Why do scientists think that RNA may have appeared on earth before DNA?

Today, many scientists believe that RNA evolved on the Earth long before DNA did. Researchers hypothesize—obviously, no one was around to write this down—that RNA was a major participant in the chemical reactions that ultimately spawned the first signs of life on the planet. Check this out,

Not only RNA has catalytic activity as pointed above, it also has the ability to control gene regulation (known as riboswitch). Taken together, it strongly suggests that RNA has a strong probability of being first.