Editing Common descent (section)

===RNA world===
{{main|RNA world}}
The possibility is mentioned, above, that all living organisms may be descended from an original single-celled organism with a [[DNA]] [[genome]], and that this implies a single origin for life.  Although such a universal common ancestor may have existed, such a complex entity is unlikely to have arisen spontaneously from non-life and thus a cell with a DNA genome cannot reasonably be regarded as the origin of life.  To understand the origin of life, it has been proposed that DNA based cellular life descended from relatively simple pre-cellular self-replicating [[RNA]] molecules able to undergo [[natural selection]].   During the course of evolution, this RNA world was replaced by the evolutionary emergence of the DNA world. A world of independently self-replicating RNA genomes apparently no longer exists (RNA viruses are dependent on host cells with DNA genomes).  Because the RNA world is apparently gone, it is not clear how scientific evidence could be brought to bear on the question of whether there was a single origin of life event from which all life descended.