The only element that has a sufficiently varied chemistry to build such systems appears to be carbon, and life as we know it is restricted to conditions under which large carbon compounds exist in a suitable balance between stability and reactivity.

The size of living organisms ultimately is tied to the lengths of chemical bonds, and the dimensions of the logical networks that can be built from molecules. Organisms smaller than bacteria are too small to display all of the properties of life. Organisms of planetary size are too large, and would suffer from severe problems of transfer of information and transport of molecules, both internally and to and from the outside world. Even in our primitive computer technology of today, we have built machines in which the limitations on computation time are the times required for electrons to flow through connecting wires from one component to another. We have evaded this difficulty by micro-miniaturization, but we still are far from achieving the level of compactness represented by the human brain. That paragon of miniaturization has 12 billion cells, each connected to at least 100 others, all within one cubic foot of folded cerebral cortex.

The study of the most complex of all chemical phenomena, living organisms, must begin with the study of the compounds of carbon. This is the subject of the next three chapters.