The main gas that makes up the Earth's atmosphere is nitrogen, which constitutes about 78% of the atmosphere by volume. Oxygen follows as the second most abundant gas, comprising around 21%. Other gases, such as argon, carbon dioxide, and trace gases, make up the remaining portion. Nitrogen is essential for living organisms as it is a crucial component of amino acids and nucleic acids, but it's relatively inert and does not easily react. The composition of the atmosphere is vital for life and influences weather patterns, climate, and the overall health of ecosystems on Earth.