There is evidence that an early form of photosynthesis may have emerged as far back as 3.5 billion years ago in single-celled organisms that produced enigmatic mounds known as stromatolites (see Chapter 3), although the precise date is still an area of active debate and research. Whatever the date, there is general agreement that a simple form of photosynthesis, using energy from the Sun to synthesise sugars from carbon dioxide, just as photosystem I does in plants today, is very ancient.
