One of my correspondents wrote to ask "What makes carbon so special?" That is, how come all the life we know about is based on it?

There are several qualities that we organic chemists (and living beings) admire about carbon. But before counting the ways, let me start by saying that I'm talking about "life as we know it." As that Steven Benner review article that I spoke about a few weeks ago makes clear, you can imagine chemical domains at other temperatures and pressures that could support life of another kind.

But for the only life we've found so far, the Earthly kind, the temperature and pressure space is roughly bounded by the territory of liquid water. Higher pressures will let it stay liquid up to higher temperatures, and we have organisms that will ride right along with them. Likewise, high ionic strength will let you keep a liquid matrix down to much lower temperatures, and we have that covered here on Earth, too.

Inside this range, carbon has a lot of advantages. It forms very stable bonds to itself, first of all. Forming and breaking them (under controlled conditions!) is one of the major challenges of organic synthesis. Carbon atoms can be strung out to give you virtually any size molecule you want; there seems to be no upper limit and there's no reason to expect one. This is important, because a likely requirement for any kind of chemical-based life is large molecules with structural diversity. Life's bound to be complex, and carbon compounds give you all the complexity you can handle - straight and branched chains, rings, whatever you want.

And those bonds come in more than one flavor. While carbon-carbon single bonds form a 3-D tetrahedral lattice (found in its pure form in diamond), double bonded carbons can all flatten out into the same plane. The best natural example is graphite, made up of flat sheets of tiled carbon rings, full of alternating double and single bonds. The sliding motion of those sheets over each other gives pencil lead its properties. And there are triple-bonded carbons, too, which end up in a straight line. Carbon gives you a wonderful 1D / 2D / 3D building set.

There's another key thing about the element. More structural (and reactive) diversity comes from all the ways that carbon can form bonds with other elements. Oxygen, sulfur, nitrogen, phosphorus and many other elements readily form carbon derivatives under Earthly conditions, and these give you the crazy variety of organic chemistry. We've got solids, liquids, and gases, acids and bases of all strengths, nonpolar compounds and polar ones fitted with all kinds of electron-rich and electron-poor zones, and reactivity all the way from rock-solid to burst-into-flames.

I think that it's much more likely that we'll find life that uses different carbon-based compounds than it is that we find life based on siloxanes or some other framework. Organic chemistry is too useful to avoid. Now, organic chemists are another matter entirely. . .