information:classification, information:extraction
Photosynthesis in the geologic past

Over the course of Earth's geologic history CO concentrations have played a role in biological evolution. 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 as sources of electrons, rather than water. Cyanobacteria appeared later, and the excess oxygen they produced contributed to the oxygen catastrophe, which rendered the evolution of complex life possible. In recent geologic times, low CO concentrations below 600 parts per million might have been the stimulus that favored the evolution of C plants which increased greatly in abundance between 7 and 5 million years ago over plants that use the less efficient C metabolic pathway. At current atmospheric pressures photosynthesis shuts down when atmospheric CO concentrations fall below 150 ppm and 200 ppm although some microbes can extract carbon from the air at much lower concentrations.