Methane (CH4): Methane was likely the major constituent of the early atmosphere.
Water Vapor (H2O): The early Earth was much hotter, allowing significant surface water to remain in vapor form.
Ammonia (NH3): Ammonia would have been present due to volcanic activity.
Carbon Dioxide (CO2): While present, CO2 levels were likely lower initially, and started building up later in Earth's history.
The concentrations of oxygen (O2) were very low or negligible in Earth's original atmosphere. The presence of free oxygen came significantly later through processes like photosynthesis after early forms of life emerged through various atmospheric and biological processes.
