The Precambrian is sometimes referred to as an "eon." However, it actually has no rank. It is simply Precambrian time. The Precambrian is that stretch of geological time from the formation of the Earth itself to the start of the Cambrian period. This immensely long stretch of time - some four billion years or more - saw the formation of the Earth as a planetary body, including geosphere, atmosphere, and hydrosphere, as well as the appearance of the biosphere and hence the transformation of the Earth from a dead planet to a living one.