Which organism is believed to have contributed to the addition of oxygen $O_2$ to Earth's early atmosphere?

• A. Algae
• B. Yeast
• C. Cyanobacteria
• D. Fungi

Answer: (C)

Verified step by step guidance

1. Understand the context: The problem is asking about the organism responsible for adding oxygen to Earth's early atmosphere.
2. Identify the process: The process responsible for adding oxygen to the atmosphere is photosynthesis, which converts carbon dioxide and water into glucose and oxygen.
3. Recognize the key organism: Cyanobacteria, also known as blue-green algae, are known for being one of the first organisms to perform oxygenic photosynthesis.
4. Differentiate from other organisms: Algae, yeast, and fungi are not known for contributing to the initial oxygenation of the atmosphere. Algae are more complex and evolved later, while yeast and fungi do not perform photosynthesis.
5. Conclude with the correct organism: Based on the process of photosynthesis and historical evidence, cyanobacteria are believed to have played a crucial role in oxygenating Earth's early atmosphere.