Which of the following best explains why the first organisms on Earth were anaerobic?

• A. The early Earth had a high concentration of $O_2$ in the atmosphere.
• B. Anaerobic organisms can survive in high temperatures.
• C. The early Earth had a low concentration of $O_2$ in the atmosphere.
• D. Anaerobic respiration is more efficient than aerobic respiration.

Answer: (C)

Verified step by step guidance

1. Understand the concept of anaerobic organisms: Anaerobic organisms are those that do not require oxygen for growth and can even be poisoned by oxygen.
2. Consider the conditions of early Earth: The early Earth had a very different atmosphere compared to today, with a low concentration of oxygen.
3. Relate the atmospheric conditions to the survival of organisms: Since the early Earth had a low concentration of oxygen, organisms that could survive without oxygen, i.e., anaerobic organisms, were more likely to thrive.
4. Evaluate the efficiency of respiration types: Anaerobic respiration is less efficient than aerobic respiration in terms of energy production, but it allows survival in oxygen-poor environments.
5. Conclude why anaerobic organisms were the first: The low oxygen levels in the early Earth's atmosphere made it conducive for anaerobic organisms to be the first to evolve and survive.