Ribonucleic acid's use of uracil instead of thymine probably developed from a genetic mutation in mRNA and from recognizing the enzyme's utility, according to the National Science Teachers Association. In its early stages, the RNA polymerase recognized uracil, and as the RNA's cells emerged and evolved, the polymerase continued usage.
It is also possible that during early cell development, the preference of RNA for uracil evolved to avoid competition with deoxyribonucleic acid polymerases for thymine, notes the National Science Teachers Association. An examination of RNA synthesis may show that it is more efficient when using uracil than it would be if it used thymine. DNA, which has greater stability than RNA, is able to use thymine efficiently.