Answer: 4000 million years back
Explanation:
The Archean eon started 4000 million years back (4 billion years) and ended by about 2500 million years back (2.5 billion years). This eon is marked by the formation of oceans and the continents on the earth's crust and also the existence of first life form on earth, commonly known as the cyano-bacteria (blue green algae).
