Geological evidence often presented in favor of an early anoxic atmosphere is both contentious and ambiguous. The features that should be present in the geologic record had there been such an atmosphere seem to be missing. Many of the features advanced in support of an anoxic model can be ascribed to diagenetic alterations, and most diagenetic environments are reducing. Recent biological and interplanetary studies seem to favor an early oxidized atmosphere rich in CO2 and possibly containing free molecular oxygen. The existence of early red beds, sea and groundwater sulphate, oxidized terrestrial and sea-floor weathering crusts, and the distribution of ferric iron in sedimentary rocks are geological observations and inferences compatible with the biological and planetary predictions. It is suggested that from the time of the earliest dated rocks at 3.7 b.y. ago, Earth had an oxygenic atmosphere.

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