Evolution of Photosynthesis.

http://en.wikipedia.org/wiki/Photosynthesis

Hydrogen and Sulfur as electron donor to Oxygen as an Electron donor.

Early photosynthetic systems, such as those from green and purple sulfur and green and purple non sulfur bacteria, are thought to have been anoxygenic, using various molecules as electron donors. 

Green and purple sulfur bacteria are thought to have used hydrogen and sulfur as an electron donor. 

Green non sulfur bacteria used various amino and other organic acids. Purple nonsulfur bacteria used a variety of nonspecific organic molecules.

  The use of these molecules is consistent with the geological evidence that the atmosphere was highly reduced at that time.

Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old.

The main source of oxygen in the atmosphere is oxygenic photosynthesis, and its first appearance is sometimes referred to as the oxygen catastrophe.

 Geological evidence suggests that oxygenic photosynthesis, such as that in cyanobacteria, became important during the Paleoproterozoic era around 2 billion years ago. 

Modern photosynthesis in plants and most photosynthetic prokaryotes is oxygenic. Oxygenic photosynthesis uses water as an electron donor, which is oxidized to molecular oxygen (O
2) in the photosynthetic reaction center.