Which one of the following gas was not present in free form at the time life originated on earth?

Options

A. Ammonia
B. Oxygen
C. Hydrogen
D. Methane

Answer

To solve the question regarding which gas was not present in free form at the time life originated on Earth, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Timeline**:
- The Earth was formed approximately 4 billion years ago.
- Life is believed to have originated around 3.5 billion years ago.

2. **Types of Organisms**:
- The first organisms were anaerobic, meaning they did not require oxygen for survival.
- Aerobic organisms, which do require oxygen, developed later.

3. **Composition of the Early Atmosphere**:
- The early Earth's atmosphere was reducing in nature, meaning it lacked free oxygen (O2).
- Instead, it was composed of gases like methane, ammonia, hydrogen, and water vapor, but not free oxygen.

4. **Role of Cyanobacteria**:
- It was only after the development of autotrophic organisms, such as cyanobacteria, that oxygen began to be produced through photosynthesis, leading to an increase in free O2 in the atmosphere.

5. **Conclusion**:
- Since the question asks for the gas that was not present in free form when life originated, the answer is free oxygen (O2).

### Final Answer:
The gas that was not present in free form at the time life originated on Earth is **Oxygen (O2)**.

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