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Precambrian time chart
Precambrian time chart (after Schopf 1999; Athena Review).
The Precambrian period (4.5 billion to 541 million years ago) is eight times as long as the remaining seventeen geological periods put together. This immense early time frame was not even sketchily known until the mid-20th century, although its existence was suspected by geologists in the early 19th century (Schopf 1999),
The Precambrian period lasted from the creation of the earth (estimated at about 4.5 billion years ago) to the beginning of the Cambrian period (541-485 million years ago), when complex marine animals began to flourish. For most of these 4 billion years, the only living inhabitants of Earth were one-celled microbes resembling bacteria. Some of these, called Archaea, were able to survive in extremely hostile environments (Cloud 1989; Margulis and Schwartz 1988). Before 3 billion years ago, photosynthesis or the harvesting of sunlight for energy was used for subsistence by cyanobacteria.
The Precambrian is divided into two stages of life, the Archean, from about 3.8 to 2.5 billion years ago, and the Proterozoic, lasting until the Cambrian period. In the Proterozoic, the first organisms with complex, nucleated cells (called eukaryotes for "true cells") appear. Algae practicing photosynthesis became widespread after 1.5 billion years ago, which increased atmospheric oxygen. The transition from the Proterozoic to the Phanerozoic ("visible life", referring to macroorganisms) is marked by the Ediacaran or Vendian stage (635-541 mya), when a small range of sponge-like complex marine organisms appear.
The Cambrian period (541-485 mya) is marked by a rapid, substantial increase in the complexity of multi-celled organisms, including a number of marine orders such as crustaceans, mollusks, and bivalves, as well as the first chordates or primitive fish.
References
Cloud, P., 1989
Margulis, L. and K. Schwartz 1988
Schopf, W. 1999
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