Why RNA is considered as the first genetic material?

The discovery of ribozymes in the 1980's led to the "RNA world" hypothesis. In the RNA world (a term coined by Nobel laureate Walter Gilbert), RNA handled the tasks now managed by DNA and proteins- storing genetic information and catalyzing biochemical reactions. One of the limitations of this idea has been the lack of a plausible route to the creation of two base chemicals that make up genetic material--cytosine and uracil (which occurs in RNA only). This led to the suggestion that alternative bases might have been used as raw material for the first genetic material.