Which was absent/least abundant in the atmosphere at the time of origin of life?

Options

A. `NH_3`
B. `H_2`
C. `O_2`
D. `CH_4`

Answer

To determine which gas was absent or least abundant in the atmosphere at the time of the origin of life, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Formation of Earth**:
- The Earth was formed approximately 4.6 billion years ago. This is crucial for understanding the conditions of the early atmosphere.

2. **Identify the Type of Atmosphere**:
- During the time of Earth's formation, the atmosphere was characterized as a "reducing atmosphere." This means it lacked certain gases that are present in today's atmosphere.

3. **Recognize the Key Components of the Early Atmosphere**:
- The primary gases present in the early atmosphere included ammonia (NH3), hydrogen (H2), and methane (CH4). These gases were abundant and played a role in the chemical processes that led to the origin of life.

4. **Determine the Absence of Oxygen**:
- A significant characteristic of the reducing atmosphere is the absence of free oxygen (O2). Unlike today’s atmosphere, which is rich in oxygen due to photosynthetic organisms, the early atmosphere had no free oxygen available.

5. **Conclude the Answer**:
- Given that the question asks for the gas that was absent or least abundant, we can conclude that oxygen was the gas that was absent in the atmosphere at the time of the origin of life.

### Final Answer:
- The gas that was absent or least abundant in the atmosphere at the time of the origin of life is **Oxygen**.

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