On Wed, 13 Aug 2008 18:50:06 -0700, Timberwoof wrote:

This suggests that the formation of such a molecule is a very rare
event.

No, it suggests that once a particular chemical basis of life gets
established, another one won't.

That is the conclusion, if I recall correctly, of a recent Scientific 
American article about this subject.  There is an effort afoot to try to 
find some evidence of a different form of organic material still on Earth.