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contradictory geochemical signals for the rise of oxygen. Silt-sized rounded grains of pyrite and uraninite, which are quickly destroyed by the faintest whiff of oxygen, are associated with the first pulses of sedimentary manganese, which normally requires molecular oxygen. This overlap interval (inside the magnifying glass) may be the hint of a manganese-precipitating photosynthetic bacterium, which would be an important evolutionary stepping-stone on the path to oxygenic photosynthesis. (Diagram courtesy of Woodward Fischer, Caltech)

    So what could the resolution of this paradox be? We think that the oxygen-emitting system of cyanobacteria had not evolved at this time (2.4 billion years ago), but that many of the evolutionary steps needed to get there had already been taken. It turns out that the actual biochemical complex in oxygen-releasing photosynthesis that collects the energy to split water, releasing oxygen, relies on a cluster of four manganese atoms, with a calcium atom thrown in for good luck. When this protein is made from scratch in living plants, the manganese atoms are sucked into the complex, one at a time, with the aid of photons that oxidize them. We suggested that these unique bursts of manganese in the sediments (not timid whiffs) might be the product of an evolutionary ancestor of the cyanobacteria that fed on reduced manganese dissolved in the water, using it as a source of the electrons needed to do photosynthesis. 8 Many primitive photosynthetic bacteria are known to do this with H 2 S, organic carbon, and ferrous iron, but none have yet been found that can use manganese. Photosynthesis of this sort would leave copious amounts of a waste product—manganese oxide—behind in the sediment, but would not release the molecular O 2 that would destroy the sedimentary pyrite or uraninite, or create an ozone screen to change sulfur chemistry. This overlap interval where sedimentary manganese exists with rounded, detrital grains of the minerals pyrite and uraninite are present happens in one—and only one—brief interval of geological time, between ~2.4 and ~2.35 billion years ago. 9 If that is indeed the time that this protein evolved, all of the other indirect suggestions for earlier oxygenic photosynthesis must be wrong. This is a new and controversial interpretation we are posing here. But we are confident it is the correct one.
    In our model, this manganese-oxidizing microbe, then new to the world through some random new mutation in all probability, dominatedthe ecosystem for a few million years until it managed to deplete the surface waters of soluble manganese. Through some biochemical rearrangement, this tiny new kind of microbe became capable of grabbing electrons directly from water molecules, releasing copious quantities of O 2 in the process. That would have been the first true cyanobacterium. Because water is basically everywhere, its growth would no longer be limited by the supply of electron donors in the environment. Only trace levels of iron and phosphate are needed for it to grow. But during this interval of time, there are clear records of glacial deposits, and those deposits contain plenty of iron, phosphate, and other nutrients for these new cyanobacteria to grow on. In fact, this glacially fertilized growth would be capable of destroying the planetary greenhouse in less than a million years by removing two important gases—CO 2 and methane—too rapidly for the system to recover. 10 The result of the sudden destruction of the greenhouse would be a global glaciation, termed a “snowball Earth” event.
    We apologize for the complex chemistry necessary in the preceding section. But to get this story right requires complexity. As we see now, the world was unalterably changed from this point onward.
A SNOWBALL FROM HELL
    In all of Earth history, we have rarely seen ocean stratification (where the ocean has a thin upper layer that is oxygenated, but a much thicker layer underneath that is