Precambrian
From EvoWiki
The Precambrian is the earliest eon of Earth history, literally meaning the time "before the Cambrian", and an abundant fossil record. As the early stratigraphers based their divisions of relative time on fossils in the rock and the Precambrian had few megascopic fossils, the vast layers of ancient rock below the Cambrian we basically lumped together and ignored. They were thought to be beyond correlating by the biostratigraphers of the 19th century. However the discovery of radiometric dating techniques of the early 20th century along with the gradual discovery and recognition of relatively abundant microscopic fossils in the Precambrian rocks has led to the increasing subdivision and correlation of the vast time represented by the Precambrian. The timespan represented by the Precambrian is enormous: from 542 million years ago back to the age of the earth, 4.5 billion years ago.
The Precambrian is currently divided into three eras: the Proterozoic (542 to 2500 million years ago), the Archean (2500 - 3800 million years ago) and the Hadean (3800 to 4500 million years ago). The Proterozoic has been subdivided into the Neo-, Meso- and Paleoproterozoic periods. The Ediacaran period (540 - 650 myrs) has recently been broken out of the Neoproterozoic period and formally recognized as the latest time of the Precambrian. The Meso- and Paleoproterozoic have also been tentatively subdivided into periods although it has been suggested by some scientists that the Precambrian should be divided into five alternative eons that are based on stages of planetary evolution rather than numerical ages which the current hadean epochs are based on. The five alternative Precambrian eons are listed below.
- Accretion The period in which the earth formed from a protoplanetary disk. The accretion period predates earths collision with the early planet Thea which created the earth's moon around 4.4 billion years ago.
- Hadean The time period that the Late Heavy Bombardment occurred in.
- Archean The period in which the earth's crust formed. The new Archean period ends with the development of of banded iron formations that resulted from the evolution of photosynthesis in early single or unicellular organisms which caused oxygen levels in the earth's early atmosphere to rise.
- Transition A period marked by the continuation of iron banded formation untill the appearence of the earliest continental red beds.
- Proterozoic The period that witnessed the rise of modern plate tectonics. The Proterozoic ended with the appearance of the earliest Animals.
The oldest subdivision of the traditional time scale is the Hadean. Zircon crystals from sedimentary rocks in Canada and Western Australia have been dated to about 4400 million years. This is assumed to be very close to the original solidification of the Earth's crust. Older samples from meteorites and Lunar samples have been dated around 4500 million. A sequence of sedimentary rocks in Greenland are intruded by an igneous dike that dates at 3900 million years. These sediments contain banded iron formation sequences and some possible organic carbon. The interpretation is based on stable carbon isotope ratios, 12C/13C, from the sediments and is controversial. The implications are that life and photosynthesis was already active in these ancient sediments.
The earliest known fossils of life forms are stromalites that have been found in fragments within a formation called the Apex Chert in northwestern Western Australia, and dated from the Archean, at 3,465 million years old.
During the Proterozoic life proliferated, and by the start of the Cambrian, soft-bodied multicellular Eukaryotes, most of which were identifiably animals, were leaving fossils.
