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...