All modern organisms have DNA as a store of genetic information, RNA as a message and Proteins as their major cellular catalyst. This shows that the organism from which all cellular life began had these properties. This is called 'LUCA' meaning last universal common ancestor. This is very complicated model and when we think of the origins of life we ask how did it all start? Where did DNA come from as Proteins are involved in its replication? But DNA codes for Proteins? What came first Polynucleotides or Polypeptides?
The RNA world is the believed hypothesis in modern genetics, that answers these questions. RNA led to Proteins which led to DNA.
Evidence supporting the RNA world
RNA can behave as both a store of genetic information and as a cellular catalyst.
RNA is capable of replicating itself without the use of enzymes.
It is more likely to suggest that a single molecule was capable of replicating itself rather than two different molecules being synthesised by random chemical reactions in the same place at the same time.
Ribosomes use RNA to catalyse pepide bond formation of amino acids in DNA.