Explain how the early atmosphere of the Earth changed to produce the atmosphere we have today, with reference to the role of photosynthesis and the formation of fossil fuels.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Scientists believe the early atmosphere of the Earth was very different from the atmosphere we breathe today. Over billions of years, a series of processes changed the composition of the atmosphere dramatically.
Model answer (4 marks)
The early atmosphere was rich in CO₂ and almost devoid of O₂.
Photosynthesis by cyanobacteria and later plants removed CO₂ from the air and released O₂ as a by‑product.
The organic matter produced was buried in sediments; over geological time it was transformed into fossil fuels, sequestering carbon and preventing it from returning to the atmosphere.
Consequently, atmospheric O₂ rose to about 21 % while CO₂ fell to ~0.04 %, giving the present‑day composition.
Photosynthesis by cyanobacteria and later plants removed CO₂ from the air and released O₂ as a by‑product.
The organic matter produced was buried in sediments; over geological time it was transformed into fossil fuels, sequestering carbon and preventing it from returning to the atmosphere.
Consequently, atmospheric O₂ rose to about 21 % while CO₂ fell to ~0.04 %, giving the present‑day composition.
Examiner tips
- Use the word ‘removed’ for CO₂ and ‘released’ for O₂ to show cause‑effect. Mention fossil fuels as a carbon sink. Show the final percentages to demonstrate understanding of the current atmosphere.
Common mistakes
- Confusing the order of events (e.g., saying fossil fuels formed before photosynthesis). Failing to mention the role of photosynthesis in producing O₂. Using vague terms like ‘changed’ without specifying CO₂ and O₂ levels.
Mark scheme (4 marks)
- The early atmosphere contained large amounts of carbon dioxide (and little or no oxygen).
- Photosynthesis by plants and algae removed carbon dioxide from the atmosphere and produced oxygen.
- The remains of ancient biomass (such as plankton) were buried in mud and over millions of years formed fossil fuels, locking carbon away from the atmosphere.
- As a result, oxygen levels increased and carbon dioxide levels decreased, giving the atmosphere its current composition (approximately 21% oxygen and 0.04% carbon dioxide).
Key terms in this question
photosynthesis · fossil fuels · atmosphere
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