The purpose of photosynthesis, if you are a plant, is twofold. One is clearly visible in the famous equation: it is to make sugars, which is done by forcing electrons onto carbon dioxide. The other, which is hidden in the detail, is to capture energy from the Sun and store it in a usable form. All life on Earth stores energy in the same way, as a molecule called adenosine triphosphate, or ATP. This suggests strongly that ATP is a very ancient ‘invention’, and the details of its production and function could provide clues as to life’s origin 4 billion years ago.
