Living systems owe their existence to a pair of information-carrying molecules: DNA and RNA. These fundamental chemical forms possess two features essential for life: they display heredity -- meaning they can encode and pass on genetic information, and they can adapt over time, through processes of Darwinian evolution. Given experts suspect RNA and DNA can't have appeared by chance, could earlier forms, like XNA, have led to their development?  http://www.sciencedaily.com/releases/2012/04/120419143117.htm