On the earth surface, reducing atmosphere changed into oxidizing atmosphere with the release of free oxygen. This took place due to the

Options

A. Evolution of oxygenic photoautotrophs
B. Evolution of anoxygenic photoautotrophs
C. Evolution of chemo-photoautotrophs
D. Evolution of anaerobic heterotrophs

Answer

**Step-by-Step Solution:**

1. **Understanding the Atmosphere**: Initially, the Earth's atmosphere was reducing, meaning it lacked free oxygen. This environment was rich in gases like methane, ammonia, and hydrogen, but had very little oxygen.

2. **Transition to Oxidizing Atmosphere**: Over time, this reducing atmosphere transitioned to an oxidizing atmosphere, which is characterized by the presence of free oxygen. This transition was significant for the evolution of life on Earth.

3. **Role of Organisms**: The key organisms responsible for this change were the cyanobacteria, which are a type of oxygenic photoautotroph. These organisms are capable of photosynthesis, a process that converts carbon dioxide (CO2) and sunlight into glucose and oxygen.

4. **Cyanobacteria's Contribution**: Cyanobacteria first appeared around 4 billion years ago and began to produce oxygen as a byproduct of photosynthesis. This release of oxygen into the atmosphere gradually increased the levels of free oxygen, leading to the oxidizing atmosphere we have today.

5. **Conclusion**: Therefore, the transition from a reducing atmosphere to an oxidizing atmosphere was primarily due to the evolution of oxygenic photoautotrophs, specifically cyanobacteria, which utilized sunlight and CO2 to produce oxygen.

**Final Answer**: The transition from a reducing atmosphere to an oxidizing atmosphere occurred due to the evolution of oxygenic photoautotrophs, specifically cyanobacteria.

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