The oldest fossils ever found were dug out of northwestern Australia in strata formed 3.5 billion years ago. They derive from a primitive bacteria composed of many connected cells. The complexity of this life form indicates that the birth of life probably happened earlier than 3.5 billion years ago. There are many theories regarding the birthplace of the Earth's first life. According to one theory, life was born deep in the seas at spots where hot water sprang forth.