text: Over the course of Earth's geologic history  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  concentrations below 600 parts per million might have been the stimulus that favored the evolution of C4 carbon fixation| plants which increased greatly in abundance between 7 and 5 million years ago over plants that use the less efficient C3 carbon fixation| metabolic pathway. At current atmospheric pressures photosynthesis shuts down when atmospheric  concentrations fall below 150 ppm and 200 ppm although some microbes can extract carbon from the air at much lower concentrations.