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BIO NMDCAT Concept of Evolution
nmdcat.online July 14, 2026

23. The first living organisms are believed to have obtained energy primarily through:

A. Aerobic respiration
B. Photosynthesis
C. Anaerobic metabolism of organic molecules
✓
D. Chemosynthesis using atmospheric oxygen

📝 Explanation

Correct: The earliest organisms were anaerobic heterotrophs because oxygen was absent. A: Requires oxygen. B: Photosynthesis evolved later. D: Atmospheric oxygen was unavailable. Concept: Earliest metabolism.

📖 Additional Information

  • Aerobic respiration
  • Photosynthesis
  • Anaerobic metabolism of organic molecules
  • Chemosynthesis using atmospheric oxygen

Correct: The earliest organisms were anaerobic heterotrophs because oxygen was absent. A: Requires oxygen. B: Photosynthesis evolved later. D: Atmospheric oxygen was unavailable. Concept: Earliest metabolism.

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