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SiriMurthy
2002-Feb-20, 08:42 PM
Is it true that we can only have Carbon based life forms and nothing else? What if ambient conditions on a different planet somewhere deep in space condusive enough for life forms other than Carbon based? How about Silicon based as we read in science fiction books?

Chip
2002-Feb-20, 09:15 PM
On 2002-02-20 15:42, SiriMurthy wrote:
Is it true that we can only have Carbon based life forms and nothing else?


Right here on Earth, the Horseshoe Crab has lysate blood (copper based) - this is quite rare. It's still a "carbon based" life form I think. /phpBB/images/smiles/icon_wink.gif

Tim Thompson
2002-Feb-20, 09:30 PM
The Horseshoe Crab is a carbon based life form, because it's basic biochemistry is the same as everything else on Earth, carbon, hydrogen, oxygen and nitrogen, often referred to in conglomeration as CHON (remember, we are carbon based, but the oxygen in our blood is carried by iron, not carbon).

I think that silicon based life forms are out of the question under any circumstances. The outer electron structure for carbon (C) and Silicon (Si) are the same, which means that they both have 4 loose electrons for binding. A naieve view of chemistry leads to the usual fiction about "silicon based" life forms, where you replace C in a chemical compound with Si. But it doesn't work.

The reason it doesn't work is that the smaller carbon atom has shorter, stronger bonds. It can double bond, so that CO2, for instance, is a closed configuration, which makes it a gas, because the C double bonds to the O. But if you try making SiO2 you only get single bonds, which means you still have two open electrons. SiO2 invariably becomes SiO4, which is a crystal lattice. Silicon chemistry is way different from carbon chemistry, and does not allow for the kind of organic chemistry that supports life as we know it. Instead, it makes sand (or computer ships).

Now, maybe some kind of "intelligent computer" could qualify as a "life form" (shades of Commander Data), but I'm not holdong my breathe waiting.

ljbrs
2002-Feb-21, 12:14 AM
There are life forms on Earth (in the deep ocean trenches) which use chemosynthesis rather than photosynthesis, so there are differences right here on Earth. However, I think that if there were other chemical possibilities, we should be able to see them evident here, as well. I believe that there are certain possibilities in nature, and if there were others, they would be evident here, as well as elsewhere.

ljbrs /phpBB/images/smiles/icon_smile.gif /phpBB/images/smiles/icon_rolleyes.gif /phpBB/images/smiles/icon_confused.gif

_________________
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<font size=-1>[ This Message was edited by: ljbrs on 2002-02-20 19:15 ]</font>

David Simmons
2002-Feb-21, 04:06 AM
On 2002-02-20 15:42, SiriMurthy wrote:
Is it true that we can only have Carbon based life forms and nothing else? What if ambient conditions on a different planet somewhere deep in space condusive enough for life forms other than Carbon based? How about Silicon based as we read in science fiction books?

Isaac Asimov's essay Planets Have An Air About Them contains information on other possible chemistry that furnishes enough energy for life with some degree of mobility.

Another Phobos
2002-Feb-22, 03:50 PM
On 2002-02-20 15:42, SiriMurthy wrote:
Is it true that we can only have Carbon based life forms and nothing else? What if ambient conditions on a different planet somewhere deep in space condusive enough for life forms other than Carbon based? How about Silicon based as we read in science fiction books?


My impression is that silicon-based life forms may be possible but carbon-based life has greater potential (more flexibility and variety in the types of molecules that be utilized). Of course, we have no scientific info about E.T. life (other than educated speculations), so we may be surprised if we ever do manage to find life elsewhere.

Azpod
2002-Feb-22, 07:43 PM
Now, maybe some kind of "intelligent computer" could qualify as a "life form" (shades of Commander Data), but I'm not holdong my breathe waiting.


Actually, based on where chips are going, it looks like chips based on carbon nanotubes will be replacing silicon wafer chips in the next 20 years or so. These same nanotubes are also being looked at to create microscopic structures that can be assembled to create light materials with a tensile strength many, many times that of steel. Since the primary application of androids probably would be to work in environments that humans cannot survive, they would likely be built of these materials. If we did manage to build an android in the next 50 years or so, it could be even more carbon-based than we are. /phpBB/images/smiles/icon_smile.gif

Wiley
2002-Feb-22, 08:03 PM
Actually, based on where chips are going, it looks like chips based on carbon nanotubes will be replacing silicon wafer chips in the next 20 years or so.


So silicon based life will have carbon based computers? /phpBB/images/smiles/icon_smile.gif

This chemical irony appeals to me.

lpetrich
2002-Feb-25, 07:21 AM
On 2002-02-20 19:14, ljbrs wrote:
There are life forms on Earth (in the deep ocean trenches) which use chemosynthesis rather than photosynthesis, so there are differences right here on Earth. ...


However, their biochemistry is still fundamentally the same as all other Earth organisms -- the difference is what their primary source of energy is. With most of the more accessible examples of Earth life, it's light. While with some deep-sea and deep-earth organisms it is certain chemical reactions, like

CO2 + 4H2 -> CH4 + 2H2O

for methanogens.

In fact, one remarkable result about Earth life is that all examples examined in detail have the same fundamental biochemistry, with only relatively minor variations. If a fully-functional microbe was discovered that lacked DNA and RNA, that would be big news. As would one with amino acids with asymmetry opposite that of all other Earth life -- or other big differences.

But such news has never happened -- DNA has been discovered in *every* cellular organism *ever* tested for the presence of some widespread gene or other. Otherwise, the gene probes would very likely fail -- it would be rather improbable for some alternative heredity molecule to make a close fit with DNA.

BADad
2002-Feb-25, 06:23 PM
To Jim Johnson: Your post of 02-20-02. I admit my chemistry is old, but isn't sand (stone, etc) made of SiO2? That's certainly a stable (almost unalterable) compound.

lpetrich
2002-Feb-25, 08:34 PM
SiriMurthy:
Is it true that we can only have Carbon based life forms and nothing else?


That does seem to be the case. One strong hint that silicon-based biochemistry is unlikely is by observing silicon-based counterparts of carbon-based prebiotic chemistry, such as volcanic activity. There is no evidence that lots of different distinct molecules form, as happens with carbon-based prebiotic chemistry. This carbon-based chemistry is observed not only in Urey-Miller experiments, but also in observations of comets, meteorites, and the interstellar medium. Instead, as lava cools, it forms lots of little crystals of various minerals, with no sign of frozen silicon molecules comparable to organic (carbon-backbone) molecules.

It might, however, be possible to produce silicon-based organisms by genetic engineering, but that will be very difficult. For starters, one needs some liquid medium in which silicon molecules can be dissolved; something like certain metal fluorides, perhaps. Such a medium would be necessary to avoid lack of molecule formation in metal silicates, but even that might not help very much.

pascoe213
2004-Apr-02, 08:00 PM
I am an amateur in this area just posting out of interest,

Quote by Tim Thompson-----
I think that silicon based life forms are out of the question under any circumstances. The outer electron structure for carbon (C) and Silicon (Si) are the same, which means that they both have 4 loose electrons for binding. A naieve view of chemistry leads to the usual fiction about "silicon based" life forms, where you replace C in a chemical compound with Si. But it doesn't work.

The reason it doesn't work is that the smaller carbon atom has shorter, stronger bonds. It can double bond, so that CO2, for instance, is a closed configuration, which makes it a gas, because the C double bonds to the O. But if you try making SiO2 you only get single bonds, which means you still have two open electrons. SiO2 invariably becomes SiO4, which is a crystal lattice. Silicon chemistry is way different from carbon chemistry, and does not allow for the kind of organic chemistry that supports life as we know it. Instead, it makes sand (or computer ships).
End Quote-----

I am no expert but silicon based self replicating chains could exist they are just a lot less likely to form naturally, due to extra complexity(extra electrons/netrons/protons).

Another quote-----
Now, maybe some kind of "intelligent computer" could qualify as a "life form" (shades of Commander Data), but I'm not holdong my breathe waiting.
End quote-------

The old question of the difference between a computer that can solve problems we can and us solving such problems. Isnt the human mind just an extremely complex machine. Then there is the issue that carbon life forms these self replicating chains, from what ive heard while they havn't created such robots self replicating silicon nanobots have been modelled mathmatically and may be created within the next few of decades(supposedly).

That silicon based lifeforms can exist ties in neatly with another theory that of the heat death of the universe(entropy) where the universe will eventually be all the same temperature with no difference between one part of the universe and another. Further, when the universe makes a choice(or choices in the case of multiple futures, or even multiple pasts) it always chooses the future(s) that lead to the shortest time to heatdeath(Entropy).

Some scientists believe that carbon life forms came into being as they spread across a planet and are good at radiating heat, meaning that they lead towards a shorter amount of time until the heat death of the universe.

So, possibbly, silicon life forms are better at radiating heat than carbon lifeforms and the only reason that carbon lifeforms formed rather than silicon lifeforms is that silicon lifeforms are more complex and less likely to form due to there extra mass/ extra electrons/protons/neutrons as mentioned earlier.

A theory that would support this is that if the universe is really searching for heat death and carbon life does radiate heat better than a lifeless planet then when carbon life happended to form the universe would want its continued existance. What then is the reason for pollution and huge amounts of carbon life being wiped out? The only answer I can see to this, is by looking at the timeline of pollution/large scale environmental damage. When did humans begin to have a significant effect on the environment? Around the time that we developed metal tools. When did environmental damage sky-rocket? around the time when we began to use electronics and mechanical devices( The equivalent of primitive lifeforms, although not self replicating... yet) The only conclusion I can see, the universe has realised we are close to achieving silicon based lifeforms and is adjusting the earth to an environment suitable for silicon based life.

a side note/question if the heat death of the universe is where every part of the universe is the same and nothing changes from one moment to the next does this mean there is no more time as time is the realationship or difference between one moment and the next.

Another perspective on this topic is that if self replicating chains(life) are a precursor to intelligence, then it is interesting to note that all attempts at artificial intelligence, so far, have been based on tree searches and we have since discovered that neurons in part of the human brain involved in problem solving are formed in chandilier(tree like) structures, suggesting that in our attempt to build silicon based intelligence we are in fact proving the existance of silicon based life forms.

Please note I am a comp sience student with little technical knowledge in this field, plus i have had a fair bit to drink. I am only posting out of personal interest and look forward to my theorys being blown away.

TriangleMan
2004-Apr-02, 09:45 PM
I am no expert but silicon based self replicating chains could exist they are just a lot less likely to form naturally, due to extra complexity(extra electrons/netrons/protons).
Welcome to the Board pascoe213. :)

I found a website (http://www.uncp.edu/home/mcclurem/ptable/silicon/si.htm) on silicon which should be of interest. Silicon has problems catenating and gets unstable once you get past Si4H8. I don't think it can form other useful organic structures such as rings (C6H6 type structures) nor can it triple bond. All of this severly limits its ability to form simple life and makes complex silicon-based lifeforms (in my opinion) impossible.

However, I think silicon can be used interspersed in carbon chains so it might be possible to have lifeforms that are part carbon and part silicon, with the majority of the structure being carbon. That could be cool.

cyswxman
2004-Apr-02, 10:07 PM
Silicon-based life? Of course, don't you remember the Horta? :wink:

Brady Yoon
2004-Apr-02, 11:03 PM
Non-carbon life forms may be possible-they even may be the rule. But since the only life we know in the universe is on the Earth, and life on Earth is carbon based, we must assume that other forms of life will also.

mike alexander
2004-Apr-03, 12:55 AM
Silicon is not a good choice for a basis of living things for a few reasons. It does not catenate well; The silanes (the structural analog of alkanes in carbon) cannot form chains of indefinite length (up to about 6) as carbon can. The main reason is that the average C-C bond has an energy of about 83 kcal/mole while the average Si-Si bond is about 42 kcal/mole.

In addition, they are unstable in a wide variety of conditions (spontaneously burn in air, spontaneously hydrolize in even the the most mildly basic water).

Silicon does not form p(pi) multiple bonds, so there are no analogs to carbon-based alkenes, imides or ketones.

Not a good choice.

TriangleMan
2004-Apr-03, 03:42 PM
Non-carbon life forms may be possible-they even may be the rule. But since the only life we know in the universe is on the Earth, and life on Earth is carbon based, we must assume that other forms of life will also.
Actually Brady many scientists assume that life will be carbon based due to the chemistry involved. Carbon can catenate and form long chains, multiple-bonds and cyclic rings - other elements cannot. Silicon can to a minor extent which is why some have conjectured about Silicon life but as explained in previous posts it does not catenate well enough and is too unstable for life. Carbon is assumed to be the normal building block for life because we know of no other element that can take its place.

ToSeek
2004-Apr-15, 04:44 PM
The Bricks of Life (http://www.space.com/searchforlife/seti_carbon_040415.html), by astronomer Seth Shostak


And by the way, if silicon is a distant second in the biology sweepstakes, the elements under it in the periodic table – germanium, tin, and lead – are worse. They’re less abundant, and less inclined to make biologically interesting compounds. The sole known example of tin-based life occurred in The Wizard of Oz, and it suffered from lack of lubricant.

pteranodon
2004-Apr-16, 01:24 PM
At least silicon based based life forms are not compatible with an atmosphere with a high ammount of oxygen. Here on Earth there is abundant silicon, but the oxygen does not allow it to form lengthy chains as we can see with carbon, otherwise we would not see so much sand on the beaches and ocean floor.

mike alexander
2004-Apr-16, 04:16 PM
In my advanced inorganic chem course my prof would repeatedly say "Silicon is not just heavy carbon!"

It's not just a coincidence that earth life uses the elements up there in the right half of the second row of the periodic table (C, O, N, plus H), with carbon occupying the central role. The combination of high bond strength with the ability to hybridize orbitals to produce tetrahedral, trigonal or linear bonds is absolutely unique. By the time you move down to the third row bond energies have dropped and hybridization schemes have changed. Silicon cannot form specific -Si-Si-Si- chains or rings of indefinite length as carbon can, even under controlled lab conditions, let alone all those C=O, C-NH- and other functional groups that carbon does so nicely.

If macromolecules are a prerequsite for life it is hard to see how carbon will not be involved. Not that it wouldn't be a rip of the first order to see!

JohnOwens
2004-Apr-16, 07:51 PM
It's not just a coincidence that earth life uses the elements up there in the right half of the second row of the periodic table (C, O, N, plus H), with carbon occupying the central role.
Don't forget the role of their relative abundance as well, due to the (presumed) means of formation of the elements heavier than H. After all, there just isn't as much P around as N, nor as much S as O. I don't know offhand how much C there is, but I know there's lots more Si, but if Si doesn't work and is all bound up tightly in SiO2, it won't be used anyway.