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
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.