350 PRECAMBRIAN/Overview PRECAMBRIAN Contents Overview Eukaryote Fossils Prokaryote Fossils Vendian and Ediacaran Overview L R M Cocks, The Natural History Museum, London, UK Copyright 2005, Natural History Museum All Rights Reserved Introduction Since the Precambrian–Cambrian boundary can now be dated at 543 Ma ago, and since the Earth is estimated to have been formed at about 4500 Ma ago, it follows that the Precambrian represents about seven eighths or 88% of geological time However, that fact was not at all obvious to early geologists who, 200 or so years ago, had no idea of the age of the Earth Between about 1820 and 1845, Earth history was divided into various named systems, with the Cambrian the oldest, based on the successive assemblages of distinct fossils contained in the sedimentary rocks and on the Law of Superposition, which states that rocks are older than other rocks now above them (assuming that they have not been structurally overturned) Igneous rocks were either undated or dated relatively as younger than the sedimentary rocks through which they had been intruded Since no unambiguous fossils were then known from rocks below the Cambrian, all such rocks were and are simply termed the Precambrian It was not until the invention and progressive refinement of radioisotopic dating in the twentieth century that the true ages of both the Precambrian and the rocks above it began to be understood Divisions of the Precambrian Unlike the Phanerozoic (Cambrian–Holocene), there are no formal subdivisions of the Precambrian However, the Precambrian is normally divided into two: the earlier Archaean and the younger Proterozoic The Proterozoic has itself been divided into three, and the Precambrian is thus divided as follows (the ages are obviously approximate): Neoproterozoic – 1000 Ma–543 Ma, Mesoproterozoic – 1600 Ma–1000 Ma, Palaeoproterozoic – 2500 Ma–1600 Ma, and Archaean – ca 4000 Ma–2500 Ma No Earthly rocks are known that are older than about 4000 Ma, so there is no universally used name for the period between the formation of the Earth at about 4500 Ma and the formation of the oldest known rocks of the Archaean, although the Hadean is a term used by some The Neoproterozoic is divided into the Riphean (1000–600 Ma) and the Vendian (600–543 Ma), and the Vendian is sometimes termed the Ediacaran The Precambrian in this Encyclopaedia A large number of articles in this encyclopaedia deal in various ways with aspects of the Precambrian, and thus this article is devised to help the reader to locate the appropriate entry through cross-referencing Thus there follows a brief guide to the key aspects of the Precambrian, under the headings the origin of the Earth, major Precambrian outcrops, Precambrian sediments and climate, the origin of life and Precambrian fossils, Precambrian orogenies and Precambrian terranes and palaeogeography The Origin of the Earth Our galaxy and the universe appear to have been formed about ten thousand million years (10 Ga) ago, but our star – the Sun – and its Solar System not seem to have been formed for a long time after that, at about 4.5 Ga The article on Earth structure and origins (see Earth Structure and Origins) describes how this process is thought to have occurred The oldest known individual minerals are found within