Most of the complex carbon compounds that exist on Earth were produced
by such organized, living chemical systems.The free energy necessary
to drive these nonspontaneous reactions "uphill" is obtained from
other carbon-based molecules in the environment. Most of this energy
results from the destruction of carbon-containing molecules and
the combination of their atoms with oxygen. These organized entities
maintain low entropy within their own boundaries by creating even
more entropy outside.
The ultimate source of free energy to drive all these chemical systems
is the sun. Living chemical systems use a minute fraction of the
solar radiation (less than one part in a billion is intercepted
by the Earth) to maintain a temporary state of high order in localized
regions of space on the surfaces of our planet. The activities necessary
for these systems to maintain themselves, to obtain free energy,
and to avoid destruction, all have the appearance of being purposeful.
Some of these localized chemical units seem more purposeful and
more adaptive to changing conditions than others. No matter what
the chemistry of our hypothetical outside observer might be, he
would undoubtedly recognize such carbon-based chemical systems as
To turn matters around and observe the observer, could such an alien
visitor exist with a chemistry based on other than carbon atoms?
The answer probably is no. We cannot give a meaningful answer to
this question until we know more about the behaviour of carbon compounds
and of living systems.