Life's impact on Earth's atmosphere

The big idea: The air you are breathing was made by living things.

When the Earth first formed, the atmosphere had almost no oxygen and a lot of carbon dioxide.

Over billions of years, living organisms changed it: they added oxygen and removed most of the carbon dioxide.

So the modern, oxygen-rich air is not a starting condition — it is a product of life acting over geological time.

Geological time

The vast timescale of Earth's history — millions to billions of years.

Early (reducing) atmosphere

The first atmosphere — almost no free oxygen and a high level of carbon dioxide, formed mainly from volcanic gases.

Modern (oxidising) atmosphere

Today's air — about 21% oxygen and very little carbon dioxide, built up by living things.

Photosynthesis

The process in which plants, algae and cyanobacteria use light to convert carbon dioxide and water into glucose, releasing oxygen as a waste product.

Great Oxidation Event

The point in Earth's history, billions of years ago, when oxygen from photosynthesis built up in the atmosphere for the first time.

Two changes — keep them straight: Life caused two opposite changes to the air, and the exam wants both:

Oxygen WENT UP ⬆️ (from almost nothing to ~21%).

Carbon dioxide WENT DOWN ⬇️ (from very high to very low).

Both were driven mainly by photosynthesis.

There are two changes to outline, and each is a clean chain of cause and effect — an organism does something, and the air changes as a result.

Take them one at a time.

Change 1 — oxygen rose (the Great Oxidation Event): The first organisms to photosynthesise were cyanobacteria (in ancient oceans).

Photosynthesis releases oxygen as a waste product.

At first the oxygen reacted with minerals (such as iron) and the air stayed low in O₂. Once those 'sinks' were full, oxygen built up in the atmosphere — this is the Great Oxidation Event.

Later, algae and then land plants kept photosynthesising, raising oxygen to today's ~21%.

Change 2 — carbon dioxide fell: Carbon dioxide dropped for two linked reasons, both caused by life.

Photosynthesis fixes CO₂ — it takes carbon dioxide out of the air and builds it into organic carbon (glucose, then the bodies of organisms).

Much of that carbon was then locked away:

Organic carbon was buried — dead organisms became fossil fuels (coal, oil, gas) and carbon-rich sediments.

Carbonate was locked in rock — marine organisms used CO₂-derived carbon to build shells, which became limestone.

With carbon trapped underground and in rock, atmospheric CO₂ fell to today's low level.

Why this mattered for life: The oxygen that life added then made more complex life possible.

Oxygen allowed efficient aerobic respiration (far more energy than anaerobic).

Oxygen high in the atmosphere formed the ozone layer, which blocks harmful UV and let life move onto land.

So life changed the air — and the new air then changed what life could become.

How this is tested: The signature ask is a 4-mark Outline on Paper 2: outline the changes living organisms have caused to Earth's atmosphere.

Four marks means four separate scoring points — not one idea written four ways.

The safe four are: oxygen rose; the cause is photosynthesis (by cyanobacteria, then algae/plants); carbon dioxide fell; the cause is photosynthesis fixing carbon that was then buried (fossil fuels) or locked in limestone. Naming the Great Oxidation Event is a nice extra.

✓ Why this scores full marks: It gives two changes, each with its cause — that is four distinct points.

A common way to lose marks is to write only about oxygen (max 2) and forget the carbon dioxide half, or to say the air changed without naming photosynthesis as the cause.