Volcanoes released gases like water vapor, carbon dioxide, methane, and ammonia.

The early atmosphere was toxic, no oxygen and not suitable for life, and no protection from the sun's radiation.

Source:https://www.usgs.gov/programs/VHP/volcanoes-can-affect-climate

This created nitrogen, hydrogen (H₂), and carbon dioxide .

Hydrogen escaped into space due to its low weight.

Water vapor stayed in the atmosphere and later helped form oxygen.

Source: https://geosci.uchicago.edu/~archer/deep_earth_readings/zahnle.2007.early_atm.pdf

These were the first organisms to do photosynthesis: using sunlight, water, and to make food and release oxygen.

Oxygen started building up in the atmosphere.

Some oxygen formed ozone in the upper atmosphere, creating the ozone layer, a protective layer from radiation from the sun.

Source: https://asm.org/articles/2022/february/the-great-oxidation-event-how-cyanobacteria-change

Life moved from oceans to land.

Green plants appeared and boosted oxygen levels.

Carbon dioxide levels dropped due to plant photosynthesis.

Source: https://www.scienceinschool.org/article/2023/when-plants-moved-ashore-and-changed-the-planet/

Atmosphere became suitable for complex animals and humans.

Current composition: 78% nitrogen, 21% oxygen, and small amounts of water vapor, and other gases.

Present day human activities are starting to deteriorate the Ozone layer.

Source: https://www.ebsco.com/research-starters/earth-and-atmospheric-sciences/atmospheres-structure-and-thermodynamics

https://www.britannica.com/video/organisms-carbon-cycling-oxygen-environment/-153055#:~:text=When%20they're%20exposed%20to,oxygen%20and%20release%20carbon%20dioxide.