The longest era in the geological history of the Earth, extending for about half its lifetime.
It began about 2600 million years ago, succeeding the Archaean (or Archaeozoic) Era, and it continued until the Cambrian Explosion began about 570 million years ago, ushering in the Palaeozoic Era, the first era of the Phanerozoic Aeon.
During the Proterozoic the dominant forms of life were unicellular, and the most abundant fossils are of cyanobacteria organised into huge mats called stromatolites.
Eukaryotes, that is organisms with nuclear DNA, arose around the middle of this era, probably by symbiosis.
Towards the end of the era there appeared an explosion in evolution, with the Vendian or Ediacaran systems appearing. These were overtaken by the Cambrian life forms that constitute our own ancestors.
[Editor's note, 7/10/2005: fixed links to point to nodes with content.]