The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution

by Richard Dawkins

Incidentally, having said that oxygen is produced by green plants and algae, it is an oversimplification to leave it at that. It is true that plants give off oxygen. But when a plant dies, its decay, in chemical reactions equivalent to burning all its carbonaceous materials, would use up an amount of oxygen equal to all the oxygen released by that plant during its lifetime. There would therefore be no net gain in atmospheric oxygen, but for one thing. Not all dead plants decay completely. Some parts of them are laid down as coal (or equivalents), other parts are consumed and bits of the consumers themselves may become locked into rocks.* The net effect is to store energy-rich compounds underground and leave some oxygen free to circulate. Releasing some of that stored energy by burning fossil fuels turns this oxygen back into carbon dioxide, taking us back towards the ancient status quo. Fortunately we are unlikely to return the atmosphere all the way to our suffocating Canterbury. But we should not forget that the oxygen we breathe exists only because of compounds tied up underground, which include coal and oil. We burn them at our peril.