It contained two deadly gases methane and ammonia including some amounts of water vapors.
Out gassing from volcano eruptions formed the first atmosphere.
Evidence of there being no oxygen in the early atmosphere of the Earth is that rock that contains iron at this time did not rust and for metal to rust it needs water and oxygen which proves this statement true.
Sunlight triggers chemical reaction creating a few different elements including Hydrogen which escapes back to space.
N CO2 N H2O N CO2 H20 C2O
The sunlight will continue to break down the elements. Turn H20 vapors into H and O . As the mixture separates the Hydrogen escapes into space while Oxygen will get stuck and accumulate at the ozone.
H2O H O H 2 0 o H2O
The Hydrogen will float to space because it is less dense than other gases.
Over time, ozone forms a blanket like layer around the Earth. Which absorbs ultraviolet rays that block them from reaching the Earth's surface.
Because of the ozone layer, microorganisms started to develop.Microorganisms photosynthesize , adding more oxygen into the atmosphere. As green plants thrive O2 contents grow tremendously. Evidence for this is red beds ( rock that contain rust which is ad indication of oxygen presence,) and stromatolites ( layered mounds formed by growth of cyanobacteria developed)
Earth's current atmosphere contains 78% nitrogen, 21% oxygen, 0.9% argon, and 1% other gases. About 600 mya, oxygen levels & carbon dioxide levels equal out and has remained constant throughout history. Evidence for this is ice cores(can compare trapped air bubbles in ice from 1 million years ago), and finding evidence of climate change in the ice by looking at the trapped pollen.