Nonchemical Life?

The argument about carbon being the likely chemical basis for life smuggles in an assumption: that life requires chemistry. Actually, life appears to require an energy source and organizational principles, and that is supplied by chemical reactions on Earth. But we cannot exclude the possibility that somewhere in the Universe some more exotic processes also qualify as life or even intelligent life. Such possibilities are beloved of science fiction writers, and ideas abound in that genre of entities that evolve into intelligent beings without benefit of chemistry as we know it.

Is Carbon-Based Life Unique?

One question that is important, but to which we don't have a solid answer, is whether life can evolve from a chemistry other than that of carbon.
The Complexity of Carbon Chemistry
In the chemistry that we understand, carbon is unique because of its atomic properties. Only carbon is capable of producing the complex long-chain compounds such as DNA and RNA that are the chemical basis of life. Thus when the Viking landers searched unsuccessfully for life on the surface of Mars, the implicit assumption of the experiments was that life would signal its presence through carbon-based chemistry (see this animated introduction to the Viking search for life on Mars).

Other Chemistries?
Though it seems highly unlikely based our present understanding of chemistry, we cannot exclude the possibility of compounds based on other elements forming the basis for life somewhere else under different conditions (see the discussion of silicon in the right panel). But, as we have already stated, being highly unlikely is not necessarily a strong argument when there are uncounted billions of chances, and it is always dangerous to generalize from what we know now to what could be possible under any circumstance. However, we may be reasonably certain that if life not based on carbon chemistry exists, it must involve a chemistry presently unknown to us that is at least as complex as that of carbon.