1. Introduction

The chemistry of Earth’s early atmosphere was an essential factor in the production of prebiotic molecules and ultimately the origin of life. Chemical processing of atmospheric gases led to various simple CHNO compounds that could react after raining out into the primordial ocean to produce a mixture of precursors that could then combine into yet larger and more complex molecules, including polymers. Various models for the formation of Earth’s earliest atmosphere have been proposed, and the current conventional wisdom of a CO2 -N2 rich atmosphere has held sway for more than fifty years. Several aspects of the nature of the early atmosphere and how it evolved through time are widely accepted, but several elements of geologically-based information have yet to be satisfactorily explained. Close consideration of the timing of primordial degassing, the nature of the degassed material, constraints imposed by Earth’s early climate and geologic deposits, and the requirement of sufficient organic precursors to make the emergence of life reasonably probable indicate that Earth’s early surface was very likely richer in reduced carbon compounds than would be likely based on the conventional wisdom of the early atmosphere. In particular, a possible mechanism for the emergence of life suggests a vast organic-rich prebiotic soup is a rather straightforward route to achieve the transition from non-living to living. The evolution of the atmosphere following the emergence and diversification of life (especially the development of oxygenic photosynthesis) is in reasonable accord with the geologic record. Recasting Earth’s atmospheric evolution in this way, when applied to Mars, can also explain certain aspects of the history and current state of that planet’s surface.