Question

Name the process that accounts for the oxygen in the earths atmosphere, oceans and rock deposition over the past 2 billion years.

Answer 1

Most scientists believe that for half of Earth's 4.6-billion-year history, the atmosphere contained almost no oxygen. Cyanobacteria or blue-green algae became the first microbes to produce oxygen by photosynthesis, perhaps as long ago as 3.5 billion years ago and certainly by 2.7 billion years