  6 P71908A 21. If catalase was acting as a photosynthetic enzyme, then it would be natural for a number of catalase molecules to cluster around the photosynthetic apparatus. In these circumstances, it would be simple for two catalase molecules to became associated as a complex: the prototype oxygen-evolving complex. At first it would have continued to use hydrogen peroxide as an electron donor, but given the right energy input, this complex could split water. We know that three small changes in the structure of bacteriochlorophyll can transform its properties, enabling it to absorb high-energy light at a wavelength of 680 nm. We now have a prototype oxygen-evoIving complex (the nutcracker that can physically split water) and a chlorophyll that can provide enough energy for it to do so (or the hand that presses the nutcracker). Thus, with no foresight and no disadvantageous steps, we have taken a path leading from anoxygenic photosynthesis to oxygenic photosynthesis. Adapted from: Nick Lane. ‘Green Planet’. In Oxygen. Oxford, Oxford University Press, 2002. pp131–146.   7 P71908A BLANK PAGE   8 P71908A BLANK PAGE