The universe began around 13.8 billion years ago  , and our solar system came into existence about 4.5 Billion years ago . The simplest form of life orignated some 3.5 to 3.8 billion years back in the cold and dark bottoms of the Earth's oceans near hot sulpher geysers . This life was not based on O2 because O2 was not there . It slowly but steadily evolved into more complex forms and crepts onto land . It also became photosynthesis - based , creating the present day greenery and it atmosphere rich in O2 .