I first heard this subject when I was in high school: hypothetical life based on silicon instead of carbon. But just how likely is it there is such life somewhere in the universe? Also, does anyone know what such life might look like? Would it be similiar to us or much different? And finally, could such life be intelligent like us?
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Silicon chemistry is very different from cabon chemistry, though, despite their continguity on the periodic table. There seem to be a lot more carbon compounds than silicon, and they’re in mre convenient form at our temperature and pressure.
Take, for instance, carbon dioxide, which, along with water, is a common product of oxidation of organic compounds. It is, conveniently, a gas. I don’t think that you have analogous oxidation products for silicon compounds and, worse, silicon dioxide is a solid.
In his story “A Martian Odyssey”, written back in the 1930s, Stanley G. Weinbaum posits a silicon-based creature who “exhaled” silicon dioxide blocks. It stacked them in pyramids around itself, until it was completerly covered, then it broke out and moved over to begin the pile again. Doesn’t sound practical.
I think the silicon based life bit is based upon merely looking on the periodic table under carbon. While carbon and silicon share many properties, carbon chemistry is far more diverse and complex. It’s just like in Evolution where David Duchovney’s character looks at the periodic table on Julianne Moore’s shirt and declares that since arsenic is ‘our poison’ and the life forms are nitrogen based (something I find more likely than silicon) that that ‘aliens’ poison’ is selenium.
Silicon-based creatures do exist, though, and they’re among the oldest species on Earth. The radiolarians are protozoans that live in the oceans and have a skeleton-like structure made of silicon dioxide. They likely never got beyond a few millimeters in size because silicon dioxide makes a pretty inefficient skeleton. They were out-competed by animals with calcium-based skeletons, which were able to grow much larger, support a wider variety of organs, and eventually leave the oceans entirely.
There are several other single cell organisms that use silica skeletons, including diatoms. But this is using inorganic silica as a structural element, not replacing carbon compounds with silicon compounds. The trouble is that silicon is much less reactive than carbon. I think you’d have to have pretty unusual conditions for silicon to form the long chains and polymers that make carbon chemistry the basis for life on Earth. Of course, you could claim that Earth has some pretty unusual conditions too…liquid water exposed to the atmosphere. We just have sample size of one.
Isaac Asimov, PhD biochemist, wrote Planets Have An Air About Them in which he explored possible forms of life. His thesis was that there is a minimum energy requirement in order to move around to get food and to reproduce. He analyzed several bases for the chemistry of life that would meet the energy requirements. Sulphur was one, carbon of course and I don’t remember the others, if any. I don’t think silicon was in the running.
Asimov also touched on the subject in various essays in The Relativity of Wrong and The Left Hand of the Electron, including discussions of what he called “thalassogens”, chemicals that could serve as a medium for life. He concluded that water was by far the best and most likely, because of a number of interesting properties, including expanding while freezing.
His nonfiction was actually a lot more interesting than his fiction, what with all those noble robots and whatnot.
Agreed, that’s the big problem. Voet and Voet’s Biochemistry, Second Edition has the best treatment of this subject that I’ve seen:
The same section of that text also goes into the problems with boron- and nitrogen-based life, for anyone who’s interested:
The part of this argument that’s missing from this text: the one example of life we have (by which I mean all life on Earth, since we’re all related and thus utilize the same general chemistry) requires complex molecules such as DNA and proteins, that require the ability to form long chains. I guess it’s possible that there could be silicon-based life somewhere that doesn’t have anything exactly equivalent to DNA, but if you don’t need long chains you might as well hypothesize iron-based life; both iron-based and silicon-based life seem highly, highly unlikely, but we only have the one example of life to base our hypotheses on.
Of course, I don’t see any reason not to throw some complete speculation in here :), so…
It might be possible for carbon-based life to evolve into carbon-and-silicon-based life, with the long-chain, complex molecules like DNA, RNA, and proteins (or their alien equivalents) linked to a silicate backbone (similar to the methyl silicones in the Voet and Voet passage). I don’t have any numbers handy to back it up, but it seems like maybe such an arrangement would be able to remain stable at higher temperatures than our standard. In fact, it’s possible that life here on Earth started with something similar to such a system (if I understand the clay-catalyzed RNA formation hypotheses correctly).
I believe scientists recently found some organisms living in the salt pan of a California desert that have a really weird biochemistry. They use arsenic to generate energy in much the same way that most organisms on earth use oxygen to obtain food. I doubt that it’s carbon-based, but it definitely shows there are alternate biochemistries out there:
I meant to say I doubt that it’s NOT carbon-based. Still, it’s got to be a different kind of chemistry – as I recall, humans generate energy by "burning Trisomethingorother down to Disomethingorother,’ combining it with oxygen and using the energy generated by the reaction. I’m wondering if M-13 uses arsenic on the same chemicals, or does it have a whole 'nother set of chemicals?