They, in turn, are subdivided into epochs and stage ages. The known geological history of Earth since the Precambrian Time is subdivided into three eras, each of which includes a number of periods.
Life on Earth originated about 2 billion years ago, but there are no good fossil remains from periods earlier than the Cambrian, which began about 490 million years ago. Our Earth acquired its present size, more or less, between 4 billion and 5 billion years ago. It is generally assumed that planets are formed by the accretion of gas and dust in a cosmic cloud, but there is no way of estimating the length of this process.
