Precambrian+Era

= __**Precambrian Era **__ = -The first two-thirds of Earth's history is called the Precambrian (meaning "before the Cambrian"), 4500 - 550 million years ago. It isknown as the Archaeozoic Era. The Earth was formed about four and a half billion years ago. Only after the Earth's crust had hardened and the atmosphere and hydrosphere were formed were the conditons created for life to appear about three billion years ago. The oldest life forms were single celled cyanobacteria called stromatolites. The next advance came about 2100 million years ago when the first single cells with a nucleus present (eucaryotes) appeared.

-The Precambrian Era spans over three billion years whereas the time since the end of the Precambrian has only been 500 million or a half billion years. This covers the time from the birth of the earth to what is known as the "Cambrian explosion" when fossils became abundant in the rocks.

-The Precambrian era contains most of earths history, but very little infomation is known about it. The Precambrian is sometimes referred to as an "eon." However, it actually has no rank. It is simply Precambrian time. The Precambrian is that stretch of geological time from the formation of the Earth itself to the start of the Cambrian period. This immensely long stretch of time some four billion years or more saw the formation of the earth as a planetary body, including atmosphere, and hydrosphere, as well as the appearance of the biosphere and hence the transformation of the earth from a dead planet to a living one.

[|www.prehistory.com/ precambrian.htm]
 * The Precambrian Time-Scale**


 * Eon || Era || Period || End-Began ||

Source: [|International Commission on Stratigraphy], 2001, published by Micropress
 * Proterozoic PR || Neoproterozoic NP || Ediacaran NP3 || 540 - 650 ||
 * ^  ||^   || Cryogenian NP2 || 650 - 850 ||
 * ^  ||^   || Tonian NP1 || 850 - 1000 ||
 * ^  || Mesoproterozoic MP || Stenian MP3 || 1000 - 1200 ||
 * ^  ||^   || Ectasian MP2 || 1200 - 1400 ||
 * ^  ||^   || Calymmian MP1 || 1400 - 1600 ||
 * ^  || Paleoproterozoic PP || Statherian PP4 || 1600 - 1800 ||
 * ^  ||^   || Orosirian PP3 || 1800 - 2050 ||
 * ^  ||^   || Rhyacian PP2 || 2050 - 2300 ||
 * ^  ||^   || Siderian PP1 || 2300 - 2500 ||
 * Archean AR || Neoarchean NA ||  || 2500 - 2800 ||
 * ^  || Mesoarchean MA ||   || 2800 - 3200 ||
 * ^  || Paleoarchean PA ||   || 3200 - 3600 ||
 * ^  || Eoarchean EA ||   || 3600 -3800? ||
 * Hadean || Early Imbrian ||  || 3800-3850 ||
 * ^  || Nectarian ||   || 3850-3950 ||
 * ^  || Basin Groups 1-9 ||   || 3950-4150 ||
 * ^  || Cryptic ||   || 4150- c. 4560 ||

- The first one-celled organisms formed during the Precambrian Era. They had an important job to do. They helped make the air and water around the Earth full of oxygen. Once there was plenty of oxygen, new life could form. This life would have many cells together to make different kinds of animals.

-The earliest living organisms were microscopic bacteria, which show up in the fossil record as early as 3.4 billion years ago. As their numbers multiplied and supplies of their chemical fuel were eaten up, bacteria sought out an alternative energy source. New varieties began to harness the power of the sun through a biochemical process known as photosynthesis—a move that would ultimately lead to simple plants and which opened the planet up to animal life.

-Some three billion years ago the Earth's atmosphere was virtually devoid of oxygen. At about 2.4 billion years ago, oxygen was released from the seas as a byproduct of photosynthesis by cyanobacteria. Levels of the gas gradually climbed, reaching about one percent around two billion years ago. About 800 million years ago, oxygen levels reached about 21 percent and began to breathe life into more complex organisms. The oxygen-rich ozone layer was also established, shielding the Earth's surface from harmful solar radiation.

**Precambrian Eon:** (4,550-543 mya) -The Precambrian eon is vast, almost unimaginably so. In fact, before some fossil discoveries were made late in the 20th century, it was considered unknowable evolution's dark ages. The Precambrian was 86 percent of the history of Earth. As its name implies, this includes all of geological time prior to the Cambrian period.