The glass apparatus used in Miller's experiment. The lower flask was designed to simulate the oceans; the upper flask, the atmosphere. Energy was supplied by sparking between the two wire electrodes. (Scripps Institution of Oceanography)
The building blocks of life, no matter what their origin are imagined by most scientists to have been swimming around in some sort of oil primordial soup! What consistutes this soup was an obviously steadily more and more complex molecule that, depending on which school of thought you listen to, this soup may be either thin or thick. All agree however that the concentrations had to be relatively high in order to get anywhere.
The composition of this soup has been guessed at using many models, the most famous of which is the Stanley Miller experiment, entitled "A Production of Amino Acids Under Possible Primitive Earth Conditions". These experiments show a remarkably diverse range of organic species ranging from sugars, amino acids and bases not to mention fatty acids and esters of various types.
However, one of the downfalls of these early experiments is that they produce such a wide variety of well everything!
Roughly speaking only 20 amino acids are used through out life on Earth and 6 nucleic acids but many more than this appear in the primordial soup and life did not just simply use the most abundant molecules or easiest to synthesise! There is however obvious reasons to why these where selected and the others where not, most of the other bases will either only complimentarily pair with themselves or form pairs with either too strong or too weak bonds which would render replication impossible.
There is a similar story with the amino acids, the amino acids used in living organisms are the ones most likely to form polymers instead of multimers and they are also the ones to have the most diversity in reactions and properties. Using anymore and you would be doubling up on properties and worse perhaps using one that was detrimental to the others.