Life's impact on Earth's atmosphere
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Question
What was Earth's early atmosphere like?
Answer
**Almost no free oxygen** and a **high level of carbon dioxide** (a 'reducing' atmosphere).
Question
What two main changes did living organisms cause to the atmosphere?
Answer
**Oxygen rose** (to ~21%) and **carbon dioxide fell** (to a low level).
Question
Which process raised atmospheric oxygen?
Answer
**Photosynthesis** — it releases oxygen as a waste product.
Question
Which organisms first added oxygen to the air?
Answer
**Cyanobacteria**, then later **algae and plants**.
Question
What is the Great Oxidation Event?
Answer
The time, billions of years ago, when **oxygen from photosynthesis built up** in the atmosphere for the first time.
Question
Why did carbon dioxide fall over geological time?
Answer
**Photosynthesis fixed CO₂** into organic carbon, which was then **buried as fossil fuels** or **locked in limestone**.
Question
Where was the removed carbon stored?
Answer
In **fossil fuels** (coal, oil, gas) and in **limestone** (carbonate from marine shells).
Question
Why did the rise in oxygen matter for life?
Answer
It enabled efficient **aerobic respiration** and formed the **ozone layer**, making **complex life** and life on land possible.
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Topic 4.11 hub
Stability and change
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