Precambrian

The Precambrian is the period of the geologic timescale from the appearance of Earth around 4.5 billion years ago (a billion being a thousand millions) to the evolution of abundant fossils, which marked the beginning of the Cambrian, some 540 million years ago. Remarkably little is known about the Precambrian, and what is known has largely been discovered in the past four or five decades.

It is thought that the Earth itself coalesced from material in orbit around the sun roughly 4500 million years before the present and may have been struck by a very large (Mars-sized) object shortly after it formed, splitting off material that came together to form the Moon. A stable crust was probably in place about 4400 million years ago. The oldest dated terrestrial rocks are about 4400 million years old. It is not known when life originated, but carbon in 3800 million year old rocks from islands off western Greenland may be of organic origin. Well-preserved bacteria older than 3460 million years have been found in Western Australia. Probable fossils 100 million years older have been found in the same area. There is a fairly solid record of bacterial life throughout the remainder of the Precambrian. Excepting a few dubious reports of much older forms from Texas and India, the first complex multicelled lifeforms seem to have appeared roughly 600 million years before the present. A quite diverse collection of soft-bodied forms is known from a variety of locations worldwide between 600 and 544 million years ago. These are referred to as Ediacaran or Vendian faunas. Hard-shelled creatures appeared toward the end of that timespan. A very diverse collection of forms appeared around 544 million years before the present starting in the latest Precambrian with a poorly understood "small shelly fauna" and ending in the very early Cambrian with a very diverse, and quite modern "Burgess fauna".

Details of plate motions and such are only hazily known in the Precambrian. It is generally believed that most of the Earth's landmasses collected into a single supercontinent around 1000 million years before the present. The supercontinent -- known as Rodinia -- broke up around 600 million years ago. A number of glacial periods have been identified going as far back as the Huronian[?] epoch, roughly 2200 million years ago. The best studied is the Sturtian-Varanger glaciation, around 600 million years ago, which may have brought glacial conditions all the way to the equator, resulting in a "Snowball Earth".

The atmosphere of the early Earth is poorly known, but is thought to have contained very little oxygen. It appears that many materials whose oxides are insoluble were present in the oceans for a few billion years after the Earth's creation. It is thought that life evolved, and developed the ability to create organic compounds from water and carbon dioxide. Oxygen was freed in the process. The oxygen was immediately tied up in chemical reactions with various materials -- primarily iron -- until the supply of iron ran out. After that the modern high-oxygen atmosphere developed. Older rocks contain massive Banded Iron Formations that were apparently laid down as iron and oxygen combined.

A diverse terminology has evolved covering the early years of the Earth's existence, but it is tending to fall out of use as radiometric dating allows plausible real dates to be assigned to specific formations and features. The terms Archean (roughly -- older than 2500 million years), Proterozoic (600 million to 2500 million), and Neoproterozoic (545-600 million years) still have some general currency. Some additional terms are included in the geological time line.

Proterozoic

As originally used, everything prior to the Cambrian boundary, which has been placed at various times by various authors. As of 2000 AD, the Cambrian boundary seems to be settling firmly around 544 million years before present (ma). Modern usage would often be as described in the previous paragraph 600ma-2500ma.

Neoproterozoic

Roughly from the Cambrian boundary back to about 900 million years. Modern usage may tend to be a shorter interval : 544ma-600ma. Coressponds to Precambrian Z rocks of older North American geology.

Mesoproterozoic

Roughly from 900ma to 1600ma. Corresponds to Precambrian Y rocks of older North American geology.

Paleoproterozoic

Roughly from 1600ma to 2500ma. Corresponds to Precambrian X rocks of older North American geology.

Archean

Roughly from 2500ma to 3800ma.

Hadean

Prior to 3800ma. This term was probably intended originally to cover the period before any preserved rocks were deposited. A very few old rock beds seem to be slightly older than 3800ma.