US scientists have discovered evidence to support the view that life may have first evolved based on RNA, rather than its better known genetic cousin DNA. RNA transcribes DNA's code, producing proteins. But in order for RNA to replicate, proteins called enzymes are needed to catalyse the formation of nucleotides, the building blocks of both RNA and DNA. This creates a paradox that researchers have puzzled over for years. Which came first, RNA or proteins? Now David Bartel and Peter Unrau from the Whitehead Institute for Biomedical Research, in Cambridge, Massachusetts have discovered a type of RNA that can synthesise its own building blocks without the help of proteins - creating the possibility that the earliest life-forms could have been based on self-replicating RNA before DNA had a chance to evolve.