First photosynthetic bacteria might have appeared around

Options

A. 3.8-4.2 billion years ago
B. 3.5-3.8 billion years ago
C. 3.3-3.5 billion years ago
D. 2.6-2.8 billion years ago

Answer

To determine when the first photosynthetic bacteria might have appeared, we need to analyze the provided options based on geological time scales and the evolution of life on Earth.

### Step-by-Step Solution:

1. **Understand the Geological Time Scale**: The Earth is approximately 4.5 billion years old. The first life forms are believed to have appeared shortly after the planet cooled enough to support them.

2. **Evaluate the Options**:
- **Option 1: 3.8 to 4.2 billion years ago**: This time frame is very early in Earth's history, around the time the solar system was forming. It is unlikely that photosynthetic bacteria existed at this time.
- **Option 2: 3.5 to 3.8 billion years ago**: This period is more plausible for the emergence of the first photosynthetic bacteria. Evidence suggests that simple life forms, including photosynthetic bacteria, began to appear during this time.
- **Option 3: 3.3 to 3.5 billion years ago**: This timeframe is close to the previous one but is more associated with the emergence of green algae rather than the first photosynthetic bacteria.
- **Option 4: 2.6 to 2.8 billion years ago**: This period marks the evolution of multicellular organisms and more complex life forms, which is too late for the first photosynthetic bacteria.

3. **Select the Correct Option**: Based on the analysis, the most accurate option for when the first photosynthetic bacteria might have appeared is **Option 2: 3.5 to 3.8 billion years ago**.

### Final Answer:
The first photosynthetic bacteria might have appeared around **3.5 to 3.8 billion years ago**.

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