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The beginnings of biospheric evolution had far-reaching biogeochemical consequences for the related evolutions of atmosphere, hydrosphere, and lithosphere. Feedback to the sedimentary record from these several simultaneously interacting aspects of crustal evolution provides the evidence from which historical biogeology is reconstructed. The interpretation of that evidence, however, is beset with pitfalls. Both biogenicity and a primary origin need to be demonstrated, or confidence limits established for each supposed morphological and biochemical fossil. Relevance to biospheric or related evolutions must be critically evaluated for every geochemical and sedimentological anomaly. Indirect evidence suggests primitive, oxygen-generating autotrophy by ∼ 3.8 × 10 9 years ago (3.8 Gyr or gigayears), while free O 2 first began to accumulate only ∼ 2 Gyr ago. Various reduced substances in the atmosphere and in solution functioned as oxygen sinks, keeping photolytic and biogenic O 2 at levels tolerable by primitive anaerobic and microaerophilic procaryotes. The oldest demonstrably biogenic and certainly primary microstructures are procaryotes from ∼ or > 2 Gyr old strata around Lake Superior. Improved biologic O 2 mediation, continued carbon segregation, and filling of O 2 sinks initiated atmospheric O 2 buildup, leading to an ozone screen ∼ or 0.7 Gyr ago and probably > 1.3 Gyr ago, perhaps simultaneously with the mitosing cell. Whatever the timing, it completed the evolution of the eucaryotic heredity mechanism and foreshadowed (given sufficient free O 2 ) the differentiation of tissues, organs, and advanced forms of life—with all their potential for biogeochemical feedback to sedimentary, diagenetic, and metallogenic processes. The first Metazoa appeared ∼ 0.7 Gyr ago. Being dependent on simple diffusion for O 2 , they lacked exoskeletons. The latter appeared, perhaps 0.6 Gyr ago, when increasing O 2 levels favored the emergence of more advanced respiratory systems.
Preston E. Cloud (Thu,) studied this question.