RNA is evolutionary older than DNA. Thymine is a methylated form of Uracil it gives a more stabilized structure of DNA, which is one of the main differences between RNA and DNA. RNA is an easily degredable molecule which needs to be produced continuously while DNA is a resistant molecule to degrade in the long term and thymine provides this protection of DNA from the nucleus of the cell and prevents mismatches in the replication. However, deamination of cytosine also gives uracil.