Earth is blessed with an abundant supply of liquid water, and is continuously bathed in light from the Sun. Nature has taken advantage of these conditions to evolve more than a trillion life forms.
Life needs chemical energy to drive its machinery along. Essentially, it requires an oxidisable chemical as a source of electrons. Early life forms exploited a number of processes, but in the end the “voltage” of these sources of electrons and the availability of the relevant chemicals were limited. So nature took another tack.
Each quantum of visible light coming from the Sun has an “energy” of more than 1 V – enough to do a lot of chemistry. Each colour of light has a different “voltage”. For example, a single silicon solar cell on your roof produces up to 0.8 V by absorbing near-infrared light, while plants use chlorophyll molecules to absorb red light, which has more energy and can generate a maximum of 1.8 V.
By inventing photosynthesis, nature provided a “voltage kick” to photoactive molecules in the photosynthetic apparatus (e.g. chlorophyll), enabling them to extract electrons from a far wider range of molecules. Some of the early photosynthetic organisms extracted electrons from hydrogen sulphide (H2S).
A monumental breakthrough occurred when photosynthetic bacteria found a way of splitting water, which is a far more abundant source of...
