Detailed Explanation:

Answer: Option 3 — 1 and 3 only

This question tests knowledge about Earth's magnetic field reversals and the evolution of Earth's atmosphere. Statements 1 and 3 are scientifically accurate, while Statement 2 contains factual errors about the early Earth's atmospheric composition.

✅ Statement 1 – Correct: The Earth's magnetic field undergoes geomagnetic reversals approximately every few hundred thousand years, a phenomenon recorded in volcanic rocks and ocean floor sediments.

❌ Statement 2 – Incorrect: The early Earth's atmosphere (over 4 billion years ago) had virtually no free oxygen and was dominated by carbon dioxide, water vapor, nitrogen, and methane—not 54% oxygen.

✅ Statement 3 – Correct: Living organisms, particularly cyanobacteria through photosynthesis during the Great Oxygenation Event (~2.4 billion years ago), fundamentally transformed Earth's atmosphere by adding oxygen.

📝 Short Notes: Earth's Atmospheric Evolution

• Primordial Atmosphere: Early Earth (4+ billion years ago) had no free oxygen; atmosphere consisted mainly of hydrogen, helium, water vapor, methane, ammonia, and carbon dioxide.
• Secondary Atmosphere: Formed through volcanic outgassing, releasing CO₂, nitrogen, water vapor, and sulfur compounds.
• Great Oxygenation Event (GOE): Around 2.4 billion years ago, cyanobacteria began photosynthesis, gradually increasing atmospheric oxygen from ~0% to ~21% today.
• Geomagnetic Reversals: Earth's magnetic poles switch positions irregularly; last reversal occurred ~780,000 years ago (Brunhes-Matuyama reversal).
• Evidence of Reversals: Preserved in magnetized minerals in volcanic rocks (paleomagnetism) and oceanic crust spreading from mid-ocean ridges.
• Impact on Life: The oxygen revolution caused mass extinction of anaerobic organisms but enabled evolution of complex aerobic life forms.