The RNA World Hypothesis
The RNA world hypothesis is the theory positing that the current DNA to RNA to protein type life evolved from simpler, RNA based organisms. Some important features shared by all modern organisms are strongly indicative of descent from RNA-only life forms.
The RNA world as the origin of life?
RNA has been suggested as the polymer that initiated life on Earth. It has been demonstrated experimentally that RNA of random sequence can be polymerized spontaneously from nucleotides on surfaces of clay. The possibility of a functional ribozyme, capable of replicating, emerging from the random sequences seems improbable; however, the sheer number of different RNA sequences generated makes this actually a fairly likely outcome. Because RNA is both informational and catalytic, there is a direct link between mutations in RNA sequence and catalytic ability. A replicating RNA sequence would reproduce and diversify rapidly, incorporating new ribozyme function (like nucleotide synthesis and energy capture) quickly. This network of ribozymes would behave similar to the network of enzymes that drive all modern life forms. Eventually the network of ribozymes would become enveloped in a membrane becoming a protocell. These protocells evolved into the huge diversity of organisms on Earth.
The RNA world as a later form of life?
Some scientists think that RNA is too complex to be the polymer that initiated life. They instead propose that a pre-RNA world came first. Candidates for a pre-RNA polymer are TNA (threose nucleic acid) that is similar to RNA except with the ribose sugar replace with threose or PNA (peptide nucleic acid) that has bases attached to a peptide backbone.
Criticism of the pre-RNA world
Initially pre-RNA was suggested because it is more stable than RNA at high temperature; however, more recent data suggests that the Earth was colder than initially reported (at least at the poles). None of these pre-RNA molecules have ever been found in nature. Further, it is difficult to see how remnants of an RNA world could be left in modern organisms while absolutely no trace of a pre-RNA world survived.
If RNA organisms came first then why does life now use DNA, RNA and proteins?
DNA is a more stable molecule for storing information and proteins make better, faster catalysts than RNA. While we think RNA came first, we also know that RNA organisms that evolved to use proteins and/or DNA would have had an advantage over those using only RNA. The purely RNA organisms were driven to extinction by lifeforms using RNA along with proteins and/or DNA.