How did the earliest single-celled microorganisms most likely produce energy to drive their metabolic processes?

• A. By anaerobic fermentation of organic molecules
• B. By photosynthesis using chlorophyll
• C. By chemosynthesis using sulfur compounds
• D. By aerobic respiration using oxygen

Answer: (A)

Verified step by step guidance

1. Understand the environmental conditions of early Earth, which lacked free oxygen in the atmosphere, making aerobic respiration unlikely for the earliest microorganisms.
2. Recognize that early single-celled organisms needed a way to generate energy without oxygen, so they relied on metabolic processes that do not require oxygen, such as anaerobic fermentation.
3. Recall that anaerobic fermentation involves breaking down organic molecules to produce energy in the absence of oxygen, which fits the conditions of early Earth.
4. Consider other options: photosynthesis using chlorophyll and chemosynthesis using sulfur compounds evolved later or under specific conditions, and aerobic respiration requires oxygen, which was not available initially.
5. Conclude that the most plausible energy production method for the earliest single-celled microorganisms was anaerobic fermentation of organic molecules.