It is generally believed that there was a time when life on earth was based entirely on RNA. The DNA/protein life form that is common to all known terrestrial biology is thought to have descended from a simpler life form that utilized RNA as both the genetic material and the instrument of catalytic function. Very little is known about this postulated RNA-based life form except what can be inferred by examining the role of RNA in contemporary organisms and by studying the behavior of RNA in the laboratory. The lack of first-hand knowledge of the RNA world has not prevented speculation concerning the chronology of events that led to its appearance and ultimate extinction. Nor has it prevented speculation concerning the degree of metabolic complexity that existed during this early period of biological evolution.

Here I consider two issues that may help to set boundary conditions to aid in speculation about the RNA world. First, I will attempt to fix a time frame for the existence of the RNA world. The dawn of the RNA world required the establishment of environmental conditions that are compatible with the solution chemistry of RNA; the end of the RNA world is demarcated by the invention of DNA genomes and/or the development of a translation apparatus. Considering the relevant evidence from geophysics, geology, paleobiology, and molecular biology, it is possible to fix the time frame of the RNA world to a 400 million year interval centered around the beginning of the early Archean (3.9 billion years ago). Second, I will examine the range of catalytic functions that are essential to the establishment and maintenance of an RNA -based evolving system. These functions constitute the minimum level of biochemical complexity that existed in the RNA world.