The rise and fall of the RNA world.

It is generally believed that there was a time when life on earth was based on RNA rather than on DNA and protein. Considering the relevant evidence from geophysics, geology, paleobiology, and molecular biology, it is possible to set the time frame for the existence of RNA-based life to a 400 million year interval beginning 4.0 to 4.2 billion years ago and ending 3.6 to 3.8 billion years ago. The minimum level of biochemical complexity that existed during this time consists of those functions necessary for the establishment and maintenance of an RNA-based evolving system, namely, an RNA unwinding activity, an RNA replicase activity, and a primitive biosynthetic apparatus leading to enrichment of the local environment with activated D mononucleotides.