Earth’s atmosphere has changed dramatically over billions of years. The atmosphere we have today was formed through volcanic activity, life processes, and chemical reactions.

Stage 1: Early atmosphere (like Mars or Venus)

• The early Earth was covered in volcanoes, which released gases
• The atmosphere was mostly carbon dioxide (CO₂) with very little or no oxygen
• It also contained nitrogen, water vapour, and small amounts of methane and ammonia
• Very similar to the atmospheres of Mars and Venus today

Stage 2: Water vapour condensed to form oceans

• As Earth cooled, water vapour condensed and formed the oceans
• CO₂ dissolved into the oceans and formed carbonates
• Some carbon was locked into sedimentary rocks and fossil fuels

Stage 3: Life begins – oxygen increases

• Algae and plants evolved and carried out photosynthesis:
  CO₂ + H₂O → O₂ + glucose
• This reduced carbon dioxide levels and released oxygen
• Over time, oxygen levels increased, allowing animals to evolve

Stage 4: Atmosphere today

Today’s atmosphere is made up of:

• ~78% nitrogen
• ~21% oxygen
• ~1% other gases (argon, carbon dioxide, etc.)

💡 Human activities (like burning fossil fuels and deforestation) are now changing the atmosphere again, increasing carbon dioxide levels.