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Abstract

The compositional evolution of Earth's early atmosphere reflected a complex interplay between the planetary processes of accretion and differentiation, massive impact events, and voluminous magmatism. Once it reached clement conditions, trace gases were introduced by the developing biosphere. In turn, the atmosphere acted as a primary reaction chamber for the evolving biogeochemical cycles, ocean chemistry, and climatic and biotic evolution. Reconstructing the concentration of individual gases in past stages of atmospheric evolution must thus be tightly coupled to an understanding of all these issues and especially the states of biological and environmental evolution. Currently, the most widely accepted characteristics of the early Archean atmosphere are a very low concentration of molecular oxygen, a higher than present day concentration of carbon dioxide and a probable prominent role for methane. Paleobiological indicators - macrofossils, microfossils and chemical fossils - that yield direct or indirect insights into the existence of atmospheric gases during the Archean and early Proterozoic are reviewed here, and they are presented in an integrated account of life's early evolution.