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OXYGEN AND PROTEROZOIC EVOLUTION: AN UPDATE 21 1 Oxygen and Proterozoic Evolution: An Update Andrew H. Knoll And Heinrich D. Holland Harvard University Complex events can rarely be reconstructed from single lines of evidence, even where the record is well preserved. Preston Cloud (1983) ABSTRACT Many authors, most notably Preston Cloud, have argued that major events in early evolution were coupled to changes in the oxygen content of the Precambrian atmosphere. Interest has focused on events close to 2000 million years ago (Ma) and 600 Ma, when increases in PO2 are thought to have stimulated the radiations of aerobically respiring eubacteria and (via endosymbiosis) protists, and macroscopic metazoans, respectively. Acceptance of these hypotheses requires (1) geochemical evidence of environmental change; (2) paleontological evidence of coeval evolutionary innovation; and (3) physiological, ecological, and phylogenetic reasons for linking the two records. New data from Paleoproterozoic weathering profiles are providing increasingly quantitative constraints on the timing and magnitude of an early Proterozoic PO 2 increase. The emerging environmental history correlates well with the known phylogeny of protists and is consistent with the fossil record. Quantitative data on possible Neoproterozoic PO2 changes remain elusive, but new geochemical data strongly indicate that the interval just prior to the Ediacaran radiation was a time of marked environmental change. An increase in PO2 may well have accompanied this event, providing oxygen levels sufficient to support the metabolism of large heterotrophs. Geochemical, paleontological, and biological data support the hypothesis that atmospheric composition significantly constrained and at times provided important opportunities for early biological evolution. INTRODUCTION The coevolution of life and its environment has long been a principal theme in interpretations of Earth's early history. In particular, molecular oxygen has frequently been singled out as a major factor in Precambrian evolution, a view argued eloquently by Preston Cloud (1968a,b, 1972, 1983). Why this coevolutionary view should be so popular is clear enough. Although the present day atmosphere con