Establishing the Synthetic Contingencies for Life: Following the Carbon from AGB Stars to Planet Surfaces

Many paradoxes currently exist in the biochemistry of living systems.
For example, why are there only twenty amino acids used in proteins?
Why does Terran genetics only use the sugar ribose in DNA? Life on Earth
probably did not have time to sample all the chemical possibilities;
therefore, modern biochemistry likely reflects what starting materials
happened to be present on Earth when life began, i.e. a question of
``synthetic contingency."  What established such contingencies?
The past 35 years of radio and millimeter astronomy have demonstrated
that a tremendous concentration of organic molecules exist in dense clouds
found throughout our Galaxy. This fact in itself is yet another paradox,
given that in interstellar gas, oxygen is more abundant than carbon.
The creation of large quantities of carbon-carbon bonds likely begins
in circumstellar envelopes of C-rich AGB stars. Recent observations
indicate that carbon-containing circumstellar molecules survive into
the planetary nebula stage and then into diffuse clouds. Dense clouds
condense out of diffuse gas, and hence, their organic chemistry may be
``seeded" by carbon fragments of AGB envelopes, thereby ``jump-starting"
complex molecule formation. Comets and carbonaceous chondrites contain
large amounts of organic material, which probably have an interstellar
origin, and may be important vehicles for bringing carbon to planet surfaces.
Thus, the ``synthetic contingencies" may have been established by
interstellar and circumstellar chemistry.