The Precambrian extended from the origin of the earth （believed to have been about 4, 600 million years ago） to about 570 million years ago, representing nearly ninety per cent of geological time. The oldest known Precambrian rocks have been dated to about 3, 800 million years old, and the earliest living organisms date from the latter part of the aeon. The Precambrian is now replaced in formal stratigraphic schemes by the Archaean, Proterozoic, and (in some schemes) Priscoan aeons.