I've never before seen such a comprehensive and well-thought-out speculation on potential alien biochemistries. I only wish I had enough of a chemistry background to properly absorb it!

Chemistry was of little use in my undergraduate astronomy, where most matter is either an ionized plasma, an extremely tenuous and very cold gas, or a lump of ice, slag or cosmic dust floating in hard vacuum and bathed by extreme radiation and intense magnetic fields. I only took one year of General Chemistry at the college level, and I forgot most of that a lifetime ago. Chemistry is the science of complexity, the chemical bond is nature's way of putting together truly elaborate structures. And complexity is just another dimension, along with space and time, along which matter organizes itself.

Having said that, I'm still a water and carbon chauvinist, although I can see other possible biochemistries can't be ruled out, and some good people have given them a lot of thought. Still, I suspect water/carbon life is probably the most likely to be found in space, its precursors have already been detected in meteorites, the interstellar medium, and in the reconstructed chemical history of the primitive earth. Of course, there are all sorts of really exotic possibilities even if we restrict ourselves to C/H20--and it seems that that arose on earth early in its history, suggesting it is probably pretty common elsewhere.

The situation may be very different if advanced alien intelligences create their own life, using exotic chemistries mentioned in the article or even others we can't imagine. Its highly possible that some forms of truly bizarre biochemistries are possible, but unlikely to arise and evolve naturally. They could be the artifacts of highly sophisticated civilizations with advanced technologies utilizing alternative biochem as weapons, data distribution or storage systems, computing machinery, construction material or heaven knows what else. That's what the two fictional examples I mentioned are all about. The possibilities are endless; maybe silicon life can't evolve naturally, but perhaps someone out there can grow it in the lab. After all, we can build thinking machines now, and soon we may be able to construct artificial intelligences that can rival our own, but those robots would not be likely to arise as a result of "natural" evolutionary processes--whatever the hell THAT means.

Remember in high school when the religious kids used to tell us; "If you find a watch (a mechanical timepiece) in the woods, its not likely it evolved naturally through natural selection. It had to have been built by somebody." This logic is supposed to lead us to the conclusion that if we find a complex artifact in nature it must have been built by God.

Of course, it doesn't. It only leads us to believe it was built by somebody, not necessarily a supernatural being. Complexity seems to be able to arise spontaneously in nature, it does not require a conscious Creator (although that does not mean that hypothesis can be ruled out). Watches, after all, serve a purpose. They were built for a specific reason, to tell time. Most of the complexity we see in nature seems to serve no purpose at all. It just is. As ER used to say, "There is a reason for everything, but a purpose to nothing."

And that, to me, is a much more wonderful and meaningful mystery than the idea that it was put together by a supernatural being of infinite power and wisdom. Maybe it just arose naturally for no good reason at all. And when complex "natural" systems evolved to the point where they were able to mobilize matter and energy in space and time, they learned to create. God did not create living, thinking beings, it was the other way round.