Evolution of the atmosphere Revision Notes for AQA GCSE Chemistry Combined Science

Evolution of the atmosphere

The early atmosphere changes

About 2.7 billion years ago, something important happened on Earth. Algae began to evolve in the oceans. This was the start of photosynthesis - a process that would completely change our planet's atmosphere.

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This moment in Earth's history marked the beginning of one of the most significant environmental changes our planet has ever experienced. The evolution of photosynthesis didn't just change the atmosphere - it made complex life as we know it possible.

When algae and later plants started doing photosynthesis, they began changing the air around Earth. This process had two major effects:

Carbon dioxide levels went down
Oxygen levels went up

How photosynthesis changed everything

Photosynthesis is the process where plants and algae use sunlight to make food. During this process:

Plants take in carbon dioxide from the air
Plants give out oxygen into the air

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Chemical Equation for Photosynthesis:

The process can be represented by the chemical equation:

$\text{Carbon dioxide} + \text{Water} \rightarrow \text{Glucose} + \text{Oxygen}$

Or in chemical notation: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

This process happened for billions of years. Slowly, the atmosphere changed from having lots of carbon dioxide to having the oxygen-rich air we breathe today.

When oxygen levels increased

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Once there was enough oxygen in the atmosphere, animals could start to evolve. This happened about one billion years after algae first appeared. Animals need oxygen to survive, so they couldn't exist before photosynthesis created it.

What happened to all the carbon?

You might wonder - if plants removed so much carbon dioxide from the air, where did all that carbon go? It ended up in two main places:

Fossil fuels

When plants and tiny sea creatures called plankton died millions of years ago, they didn't just disappear. Their remains got buried and compressed over time. This formed:

Coal (from dead plants)
Crude oil and natural gas (from dead plankton and sea creatures)

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The carbon that was originally in the atmosphere when these organisms were alive became "locked up" in these fossil fuels. This is why burning fossil fuels today releases ancient carbon dioxide back into the atmosphere.

Limestone rocks

Carbon dioxide dissolves in seawater. Sea creatures use this dissolved carbon dioxide to make their shells and skeletons. When these creatures die, their shells sink to the seabed and eventually form limestone - a type of sedimentary rock.

This is another way carbon dioxide gets removed from the atmosphere and becomes "locked away" for millions of years.

The complete cycle

Here's how the whole process worked:

Photosynthesis by plants removes carbon dioxide from air
Some plants and sea creatures die and form fossil fuels
Carbon dioxide dissolves in oceans
Sea creatures use dissolved carbon dioxide to make shells
Dead shells form limestone on the seabed

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All of these processes helped reduce the amount of carbon dioxide in our atmosphere over billions of years, creating the conditions necessary for complex life to thrive.

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Key Points to Remember:

Algae evolved 2.7 billion years ago and started photosynthesis
Photosynthesis decreased carbon dioxide and increased oxygen in the atmosphere
Animals could only evolve once there was enough oxygen in the air
Carbon from the atmosphere got locked up in fossil fuels and limestone rocks
The oceans help remove carbon dioxide by dissolving it and allowing sea creatures to use it for shells

Evolution of the atmosphere (AQA GCSE Chemistry Combined Science): Revision Notes