Carbon dioxide, ammonia and the origin of life

Abstract

Stellar evolution theory predicts that the luminosity of the Sun has increased by ∼30% over the past 4,000 Myr. Yet geological and biological evidence indicates that the climate of the Earth between 3,000 and 4,000 Myr ago was as warm as, or warmer than, today. This apparent contradiction, the ‘faint Sun paradox’, has been resolved by invoking the greenhouse effect of radiatively active gases in the early Earth atmosphere. Sagan and Mullen¹ first suggested that the concentration of ammonia in the early atmosphere was around 10–100 p.p.m., sufficiently high to counteract the reduced luminosity. However, because ammonia photodissociates readily and has a short atmospheric residence time^2,3, such a concentration could be maintained only by a large continuous ammonia source. For this reason, carbon dioxide is now considered to have been the radiatively active gas^4–7. Some atmospheric ammonia is, nevertheless, required to provide conditions conducive to the origin of life⁸. We now show that, if the early Earth's atmosphere contained high concentrations of CO₂, as suggested above, then the chemical conditions required for life to begin can be maintained by very low ammonia partial pressures, rather similar to those observed today.