It has become rather widely accepted in recent years that (1) during the geologic past, the Earth's atmosphere evolved from an initial “oxygen deficient” to a later “oxygen-rich” state; that (2) this change was a result chiefly of the cumulative effects of O2-producing “green plant-type” (including cyanobacterial) photosynthesis; and that (3) the transition occurred during the Precambrian, with stable oxygenic conditions having probably become established during the Early Proterozoic (viz., 2.5 to 1.7 Ga). Lines of evidence in support of these suppositions have been drawn from paleobiology, geology, mineralogy, isotopic and organic geochemistry, and comparative planetology – data well familiar to many paleontologists (for detailed discussion including a reference list of some 1,700 entries, see articles in Schopf, 1983). In addition, however, and although it is perhaps not widely recognized, it is of interest to note that the occurrence of such a transition seems also reflected by the nature and structure of the metabolic and biosynthetic pathways of extant living systems. To a paleontologist, this observation should not be surprising. Just as comparative studies of the embryology and structural adult anatomy of extant organisms can provide reliable evidence of phylogenetic relationships, comparative studies of biochemistry –in this case, of evolutionarily conservative, widespread, biochemical pathways – can similarly provide evidence of evolutionary derivation. And just as the structural morphology of currently living systems is a product of, having been influenced by, environments inhabited by evolutionary precursors (e.g., the limb structure of both aquatic and flying mammals reflecting derivation from lineages earlier adapted to the land environment), metabolic and biosynthetic pathways, especially required pathways involved in processes or producing products fundamental to survival, can be expected to provide evidence of past environmental conditions.