The Archaean (or Archean) eon is the geological period after the Hadean and before the Proterozoic. It is one of the four main time periods (eons) of Earth history. The Archaean lasted from 4,000 million years ago (mya) to 2,500 mya. It contains the first sedimentary rocks, and the first fossil life forms, which were cyanobacteria, and acritarchs.