Scientists estimate that the earth formed some 4.6 billion years ago with the first forms of life appearing around 3.5 billion years ago. these first forms of life were simple prokaryotes. later, the first photosynethic organisms resulted in more in the atmosphere. this allowed for more higher level organisms to appear on earth.

Oxygen in the atmosphere nearly 21 % other wise there was no oxygen in the atmosphere

when the first prokaryotic species occurred in the earths  history some .5 to 2 billion years ago oxygen started building up in the atmosphere as O2 was released as by product in photosynthesis reaction

Scientists estimate that the Earth formed some 4.6 billion years ago with the first forms of life appearing around 3.5 billion years ago. These first forms of life were simple prokaryotes. Later, the first photosynethic organisms resulted in more oxygen in the atmosphere. This allowed for more higher level organisms to appear on Earth.

Explanation:

The primitive atmosphere was absent of oxygen. The photosynthetic organisms were the ones who gave their origin. These organisms are cyanobacteria that are photosynthetic and prokaryotic. They organize photosynthesis from the breakdown of water molecules. Thus during millions of years molecular oxygen was increasing its concentration until it changed its character from reductant to oxidant.