Re: Is carbon-based life the only possibility?

Good question! The answer is that we only have one example to go by and 
it seems to be based - predominantly - on carbon, oxygen, hydrogen, and 
nitrogen. With a data set of "1", it is hard to come to any definitive 
conclusions!

However, let us explore what is necessary. For example, if the message 
attached to your question, you point out that carbon "cannot perform as 
well at temperatures below -100 C or above about 200 degree C". This 
isn't quite correct. Carbon doesn't care what temperature it is. 
Molecules, such as methane, are methane from close to absolute zero until 
they decompose at some relatively high temperature. The existence of 
carbon dioxide is virtually temperature invariant. Carbon based compounds 
are fine at any temperature.

But then why don't we find lots of organisms in really cold or really hot 
places? It isn't the compounds that matter (although there is some 
sensitivity to elevated temperatures) but the chemistry that matters. 
That is, life is more than just carbon compounds. It is carbon compounds 
interacting. It is metabolism. Oil, for example, comes from living things 
but it is not "living" because it is not engaged in a myriad of chemical 
reactions that result in the constant flow and ebb of compounds 
throughout the system. 

So, this is really what life is all about - chemical reactions. We are a 
complex web of chemical compounds reacting in a chaotic manifold of 
kinetic rate equations. Not maybe the prettiest picture of a person but 
certainly one way of thinking about us or any living organism.

Why use carbon? Well, for one thing, it is extremely abundant. All of the 
atoms that compose the planet earth were born in the nuclear furnace of a 
distant star that burnt out billions of years ago. The star exploded and 
spread its ashes throughout the region and they eventually coalesced to 
give you and me.

But abundance isn't the only reason. Carbon has some rather unique 
properties. First up, it is capable of forming four bonds and adopting a 
tetrahedral geometry. This allows for, among other things, chirality - 
that is, left and right handed molecules. These are critical for life as 
we know it. Second, carbon undergoes catanation better than any other 
element. It is quite happy bonding to itself in long, long chains. 
Consider that the average molecule of polyethylene involves a 100,000 
carbon atoms all linked together and you can appreciate carbon's ability. 
It is not the only element that is capable of doing this. Silicon and the 
rest of the group four elements can also form chains. But carbon is the 
champion. Third, carbon does not have energetically accessible d-
orbitals. That is, the chemistry of carbon is limited to the s- and p-
orbitals which means that it does not form anything more than a 
tetrahedral structure or a structure containing four bonds very easily. 

The consequence of this last point is often lost in discussing the 
chemistry of life but it is, in many ways, the most important of all. 
Carbon does not like to have more than or less than four bonds to it. 
This means that it is hard to oxidize. That may not seem to be the case 
when you are gathered around the yuletide fire, but it is. And it is 
important because most carbon compounds are thermodynamically unstable. 
This is why you can sit around the yuletide fire. With enough activation 
energy, pretty much any and every carbon compound will combust to give 
carbon dioxide. It is kinetics that saves us from spontaneous combustion 
(articles in the National Inquirer aside!) and the kinetics are inhibited 
because carbon is a second row element with only a limited ability to 
interact with other compounds.

What this means for other elements is that silicon, for example, is too 
reactive to sustain long term and controlled chemical reactions. The 
likelihood of life being based on silicon (outside of the concept of 
artificial life based on the machine model) are slim. Silanes - the 
equivalent of the carbon based alkanes - are spontaneously combustable in 
the presence of oxygen (or react rapidly with nitrogen or the halides). 
They would not work as they would not provide a stable base. Other 
elements, such as nitrogen or oxygen, have unfilled valencies and would 
also be too reactive. Ditto with boron. Of the second row, carbon is the 
only one that makes kinetic sense - the only one that reacts at rates 
that would allow a metabolism to develop. The same is true of the group 
four elements.

So, no, I don't think that there is another element that could replace 
carbon as the "element of life" - although it is carbon chemistry that is 
important and not the atom per se. But having said that, I refer you back 
to the first paragraph. With a data set consisting of "one" result - life 
on the planet Earth - it is hard to come to a definite conclusion. Given 
the composition and chemistry of the Universe, though, it is likely a 
correct conclusion.

Hope this helps!