A key problem for the interpretation of the evolution of microbial ecosystems and physiologies in the Precambrian is that of the time of onset of an oxygenic atmosphere. This onset constrains the time of appearance of O2-producing photosynthesis as well as of the various structural and enzymatic protective devices enabling cells to function aerobically. It is suggested that evidence bearing on this matter can be derived by examining the distribution of carbon isotope values in samples obtained from rock formations of a given period. Prior to the appearance of free O2, in large, stable quantities, microbial diversity could have been greater than it was subsequently and this might be reflected in a wider variability in the carbon isotopic composition of organic carbon in sediments from the preoxygenic Earth. Examination of the available isotope data offers support for this picture. Whatever the ultimate cause, microbial variability appears to have diminished markedly by about 1.5 Ga and to have remained relatively low thereafter

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