Turning Back the Clock: What Modern Life Tells us About the First Life
RNA Orchestrates Information Flow
The typical information flow in living organism is DNA to RNA to protein. There is also DNA replication (i.e. DNA to DNA), which requires RNA to get the process started. Some special cases also exist; for example, viral enzymes that can copy RNA into DNA or RNA into more RNA. It is important to note that protein is never copied back into RNA or DNA; this is impossible for all known organisms. Proteins also cannot synthesize other proteins from scratch; they can only modify other proteins.
Proteins catalyze almost all chemical reactions in a cell so it is easy to think of them as the puppet masters. When we stop and think about how information flows in a living system, a different picture takes shape. All of the information transfers require RNA, whether it is the catalytic RNA in the ribosome to make proteins, the small RNA primers need for DNA replication, or the mRNA intermediate that codes for proteins. It all comes down to RNA: the real puppet master.
What Does this Tell us About Ancient Organisms?
RNA is the most fundamental polymer for life, all other polymers are secondary. The fact that RNA is responsible for the manufacture of DNA and protein, as well as informational transfer between the two, suggests that life used RNA first, evolving the ability to make DNA and protein later. This idea is the RNA World Hypothesis.