In the 4.6 billion years of Earth’s history, the composition of the atmosphere has changed from a hazy, unfamiliar mix to today’s mostly blue skies. As the atmosphere developed, life began and evolved. The evolution of living things changed the atmosphere, and those changes in turn altered life. As far as we know, the relationship is unique to our planet.
To deduce what the atmosphere has been like for billions of years, paleontologists, geologists, and paleoclimatologists study rocks, ancient soils, and fossils. With every new find, they improve and refine their understanding of ancient atmospheres.