The Precambrian Supereon was the longest single time period in Earth's history. Beginning 4.6 billion years ago, and lasting for over 4 billion years, the Precambrian era saw the formation of the Earth, origin of life, rise of multicellular organisms and development of an oxygen-rich terrestrial atmosphere. The Supereon ended with a change to the fossil record, caused by the development of hard-shelled organisms of the Phanerozoic era.
According to the Wikipedia geologic time scale, the first of several divisions of the Precambrian Supereon is known as the Hadean Eon. During this 600 million-year stretch of time, Earth was struck by a large impactor that eventually formed the Moon. The surface of the early Earth cooled sufficiently to support liquid water shortly after the impact, and the primordial atmosphere was lost to space to be supplanted with hydrogen-rich air out-gassed by volcanoes.
Life arose early on Earth. Michigan State University Geology Department notes that the oldest sedimentary rocks ever discovered date to around 3.5 billion years ago, and they show chemical traces of organic chemistry consistent with life. For much of the Precambrian, life consisted of cyanobacteria and primitive multicellular organisms such as Cnidarians.
Toward the end of the Precambrian, during the Ediacaran Period, complex multicellular life arose and left enigmatic fossils on the sea floor. The Phanerozoic Supereon began around 546 million years ago at the dawn of the Cambrian, which was characterized by the fossils of hard-shelled and more recognizably modern organisms.