I've just read a paper which discusses bottlenecks in the evolution of
intelligent life. It points out that oxygen-producing photosynthesis
evolved only once and might be one of the less likely steps towards
the modern biosphere. So, suppose it never evolved. The productive
base of the biosphere becomes the other types of photosynthesis, which
don't use water as an electron donor and don't produce oxygen. After
billions of years of evolution, what does life on Earth look like? One
possibility is a sulphur-hydrogen sulphide cycle. There are already
bacteria that use photosynthesis to release sulphur from hydrogen
sulphide, and other bacteria that "burn" glucose using sulphur or
sulphates instead of oxygen, producing hydrogen sulphide. You could
easily imagine "plants" doing the former and "animals" doing the
latter - except the amount of energy involved is a fraction of what
you get with oxygen and water. Any other suggestions?
