Cyanobacteria can oxidize water using light energy, and which causes evolving oxygen molecules. In the history of life on the Earth, the origin of cyanobacteria is not so old, and anoxygenic photosynthesis has been established prior to the first appearance of oxygenic photosynthesis. Such phototrophs that photosynthesize without oxygen production still survive up to now and frequently found in various environments. Anoxygenic photosynthetic bacteria are taxonomically divided into four groups: filamentous anoxygenic phototrophic bacteria (FAPB), purple bacteria (PB), green sulfur bacteria (GSB), and heliobacteria (HB). Cyanobacteria are known to use two different types of photopigment-containing protein complex, i.e., photosystem I (PS I) and photosystem II (PS II), in their oxygenic photosynthesis, whereas anoxygenic phototrophs possess only one of two photosystem-types. GSB and HB contain a PS I-type photosystem in their photosynthesis, while FAPB and PB contain a PS II-type photosystem. The phylogenetic analysis based on 16S rRNA sequence and comparison of the photochemical structure among the phototrophic bacteria suggested that the origin of anoxygenic photosynthesis is much older than that of oxygenic photosynthesis and a photosynthetic core complex in the first phototroph appeared to be similar to a PS II-type photosystem. In the evolutionary pathway of photosynthesis, a PS II-type phototroph emerged first, and then a PS I-type phototroph came out. Oxygenic photosynthesis that appeared subsequently to anoxygenic photosynthesis originated from genetic incorporation of these two different types of photosystems.

Major events concerning the early evolution of life have occurred in the pre-Cambrian era. Quality and quantity of the fossil records with important information regarding the early evolution of life have been insufficient. The introduction of geochemical methods and the recent discoveries of well-preserved fossils that have provided high resolution data for the discussions on the early evolution of life are reviewed. The major events of the early evolution of life are the emergence of life, photosynthetic bacteria, and Metazoa. Studies on the early evolution of life with fossil records began with discussions on the 1.9 Ga fossil bacteria. Currently, oldest fossil records are traced back to 3.5 Ga. Recent controversies on these fossils have been made on the basis of several geochemical approaches. Some interesting attempts for the detection of the photosynthetic activity from sedimentological setting of the strata-containing bacteria fossils should be noted. Ediacaran fossils are regarded as ancestors of Metazoa, but some researches proposed that apparent difference exists between those fossils and Metazoa. Recent discoveries of well-preserved three-dimensional Ediacaran fossils will provide insight into the evaluation of such unique organisms.