18 Oct—Photo­synthesis

Many forms of life get their energy directly from the Sun in a chemical process named photosynthesis. They create their own food from sunlight. Cyanobacteria first learned this trick about 2,800 million years ago, the 18^th of October on the Cosmic Calendar.

Actually, that's not entirely true. Photosynthesis probably began much earlier—about 3,500 million years ago, the 29^th of September on the Cosmic Calendar—with bacteria that evolved to use hydrogen and sulfur or other chemicals to convert light into food. But unlike the photosynthesis that cyanobacteria developed much later, that early anaerobic form of photosynthesis didn't create oxygen.

The oxygenic form of photosynthesis that we can thank cyanobacteria for is particularly important to us because without it, we couldn't exist. Oxygen helps us to create and store enough energy to sustain our complex and fast-paced life. Without oxygen, life would be stuck in the slow lane.¹

Life has only discovered oxygenic photosynthesis once. The fundamentals of the process have remained unchanged ever since: the photosynthesizer takes in molecules of carbon dioxide and water and absorb the energy of photons from sunlight. The end product is sugar (glucose) and oxygen. For the photosynthesizer, oxygen is a toxic waste product which is expelled from the organism.

Green algae and plants also photosynthesize today, but they didn't learn how independently. They came by this ability in a surprising way.

Rather than evolve photosynthesis for themselves, these organisms' ancient ancestors imprisoned cyanobacteria inside their own cells.² The cyanobacteria continued to do the work of photosynthesis, producing food for their kidnappers.

Ever since that time long ago, some cyanobacteria have lived inside of green algae and plants. They have adapted to life within their hosts and formed a symbiosis with them, but they are still recognizable as separate organisms. They have their own genome that closely resembles that of their free-range cyanobacteria cousins and which gets passed down from generation to generation inside of algae and plant cells. They are known as the little organelles called chloroplasts inside of every green algae and plant cell.

Without this symbiosis, plants couldn't get their energy from the Sun and we wouldn't exist.

1. More will be said about this when we discuss the Oxygen Catastrophe.

2. We will discuss discuss this again when we come to the development of eukaryotes.