The Story of O
A Subversive Element
Diatomic O2 apparently was not abundant early on in the history of the Earth, based in part on the age of oxidized iron deposits. So life, evolving as it did to utilize oxygen atoms in almost every molecule of consequence, got those atoms from carbon dioxide and carbonates. But the energy—and the electron donors and acceptors needed to run reduction-oxidation chemistry—came from elsewhere, likely first from reactions in the absence of light, such as H2 reacting with CO2. Later, with light but still without free O2, the energy and electron sources may have been bacterial photosynthesis, described by the schematic equation
2H2X + CO2 [CH2O] + 2X + H2O
About two billion years ago, oxygen-releasing photosynthesis spread widely among biota, introducing vast amounts of O2 into the atmosphere. One might look at the 2X in the generic equation above as just a waste product. But nature is opportunistic. The leavings of one organism, even if dangerous, provide food for another. Or (for 2X= O2) a breathable gas.
Yes, oxygen drove many of the early prokaryotes into anoxic niches, where they survive perfectly well to this day. Other survivors, prokaryotic and eukaryotic, evolved O2 detoxification strategies. These schemes included enzymes such as superoxide dismutase, classes of molecules such as luciferins, and some isoprenoids. With these and other molecular constables, aerobic organisms created an intracellular balance (perhaps upset as we age): Some oxygen-containing radicals were kept dangerous, even as aerobes evolved the biochemistry to control them.
And organisms did evolve to use O2. What was waste, now nourished. Yet oxygen does more than fuel cellular fires. The immune system is actively involved in making killing forms of oxygen, such as peroxide and ozone.
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