Takahiko: There is considerable literature now on alternatives to RNA
(and DNA) as genetic materials
very early in the evolution of life. It is thought that an "RNA World"
preceded the current RNA +DNA
+Protein biosphere, but since RNA itself is a rather complex molecule,
something simpler must have
preceded it. Also, ribose (and deoxyribose) are unstable under presumed
primitive earth conditions
as well as being difficult to make prebiotically. These sugars are also
For a number of reasons, 4 carbon sugar phosphates and peptide
backbones have been considered for
this hypothetical pre-RNA. Much of this literature has appeared in
"Origins of Life and Evolution of the Biospher," Nature, Science, J.
Molecular Evolution, Nucleic Acids Research, and P.N.A.S.
In addition to be very hard to recognize as Mary Kuhner has pointed out,
organisms whose genetic systems
were not based on DNA or RNA might have trouble competing with
terrestrial life (I think Darwin himself
made a similar argument for why new types of life are not appearing on
the earth today).