Life first evolved in the primordial oceans of Earth approximately four billion years ago. The first life forms were prokaryotes, or non-nucleated unicellular organisms, which divided in two domains, the Bacteria and Archaea . They lived around hot sulfurous geological and volcanic vents on the ocean floor, forming distinct biofilms, organized in multilayered symbiotic communities, known as microbial mats. Fossil evidence suggests that these first communities were not photosynthetic, i.e., did not use the energy of light to convert carbon dioxide and water into glucose, releasing oxygen in the process. About 3.7 billions years ago, anoxygenic photosynthetic microorganisms probably appeared on top of pre-photosynthetic biofilms formed by bacterial and Archaean sulphate-processers. Anoxygenic photosynthesizers use electrons donated by sulphur, hydrogen sulfide, hydrogen, and a variety of organic chemicals released by other bacteria and Archaea. This ancestor species, known as protochlorophylls, did not synthesized chlorophyll and did not release oxygen during photosynthesis . Moreover, in that deep-water environment, they probably used infrared thermo taxis rather than sunlight as a source of energy.
Protochlorophylls are assumed to be the common ancestors of two evolutionary branches of oxygenic photosynthetic organisms that began evolving around 2.8 billion years ago: the bacteriochlorophyll and the chlorophylls. Bacteriochlorophyll gave origin to chloroflexus, sulfur green bacteria, sulfur purple bacteria, non-sulfur purple bacteria, and finally to oxygen-respiring bacteria. Chlorophylls originated Cyanobacteria, from which chloroplasts such as red algae, cryptomonads, dinoflagellates , crysophytes, brown algae, euglenoids, and finally green plants evolved. The first convincing paleontological evidence of eukaryotic microfossils (chloroplasts) was dated 1.5 at billion years old. In oxygenic photosynthesis, electrons are donated by water molecules and the energy source is the visible spectrum of visible light. However, the chemical elements utilized by oxygenic photosynthetic organisms to capture electrons divide them in two families, the Photosystem I Family and the Photosystem II Family. Photosystem II organisms, such as Chloroflexus aurantiacus (an ancient green bacterium) and sulfur purple bacteria, use pigments and quinones as electron acceptors, whereas member of the Photosystem I Family, such as green sulfur bacteria, Cyanobacteria, and chloroplasts use iron-sulphur centers as electron acceptors.
It is generally accepted that the evolution of oxygenic photosynthetic microorganisms was a crucial step for the increase of atmospheric oxygen levels and the subsequent burst of biological evolution of new aerobic species. About 3.5 billion years ago, the planet atmosphere was poor in oxygen and abundant in carbon dioxide and sulfuric gases, due to intense volcanic activity. This atmosphere favored the evolution of chemotrophic Bacteria and Archaea. As the populations of oxygenic photosynthetic microorganisms gradually expanded, they started increasing the atmospheric oxygen level two billion years ago, stabilizing it at its present level of 20% about 1.5 billion years ago, and additionally, reduced the carbon dioxide levels in the process. Microbial photosynthetic activity increased the planetary biological productivity by a factor of 100–1,000, opening new pathways of biological evolution and leading to biogeochemical changes that allowed life to evolve and colonize new environmental niches. The new atmospheric and biogeochemical conditions created by photosynthetic microorganisms allowed the subsequent appearance of plants about 1.2 billion years ago, and 600 million years later, the evolution of the first vertebrates, followed 70 million years later by the Cambrian burst of biological diversity.
See also Aerobes; Autotrophic bacteria; Biofilm formation and dynamic behavior; Biogeochemical cycles; Carbon cycle in microorganisms; Chemoautotrophic and chemolithotrophic bacteria; Electron transport system; Evolutionary origin of bacteria and viruses; Fossilization of bacteria; Hydrothermal vents; Plankton and planktonic bacteria; Sulfur cycle in microorganisms