Chapter 22: The Evolution of Photosynthesis

Photosynthesis probably began on the Earth soon after life itself, as the early sources of chemical energy (geothermal, reducing compounds, hydrogen) were exhausted [about 3.7 giga years (Ga) ago]. However, a primitive form of respiration is likely to have preceded photosynthesis, providing complexes such as cytochromes and iron-sulfur centers. Anoxygenic photosynthetic (PS) bacteria were the first organisms involved, but it should not be assumed that these were similar to those found today, and it is possible that they used both chlorophylls and bacteriochlorophylls, in simple light-driven reactions. This was followed by the evolution of complex reaction centers and light-harvesting complexes, which could cope with light and oxidative stresses. This leads to the development of reaction centre I-type (RCI-type) centers, with iron-sulfur centers as secondary electron acceptors, and RC-II-type centers, with quinones as the secondary electron acceptors. These events may have occurred in two lines of PS bacteria, with the later development of cyanobacterial-like organisms, with both types of RC, brought about by fusion (Fusion Hypothesis). However, it is also possible that the two types of RC developed in one organism and later separated (Fission hypothesis), giving rise to the present situation with anoxygenic photosynthetic bacteria with either one or the other type of RC but not both. The next big development was the ability to split water to gain hydrogen that is vital for the reduction of inorganic carbon (CO₂). This developed in the forerunners of present day cyanobacteria about 2.2–2.8 Ga. This was one of the most revolutionary events to happen on the Earth: it turned the atmosphere into an oxidizing rather than a reducing one, allowed the evolution of oxidative bacteria, allowed the evolution of mitochondriate protists, which in turn paved the way for the evolution of algae (protists with mitochondria and chloroplasts). The final step, multicellularity, led to the three multicellular domains dominant on terrestrial parts of the Earth today, the animals, plants and fungi.