Viroids are strands of RNA that can perform chemical reactions on their own and act as pathogens for crop plants. Some scientists are proposing that these viroids may be the first life forms that evolved on Earth, predating even the development of proteins and DNA.
http://www.nytimes.com/2014/09/25/s...y-from-the-ancient-past.html?ref=science&_r=0
http://www.nytimes.com/2014/09/25/s...y-from-the-ancient-past.html?ref=science&_r=0