The central dogma of Molecular Genetics is that information flow is unidirectional:  DNA to RNA to PROTEIN.  That is, DNA holds the blueprints, RNA is the messenger, and Proteins are the constructed functional units of life.

This dogma seems to hold for most of the species on the planet.  From bacteria to humans to insects, the central dogma acts as a unifying theory of life’s architecture.   But, there are a few key exceptions.

The first is that in some viruses, there is no DNA at all!  Instead, they used RNA exclusively for their coding.  Among these are the viruses that cause the common cold, flu, polio, and hepatitis.  In these, the flow of information is simply:  RNA to PROTEIN.

Another remarkable exception to the central dogma is that of retroviruses.  These break a big rule!  (Namely, that flow of information is unidirectional.)  A retrovirus is so called because it, like the above viruses begins with RNA, but it is able (by way of an enzyme called, appropriately, “reverse transcriptase”) to catalyze the formation of DNA from its RNA molecule.  The flow of information is then:  RNA to DNA to RNA to PROTEIN.   HIV is an example of a retrovirus.  It gets into the cell, creates new DNA that then incorporates itself into the hosts chromosome and gets nasty.

Why is this important to the discussion about evolution and life’s origins?  Because it is highly unlikely that life began with DNA.  DNA is far too complex, secure, and doesn’t have the enzymatic properties of RNA.  DNA is double stranded where RNA is single stranded.  This makes RNA more able to react chemically with other molecules in the environment.

In spite of how remarkably important it is to life on the planet today, DNA at one time didn’t exist.  But, that doesn’t mean life didn’t exist.  The first complex organisms may have been RNA carrying viruses. 

Could a retrovirus be responsible for creating the first DNA carrying organism?  I dunno, but it’s an interesting question.