Evolution of Photosynthesis
Photosynthesis is an ancient process dating back to about 3.2 to 3.5 billion years ago. Anoxygenic photosynthesis originated in early green and purple sulfur and green and purple non-sulfur bacteria, that flourished in an atmosphere composed mostly of nitrogen and carbon dioxide gas. These bacteria were anoxygenic organisms that did not produce oxygen through photosynthesis and could be harmed by an oxygenic atmosphere. Early forms of cyanobacteria, which slowly evolved into the chloroplast found in plants today, changed the atmosphere of the Earth into an oxygenic atmosphere. This allowed organisms that required oxygen to grow and to evolve. The event when oxygenic photosynthesis began to occur is generally referred to by scientists as the "oxygen catastrophe" because the oxygen gas produced was highly toxic to the previously dominant anoxygenic organisms. This event occurred during the Paleoproterozoic era, 2 billions years in the past, when the atmosphere of the Earth was mostly composed of nitrogen and carbon dioxide gas.
Over billions of years, as plants, animals, and bacteria evolved, the cyanobacteria that caused the "oxygen catastrophe" became the plastids, such as chloroplasts, found in the plants and algae we see today. Cyanobacteria continue to be major producers of oxygen throughout the ocean today, producing large amounts of oxygen necessary for the survival of the many oxygenic species that inhabit the Earth.