Evolution of the atmosphere (AQA GCSE Chemistry): Revision Notes
Evolution of the atmosphere
How Earth's atmosphere changed over time
The atmosphere we breathe today is very different from Earth's early atmosphere. About 2.7 billion years ago, tiny organisms called algae first appeared. These algae started a process called photosynthesis, which completely changed our planet's atmosphere over millions of years.
This transformation of Earth's atmosphere was one of the most dramatic changes in our planet's history, taking hundreds of millions of years to complete and making complex life possible.
The role of photosynthesis
Photosynthesis is a process where plants and algae use sunlight to make food. This process had a massive impact on our atmosphere.
What photosynthesis does:
- Takes in carbon dioxide from the air
- Releases oxygen into the air
- Uses water and sunlight to make glucose (sugar)
The photosynthesis equation:
carbon dioxide + water → glucose + oxygen
As photosynthesis became more common on Earth, two major changes happened to the atmosphere:
- Carbon dioxide levels went down (because plants were using it up)
- Oxygen levels went up (because plants were releasing it)
This increase in oxygen was essential for complex life to develop. Without photosynthesis, animals as we know them could never have evolved on Earth.
When animals could survive
About 1 billion years after algae first appeared, there was finally enough oxygen in the atmosphere for animals to evolve and survive. This shows just how important photosynthesis was for life on Earth.
Formation of fossil fuels and limestone
Over millions of years, dead plants and tiny sea creatures called plankton got buried and compressed. This created our fossil fuels:
- Coal - formed from dead plants (coal is a type of sedimentary rock)
- Crude oil and natural gas - formed from dead sea creatures and contain hydrocarbons
The carbon in these fossil fuels originally came from carbon dioxide in the atmosphere when these organisms were alive.
This process of fossil fuel formation took millions of years and required very specific conditions of burial, pressure, and heat. The fossil fuels we use today are essentially stored ancient sunlight energy.
The carbon cycle
Carbon moves around our planet in a continuous cycle:
- Photosynthesis - plants take CO₂ from the air
- Animals eat plants - carbon moves to animals
- Death and decay - dead organisms can form fossil fuels over time
- Dissolving in oceans - CO₂ dissolves in seawater
- Formation of carbonates - sea creatures use dissolved CO₂ to make shells
- Limestone formation - when these creatures die, their shells form limestone rock
Key example: oceans and carbon dioxide
Worked Example: How Oceans Remove Atmospheric CO₂
The oceans play a huge role in removing carbon dioxide from the atmosphere. Here's the step-by-step process:
Step 1: Dissolution CO₂ dissolves in seawater
Step 2: Biological uptake
Sea creatures use this dissolved CO₂ to make their shells and skeletons
Step 3: Death and sedimentation When these creatures die, their shells sink to the seabed
Step 4: Rock formation Over time, this forms limestone rock
Step 5: Carbon storage The CO₂ becomes "locked away" in the rock as calcium carbonate
This process also helps create crude oil and natural gas deposits.
Key Points to Remember:
- 2.7 billion years ago - algae evolved and photosynthesis began
- Photosynthesis decreased CO₂ and increased O₂ in the atmosphere
- Animals could evolve once there was enough oxygen (1 billion years later)
- Fossil fuels formed from dead plants and sea creatures over millions of years
- The carbon cycle shows how carbon moves between air, plants, animals, oceans and rocks
- Oceans remove CO₂ by dissolving it and turning it into limestone through sea creatures