Photosynthesis: The Game Changer
The exhaustion of the prebiotically produced food supply by the earliest organisms not only led to competition between and selection for the more efficient forms, but gave impetus to development of biochemical pathways that could use simple inorganic energy sources such as molecular hydrogen, or direct support of biochemical synthesis using sunlight. Pigments originally useful in protecting simple cells from the destructive effects of solar radiation turned into traps and converters of solar energy to drive useful biochemical reactions. This direct access to energy resources freed these cells from dependence on external food sources that were becoming short in supply. Initially these photosynthetic process used reduced compounds including molecular hydrogen, ferrous iron, hydrogen sulfide and even some simple organic compounds to serve as sources of hydrogen (and reducing capacity) to convert carbon dioxide into energetic compounds (e.g. sugars) to be used as food by the cell. At some point a pigment system arose that could use water as the hydrogen source, with free oxygen as the waste product. This led directly to a major change not only to life, but to conditions on Earth’s surface. These changes are recorded in the complexity of the phylogenetic trees showing how various organisms are related to one another.