The atmosphere became oxidising with the appearance of

Options

A. Autotrophic bacteria
B. Autotropic cyanobacteria
C. Land plants
D. Algae

Answer

**Step-by-Step Solution:**

1. **Understanding Early Earth Conditions**: The Earth was formed around 4 billion years ago, and the first life forms appeared approximately 3.5 billion years ago. Initially, the atmosphere was reducing in nature, characterized by the absence of free oxygen (O2) and the presence of gases such as methane (CH4) and ammonia (NH3).

**Hint**: Consider the timeline of Earth's formation and the conditions that existed before life emerged.

2. **First Life Forms**: The first organisms to appear were anaerobes, which are organisms that can survive without oxygen. These anaerobic organisms were primarily heterotrophs, meaning they obtained their food from organic materials rather than producing it themselves.

**Hint**: Think about the types of organisms that can thrive in environments without oxygen.

3. **Transition to Autotrophs**: As the anaerobic organisms consumed available organic materials, these resources became exhausted. This led to the evolution of autotrophs, which are organisms capable of producing their own food.

**Hint**: Reflect on how the depletion of resources can drive evolutionary changes in organisms.

4. **Types of Autotrophs**: The first autotrophs were chemoautotrophs, which synthesized organic molecules from inorganic materials through chemosynthesis. However, they remained anaerobic.

**Hint**: Differentiate between the types of autotrophs and their methods of obtaining energy.

5. **Emergence of Photoautotrophs**: Eventually, photoautotrophs emerged, which developed chlorophyll and could produce organic food using solar energy. Initially, these organisms did not release oxygen.

**Hint**: Consider the role of sunlight in the evolution of life and how it affects energy production.

6. **Oxygen-Producing Organisms**: Over time, oxygen-producing organisms, such as cyanobacteria, evolved. These organisms utilized water (H2O) to release oxygen (O2) as a byproduct of photosynthesis.

**Hint**: Think about the significance of water in the process of photosynthesis and the release of oxygen.

7. **Impact on the Atmosphere**: The oxygen produced by cyanobacteria combined with methane and ammonia in the atmosphere, leading to the formation of carbon dioxide (CO2) and nitrogen compounds. This process contributed to the transition of the atmosphere from a reducing to an oxidizing state.

**Hint**: Consider how the chemical reactions involving oxygen transformed the early atmosphere.

8. **Conclusion**: The atmosphere became oxidizing primarily due to the appearance of autotrophic cyanobacteria, which were capable of producing oxygen through photosynthesis.

**Final Answer**: The atmosphere became oxidizing with the appearance of autotrophic cyanobacteria.