The Earth's early atmosphere was different to Earth's atmosphere today. Scientists think that the Earth's early atmosphere was like the atmosphere found on Venus today. Table 7 shows the amounts of carbon dioxide and oxygen in the atmospheres of Venus and Earth today.
The percentages of carbon dioxide and oxygen have changed from Earth's early atmosphere to Earth's atmosphere today. Explain the processes that led to these changes.

This question requires an explanation for two major changes in Earth's atmosphere over billions of years. **1. The Decrease in Carbon Dioxide:** The early atmosphere, like that of Venus, was dominated by carbon dioxide (approx. 96.5%). The level has dropped to just 0.04% today. This happened through several key processes: - **Formation of Oceans:** As the Earth cooled, water vapour in the atmosphere condensed and formed vast oceans. A large amount of carbon dioxide from the atmosphere dissolved into these oceans. - **Formation of Sedimentary Rocks and Fossil Fuels:** In the oceans, the dissolved carbon dioxide reacted to form carbonate precipitates. These built up on the seabed and, over geological time, formed sedimentary rocks such as limestone (calcium carbonate). This process locked away huge quantities of carbon dioxide. Additionally, carbon from dead marine organisms was buried and compressed, forming fossil fuels. - **Photosynthesis:** About 2.7 billion years ago, algae evolved and began to photosynthesise. Photosynthesis uses carbon dioxide from the atmosphere (or water) and converts it into organic compounds (glucose) for growth. This biological process was a major factor in reducing atmospheric CO₂. **2. The Increase in Oxygen:** The early atmosphere was virtually free of oxygen (0.00%). The rise to today's level of about 21% is due to the process of **photosynthesis**. - Photosynthesis, carried out first by algae and later by plants, releases oxygen as a waste product. The equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ - Over billions of years, the continuous release of oxygen by these organisms gradually changed the composition of the atmosphere, allowing for the evolution of aerobic life, including animals.