Although the known fossil record of cellularly preserved microbes extends deep into the Precambrian—throughout all of the Proterozoic and much of the Archean—in units older than ~2,000 Ma

it becomes increasingly sparse and patchy, and the history of the various microbial lineages becomes increasingly difficult to PD0325901 cost decipher. The great oxidation event Despite the problems posed by the petering-out of the rock and fossil records over geological time, the record that has survived is sufficient to establish the presence of molecular oxygen and, by implication, of oxygen-producing photoautotrophs, at least as early as ~2,450 Ma ago. As summarized by Holland (2002) and Canfield (2005), beginning about 2,200 Ma ago and continuing to the present, sandstones known as red beds have been deposited on land surfaces by meandering rivers and windblown dust. The beds are colored red by the presence of the mineral hematite (Fe2O3), iron oxide that typically forms

a thin veneer on individual quartz sand gains and the presence of which indicates that the atmosphere at the time was oxidizing. In contrast, in numerous terrains older than about 2,400 Ma, conglomeratic buy CHIR-99021 rocks GNE-0877 occur that contain detrital grains of pyrite and uraninite deposited in shallow-water deltaic settings, minerals that in the presence of molecular oxygen

are rapidly converted to their oxidized forms—for pyrite (FeS2), to the mineral hematite (Fe2O3); and for uraninite (UO2), to its soluble more oxidized form, UO4. If there had been appreciable oxygen in the overlying atmosphere when these sediments were laid down, hematite, rather than pyrite, would occur in such conglomerates and uraninite would have oxidized and been dissolved. The temporal distributions of red beds and of pyritic uraniferous conglomerates thus indicate that there was an increase in the amount of oxygen in Earth’s atmosphere some 2,200–2,400 Ma ago, a date that has recently been more firmly set by studies of sulfur isotopic ratios preserved in the rock record that evidence a major rise in atmospheric O2-content at ~2,450 Ma ago (Farquhar et al. 2000, 2007). Since photosynthesis produces well over 99% of the oxygen in the atmosphere, and since no other large-scale source of free oxygen is known, this increase of atmospheric O2 can be firmly attributed to the activities of oxygenic photosynthesizers.