Because it is present in the earliest cell that lived in earth
The main difference between DNA and RNA is that DNA contains double-stranded molecules and RNA contains single-stranded molecules. DNA stands for deoxyribonucleic acid and RNA stands for ribonucleic acid. Both DNA and RNA carry genetic information, but there are a few major differences between them. DNA consists of the sugar deoxyribose, and RNA contains the sugar ribose. Ribose contains one more -OH group than deoxyribose. Another difference is that DNA is stable under alkaline conditions while RNA is not stable.