Photosynthesis is the process used by plants, algae and certain bacteria to harness energy from sunlight and turn it into chemical energy. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water. In most cases, oxygen is also released as a waste product. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies all of the organic compounds and most of the energy necessary for life on Earth.
The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons. Cyanobacteria appeared later; the excess oxygen they produced contributed to the oxygen catastrophe, which rendered the evolution of complex life possible. The study of photosynthesis began in the 17th century with Jan Baptist van Helmont and continued with major discoveries by Joseph Priestley, Jan Ingenhousz, and Melvin Calvin (Calvin Cycle).
Photosynthesis is fundamental to life on Earth. It is the primary source of energy for nearly all organisms, either directly (in plants/algae) or indirectly (in animals that eat plants). Research into photosynthesis continues today, especially in the context of increasing crop yields to feed a growing population and developing 'artificial photosynthesis' systems to generate clean fuel (like green hydrogen) from sunlight and water, mimicking the natural process.