How life originated on earth and not on other nearby planets? Why life chosen
our climatic conditions?
We do not know if life developed on other planets or not. That is why we are checking out Mars right now. We don’t know what we don’t know; something that qualifies as life may exist on the moons of Jupiter or even on one of the gas giants for all we know. It may not remotely resemble us, but may be none the less be alive.
The biggest factor for life as we know it is the presence of liquid water; Mars and maybe even Venus had liquid water at one time but Earth has sustained it for the time needed for life to evolve, at least more complex life (there is evidence for liquid water on some other plants/moons, like Europa, but it would appear to be much harder for life to develop than on Earth).
The other planets had instain mothers who kill their babbies.
I’ve always found the “liquid water” criterion to be somewhat parochial. Life on earth needs liquid water; why should life elsewhere conform to that measure? “Life as we know it” might be very small, compared to “life as we don’t know it.”
If we stick to what seems possible based on what we know about chemistry, I believe that carbon and silicon are the only two elements that can form the basis of molecules of sufficient complexity and diversity to make life possible. Someone who knows more than me can probably explain why carbon-based life forms pretty much require liquid water. As far as any hypothetical silicon-based life forms, I have no idea how much work has been done on predicting the kind of conditions that would be necessary.
A doctor (M.D.) told me once that carbon bonds can “flex” a bit, so that proteins and other complex carbon molecules can fold up in ways which the stiffer silicon bonds don’t permit.
(No better cite than that, alas…)
because it’s generally thought that we’ll need life to be pretty similar to the ones we have already seen for us to understand it - i.e. :
The majority of ‘life’ which exists is completely alien to us, that’s why we want to find life as we know it instead of life as we don’t, which we cannot recognize
True. But it’s hard to imagine another equivalent medium that’s so useful for the creation of life. You want something that will allow complex molecules to move around in it and have reactions to each other. Solids and gases aren’t going to work. And other than water, what common liquids are there? I guess liquid ammonia would be the closest equivalent.
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 to make up their grand total of 2 since their incept date.
Just sayin. Carry on.
Water really has very unusual and unique chemical properties. There is no other known substance quite like it, and life as we know it certainly depends on those properties in many very fundamental ways. It is far, far more than the mere fact that water is a liquid. The shape and the unique electrochemistry of the water molecule are also crucial. Furthermore, if there were any other relatively chemically simple substance anything like it then we would know about them. There are fewer than 90 reasonably stable chemical elements and we know their chemical properties and the simpler compounds that they form quite well. (Also, most of the elements, for good astrophysical reasons, are very rare in the universe, whereas water and living things are made up of almost entirely of relatively common elements like hydrogen, oxygen, carbon and nitrogen.) I suppose it is just about conceivable that some complex chemical compound might have properties relevantly similar to those of water, but no such complex substance is going to exist prebiotically anywhere in the universe in large enough quantities for life to originate in it, or for it to sustain an ecosystem. Anyway, it is inevitably going to be a large molecule, and you really do need a small one, like water, to play water’s role in sustaining a biochemistry.
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.
People who say things like “we just don’t know; it is just science being parochial and unimaginative to say life anywhere must depend on similar basic chemistry to that on Earth” are, frankly, just showing their ignorance of chemistry, and of what life is and how it arises from complex biochemical processes. The possibilities are very far from being limitless. In fact, they are tightly constrained by basi chemistry, which in turn is constrained by basic physics. Although there is probably much room for variation at the level of macromolecular structure, at the basic chemical level, the level of what elements and what simple, common compounds are essentially involved, there are really very few, if any, viable alternatives at all. Alien biochemistries are almost certainly possible (actually, I would say that, given the huge size of the universe, they almost certainly exist) but it is also very close to being certain that they will all be based upon the properties of carbon and of water. (Luckily, carbon, oxygen and hydrogen are amongst the commonest elements in the universe.)
Hmmm. Really? So what do you think of xeno nucleic acids?
I think that they are entirely consistent with what I said: they are macromolecules based on the chemistry of carbon. I explicitly allowed for the fact that alien life might be based upon organic macromolecules different from the nucleic acids and proteins with which we are familiar. These are still organic (i.e., carbon based) molecules, and they will still function (if they really do function) in an aqueous medium.
If there really were life based upon the chemistry of such compounds (something that has in no way been demonstrated to actually be possible, by the way, but I do not say it is imposible) its existence would not in any way contradict the claims in my previous post.
Xeno nucleic acids are regular old organic chemicals composed of good old carbon, hydrogen, nitrogen, and oxygen, with a tiny dusting of phosphorus. All life on Earth uses the exact same biochemical building blocks of DNA and RNA, and this biochemical machinery is highly, highly conservative. But there’s no particular reason an alien biochemistry couldn’t use very different molecules. But these xenonucleic acids are actually extremely similar to regular old DNA and RNA. Like, really really similar. So alien life that used these molecules would be alien life, but still “life as we know it”.
Hydrogen, helium, carbon, nitrogen and oxygen are the most common elements in the universe. Helium is not chemically reactive. So any life that isn’t composed of CHON would have to be composed of very rare materials in the universe. It’s completely plausible to imagine an alien biochemistry that substitutes chlorine or fluorine for oxygen, this was a staple of science fiction in the 1950s. The problem is that chlorine is about 100 times less common in the universe than oxygen, and there’s no real plausible physical mechanism that would concentrate chlorine on a planet and exclude oxygen. So an alien that breathes chlorine gas and drinks carbon tetrachloride and pisses hydrochloric acid is chemically possible, but where does the chlorine on his planet come from? The earth’s crust is mostly oxygen by weight, oxygen is everywhere.
Since carbon and nitrogen are also very common, it’s plausible to imagine life using ammonia or methane or ethane as a solvent. Another possibility might be carbon dioxide, but carbon dioxide doesn’t have a liquid phase, so that’s not a good candidate. Ammonia or methane or ethane would require lower temperatures…so a place like Titan is a possible candidate for life. But the problem is that photosynthesis wouldn’t work very well on Titan, it gets much less energy from the Sun than Earth does. So life on Titan would have to rely on chemosynthesis. Which means the biosphere would have less available energy by several orders of magnitude. Which means that complex life is probably impossible. So simple organisms similar to chemosynthetic bacteria might exist, albeit with an exotic biochemistry. But they’d still be composed of the familiar elements of organic chemistry.
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.
The comment I was mainly addressing therefore was this: " at the basic chemical level, the level of what elements and what simple, common compounds are essentially involved, there are really very few, if any, viable alternatives at all."
Well, the backbone structure of the molecule that transmits our genetic information and allows us to evolve and change our genetic code is pretty damned basic and with XNA’s you have an example of something very alien from both DNA and RNA doing that.
Now even I’ll admit that’s not really mind bending. You’re still using a 3 nucleotide sequence to code a protein and sure you can pop in some different nucleotide pairs for new proteins, but meh, that really goes to higher level functions.
Even so, I don’t really have a great imagination and even I can imagine life forms that might go far beyond this so maybe I just missing your point.
I’m sorry but that’s a bit presumptuous. A lot of how DNA works is bound up in it’s structure - how it coils and uncoils - which will be different here, how it is read, which is difference here as evinced by the fact that new enzymes had to be created, the number and type of proteins that can be encoded. There are countless subtleties that would shape evolution based on such structures in ways that it would be impossible to predict.
Yes, you are absolutely correct. It is completely plausible to me that an alien biosphere would have a completely alien biochemistry, using completely different methods of encoding genetic information, different types of proteins or maybe no proteins at all, different energy storage molecules, different photosynthetic pathways, different everything.
But what you’re not understanding is that this would still be familiar organic chemistry, with carbon, oxygen, nitrogen and hydrogen. Not boron, beryllium, xenon and tungsten. Yes, you can stick a nitrogen or oxygen in place of a carbon atom in a ring. But you can’t make a complex molecule without carbon, because only carbon (and silicon) can create four bonds. There’s a reason that carbon chemistry is called “organic chemistry”. You can make all sorts of molecules do all sorts of things, but you can’t create millions of different complex chemicals without carbon. It’s not a matter of lack of imagination, it’s a simple matter of the physics of the chemical bond.
I don’t think you understand why the chemistry of carbon is different than the chemistry of all other elements. But the point is, a completely and utterly alien biochemistry that uses none of our familiar biochemical building blocks of protein, lipid, carbohydrate, or nucleic acids would still be organic chemistry based on the unique physical properties of the carbon atom, and very likely the unique physical properties of the water molecule.
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.
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?
No, I get that. I’m pretty light in the area of chemistry but working on it, but I get it - valance electrons, binding energy, other buzz words I don’t really understand . . . so just let me strike a Captain Morgan pose for a moment here. 
My point is, that’s a constraint Ms. bitchy pants universe places on us. ‘Oh, you’d like another light, tetravalent non-metal would you? Well when I start seeing some of those sweet ass multiple universes your people promised me, get back to me. Til then, piss off!’
Personally, I would consider something radically different if it it used compounds other than proteins (even if many are intrinsically disordered and therefore somewhat, if not largely amorphous) and simple enzymatic reactions for so many processes.
Go a head and scrape up what you need from the periodic table’s meager junk yard and find a way to create movement, all the way from the cell to multicellular forms using the transfer of coherent energy. It’s possible. Encode structural components in autonomous nodes that only assemble in the presence of a competing intermixing chemical gradient inside of an induced EM field.
Then make the sturctural components from repeating units like the beta sheets and alpha helices of our proteins, but so that they assemble a 3d topology not based on random hydrophobic/philic, VanderWaals, and other forces, but based on whatever would work like enzymantic stablilization, like methylation of phosphorylation (hard to describe). the point is that components could be build to an evolutionary specification without the trial and error of finding just the right beta sheet with just the right side chains to get just the right topology.
Now THAT would be different and they would rule our crusty homey sapien asses in a few thousand generations.