What was the composition of the early Earth's atmosphere?

Evolutionary Biology is the study of history of life forms on earth. What exactly is evolution? To understand the changes in flora and fauna that have occurred over millions of years on earth, we must have an understanding of the context of origin of life, i.e., evolution of earth, of stars and indeed of the universe itself. Understanding the evolution of earth, stars, and the universe is essential for comprehending the changes in flora and fauna over time.The origin of life and evolution of life forms is referred to as the longest of all construed and conjectured stories. In the solar system of the milky way galaxy, earth was supposed to have been formed about 4.5 billion years back. There was no atmosphere on early earth. Water vapor, methane, carbon dioxide and ammonia released from molten mass covered the surface. The UV rays from the sun broke up water into Hydrogen and Oxygen and the lighter H₂ escaped. Oxygen combined with ammonia and methane to form water, CO₂ and others. The ozone layer was formed. As it cooled, the water vapor fell as rain, to fill all the depressions and form oceans. Life appeared 500 million years after the formation of earth, i.e., almost four billion years back.