Name the first oxygenic photoautotrophic organism. What was the effect of evolved oxygen on atmospheric gases of reducing atmosphere of that period ?

Explanation

**Step-by-Step Solution:**

1. **Identify the First Oxygenic Photoautotrophic Organism:**
The first organism that is recognized as an oxygenic photoautotroph is **cyanobacteria**. These organisms are capable of photosynthesis, using sunlight to convert carbon dioxide and water into glucose and oxygen.

2. **Understand the Early Earth's Atmosphere:**
Approximately 4.5 to 5 billion years ago, the Earth's atmosphere was primarily reducing, meaning it lacked free oxygen. The environment was anaerobic, and the first organisms, such as archaebacteria, thrived without oxygen.

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