Which molecules are believed to be absent in the atmosphere of the Earth before living cells were formed?

The correct answer is Oxygen.

Key Points

• Before the formation of living cells, the Earth's atmosphere lacked free oxygen (O2).
• This early atmosphere was primarily composed of methane (CH4), ammonia (NH3), water vapor (H2O), and hydrogen (H2).
• The absence of oxygen created a reducing atmosphere, which was crucial for the synthesis of organic molecules.
• Oxygen began to accumulate in the atmosphere around 2.4 billion years ago, during the Great Oxidation Event, due to the photosynthetic activity of cyanobacteria.

Additional Information

• Reducing Atmosphere
  • An atmosphere with low oxygen and high levels of reducing gases such as hydrogen, methane, and ammonia.
  • Such conditions are thought to be favorable for the formation of organic molecules, essential for the origin of life.
• Great Oxidation Event (GOE)
  • Occurred around 2.4 billion years ago.
  • Marked the transition from a reducing to an oxidizing atmosphere due to the biological activity of photosynthetic organisms.
  • Led to the accumulation of free oxygen in the atmosphere.
• Cyanobacteria
  • Photosynthetic microorganisms that played a significant role in oxygenating the early atmosphere.
  • Often referred to as blue-green algae, they were among the first organisms to perform oxygenic photosynthesis.
• Early Earth's Atmosphere
  • Comprised primarily of volcanic gases such as carbon dioxide, water vapor, methane, and ammonia.
  • Lacked free oxygen until the advent of photosynthetic life forms.