4.5 bya, the newly formed Earth was under constant tectonic motion, leading to volcanic eruptions which introduced carbon dioxide to the atmosphere
3.8 bya, photochecmical reactions triggered by solar heat caused poisonous methane and ammonia to become nitrogen, hydrogen, and carbon dioxide gases.
CH4 & NH3
N2, H2, & CO2
3.6 bya, decrease in the frequency of geothermal events led to drops in global temperatures, which caused water vapor to condense into liquid water, resulting in only a bit of vapor left in the atmosphere.
Roughly 3.5 bya, the atmosphere had low amounts of H2, but whatever was present often reacted with CO2 to produce methane, resulting in a slight increase of the compound in the atmosphere.
CO2 + H2 = CH4 + H2O
As plant life started to become much more complex and abundant, oxygen levels steeply increased due to widespread photosynthesis
Today, atmospheric composition hasn't changed much. However, since the industrial revolution, harmful greenhouse gases like CO2 and CH4 have become concentrated.