Potted history of atmospheric oxygen
The most likely hallmark of an inhabited planet is an atmosphere that contains oxygen; a simple rule of thumb made popular by James Lovelock. By assembling complex molecules based on carbon, life increases the degree of chemical reduction in its environment. Effectively it draws in electrons, and the counterpart of that must be that some other component loses them through oxidation. On Earth the source of electrons needed to make organic molecules through the action of photosynthesis is predominantly the oxygen atoms locked in molecules of water and carbon dioxide. By losing 4 electrons, 2 oxygens bonded in those two simple compounds are oxidised to become the gas O2, which itself has become the commonest and most active acceptor of electrons from reduced ions and compounds. Oxygen gives its name to oxidation, which is the inevitable fate of most organisms, thereby reversing the process of photosynthesis. A planet whose surface topography is continually changing, because more radioactive energy is produced in its mantle than can be lost to space by simple conduction, generates physical conditions that continually bury and store some unoxidised carbon compounds. Carbon burial together with continued living processes keeps the photosynthetic chemical equation weighted in favour of free oxygen.
Since the domain of living things to which we and all advanced organisms belong, the Eukarya, is almost wholly one to which oxygen is vital in metabolism, there can be few more important geoscientific topics than how and when oxygen emerged as a free element. There have been major recent developments in addressing these questions, so it is useful and fascinating to find an up-to-date and easily read review (Kerr, R.A. 2005. The story of O2. Science, v. 308, p. 1730-1732). Among its highlights is evidence that although cyanobacteria (the most primitive oxygenic photosynthesisers) were definitely around at 2.7 Ga, they may not have produced oxygen until about 300 Ma later, when the first signs of free environmental oxygen appear. Photosynthetic release of oxygen during life’s early period was not the only reduction-oxidation regime adopted by organisms. Another of huge importance was generation of methane, which can rise to the limits of the atmosphere unlike the other major hydrogen-bearing gas, water, which is condensed out at quite low altitudes. Photochemical breakdown of methane at the limits of outer space would release hydrogen to leak away from the Earth, removing a reductant gas that would otherwise consume highly reactive oxygen: without this process, modelling suggests that Earth’s atmosphere would never have accumulated free oxygen, even had primitive life emerged.
Once free oxygen appeared, about 2.4 Ga ago, it took almost 2 billion years for enough to accumulate so that complicated, multicelled Eukarya could use its potential (see The Malnourished Earth hypothesis – evolutionary stasis in the mid-Proterozoic in EPN of September 2002). What kept the levels down? Quite probably it was oxidation of sulfide minerals on exposed land. That supplied sulfate ions to a still reducing ocean, so that sulfide ions formed again to become metal sulfide precipitates, which drew from ocean water several essential nutrients for Eukarya. Oxygen-producing Eukarya (algae) would not be able to bloom because of this ‘starvation’. Nonetheless, about 600 Ma ago, surface oxidation potential soared to almost modern levels, sufficient for large organisms to appear and evolve, to lead to life as we know it. Another series of questions surrounds this tremendous event, but they remain to be answered convincingly.