the biological macromolecules, which are stable structures with thousands or even millions of atoms in them. I dare say you could string a few boron based subunits together, but how many? How much structural variation could you introduce (whilst keeping everything stable)? ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [March 26, 2013, 7:31pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/35 "2013-03-26T19:31:11Z")
> [@eltro102](#): > > all of these may be necessary or may not, we know they are for _life as we know it_ because if life arose without those requirements, it would be \*life as we don’t know it \* and very well unrecognizable to us - anything can be defined as life if you extend time scales, allow non-organic materials, generic non-biological processes etc, (eg cellular automatons) As I said, the more ignorant you are of the relevant science, of both basic chemistry and of the actual chemistry of life, the more anything seems possible. (Actually this works for most things, not just alien life.) ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [March 26, 2013, 9:35pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/36 "2013-03-26T21:35:01Z")
> [@njtt](#): > > “Fairly complex” molecules, with numbers of atoms in the teens like the ones shown there, are a _very_ long way from the biological macromolecules, which are stable structures with thousands or even millions of atoms in them. I dare say you could string a few boron based subunits together, but how many? How much structural variation could you introduce (whilst keeping everything stable)? What makes you think there’s a limit? ---
**Author:** ![eltro102](https://avatars.discourse-cdn.com/v4/letter/e/7cd45c/32.png) [@eltro102](https://boards.straightdope.com/u/eltro102)\ **Post date:** [March 26, 2013, 10:47pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/37 "2013-03-26T22:47:29Z")
> [@njtt](#): > > As I said, the more ignorant you are of the relevant science, of both basic chemistry and of the actual chemistry of life, the more anything seems possible. (Actually this works for most things, not just alien life.) for _life as we know it_, yes we will need carbon, yes we will (most probably) need water what I meant with _life as we don’t know it_ is not life based on slightly different chemistries, I meant in a more life which we would not normally regard as life because our definition of life is very recursive, sorry if I wasn’t very clear. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [March 27, 2013, 6:16pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/38 "2013-03-27T18:16:04Z")
Our defininition of life isn’t recursive, it’s _fundamental._ If we encountered something we couldn’t possibly detect as being alive, by virtue of it’s complete non-resemblance to any kind of living organism, or the criteria by which we might describe such a thing, that would be because _it isn’t alive_. I realise we don’t know what we don’t know, but what sort of thing could you possibly be talking about? Can you describe a hypothetical encounter with a hypothetical undetectable-as-life-but-somehow-still-alive thing? ---
**Author:** ![Si\_Amigo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/si_amigo/32/2877_2.png) [@Si\_Amigo](https://boards.straightdope.com/u/Si_Amigo)\ **Post date:** [March 27, 2013, 6:23pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/39 "2013-03-27T18:23:58Z")
> [@Mangetout](#): > > I realise we don’t know what we don’t know, but what sort of thing could you possibly be talking about? Can you describe a hypothetical encounter with a hypothetical undetectable-as-life-but-somehow-still-alive thing? I am thinking that they (not I) may be considering God’s existence. Either that or some theory that somehow the Earth is alive and forms a symbiotic relationship with the flora and fauna that it supports. The Force be with you. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [March 27, 2013, 9:53pm UTC](https://boards.straightdope.com/t/life-on-earth/653932/40 "2013-03-27T21:53:50Z")
Yeah, I thought maybe something like that, but if it’s there, and alive, we should be able to detect it, or else maybe it’s not alive (or not there). [Previous page](https://boards.straightdope.com/t/life-on-earth/653932.md?page=1) [Next page](https://boards.straightdope.com/t/life-on-earth/653932.md?page=3)