Why do scientists think that RNA may have appeared on the 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,

http://publications.nigms.nih.gov/thenewgenetics/chapter2.html