Precambrian

Precambrian, or more fully Precambrian time, in geology, term now used mainly informally to describe the longest period of the geological timescale, covering the estimated 4 billion years between the formation of the Earth and the start of the Phanerozoic eon, 575 million years ago. When the geological timescale was first developed in the 19th century two major divisions of Earth's history were discerned, based on the fossil content of rock strata (layers) of particular ages: the Precambrian, when fossils were rare or absent; and the Phanerozoic, when they were relatively abundant. In recent decades, however, research using modern technology has led to the discovery of the remains of primitive unicellular organisms dating back more than 3 billion years. As a result, more sophisticated subdivisions of the period before the Phanerozoic have been developed, and today Earth scientists generally divide the Precambrian into the Priscoan, the Archaean, and the Proterozoic eons. The Priscoan eon, the earliest of the three divisions, strictly covers the period from the formation of the universe until the start of the Archaean eon about 4 billion years ago. For this reason it tends to be divided into two eras: the Hadean, running from the formation of the Earth about 4.6 billion to 4.5 billion years ago to the start of the Archaean; and the pre-Hadean, covering the time between the origin of the universe and the formation of the Earth. The Archaean, during which much of the Earth's crust was formed and life first began, runs from about 4 billion years ago until the start of the Proterozoic eon about 2.5 billion years ago. The Proterozoic was a period in which life was rapidly developing and during which the first multicellular animals appeared—leading up to the emergence of the first organisms with hard parts (shells and skeletons) about 570 million years ago, and the start of the Phanerozoic.

Over the course of the three eons of Precambrian time, the Earth's surface underwent many important changes. At some early stage the crust became differentiated into the dark, heavy “simatic” rocks that floor the huge basins in which the first oceans began forming, and the lighter “siliac” rocks that float on the sima and form the continents. At the same time, the crust became broken into plates, and the action of plate tectonics gave rise to continental drift. The first oceans became home to the newly evolved aerobic bacteria and cyanobacteria. These early marine life forms are thought to have been responsible for generating oxygen, pouring the gas into the primitive atmosphere for millions of years and thereby setting the stage for the evolution of oxygen-dependent marine creatures in the following Cambrian period of the Palaeozoic era.

Precambrian rocks consist in general of (1) an extensive series of metamorphosed igneous and sedimentary strata, such as gneisses, schists, slates, quartzites, and crystalline limestones; (2) igneous rocks, slightly altered; and (3) recognizable sedimentary rocks that contain vital clues for researchers in palaeontology. Such clues include fossils of primitive uni- and multicellular marine life such as algae, traces of more primitive life, such as bacteria, and (from the youngest Precambrian rocks) the Ediacaran fauna, an assemblage of complex, soft-bodied marine invertebrates that may represent an evolutionary dead end.

Precambrian rocks are rich in ores and other minerals, including iron ore, gold, nickel, and copper, as well as building materials such as granite and marble. Other minerals of economic importance found in such rocks include graphite, garnet, apatite, talc, emery, and feldspar.