18. During the early origin of earth oxygen was absent in environment. Later on the oxygen increased and reached to present level. The main source of oxygen was
    (1) Photosynthesis
    (2) Released from CaCO₃
    (3) Escape of CO₂to environment
    (4) Escape of oxygen from internal sources

The Early Earth: A World Without Oxygen

Early Earth, during its first two billion years, had an atmosphere that was very different from today’s. The air was rich in gases such as methane, ammonia, hydrogen, and water vapor, but it was almost entirely devoid of free oxygen (O₂). This “reducing” atmosphere was suitable for the chemistry that led to the origin of life, but it could not support oxygen-dependent (aerobic) organisms.

Free oxygen is highly reactive and would have quickly combined with other elements if it had been present in significant amounts. The absence of oxygen allowed organic molecules to accumulate and interact, setting the stage for the emergence of the first living cells.

The Rise of Oxygen: The Great Oxygenation Event

About 2.3 to 2.4 billion years ago, a dramatic change occurred in Earth’s atmosphere. Oxygen began to accumulate, marking the start of the Great Oxygenation Event (GOE). This event was not instantaneous but unfolded over hundreds of millions of years, fundamentally altering the planet’s chemistry and biology.

The GOE had several profound effects:

• Extinction of Anaerobic Life:

  • The rise of oxygen was toxic to many early microorganisms that had evolved in its absence, leading to a mass extinction of anaerobic life.

• Evolution of Aerobic Organisms:

  • The presence of oxygen allowed the evolution of new life forms that could use oxygen for respiration, a much more efficient way to generate energy.

• Formation of the Ozone Layer:

  • Oxygen in the upper atmosphere formed ozone (O₃), which absorbs harmful ultraviolet radiation and protects life on the surface.

The Main Source of Atmospheric Oxygen

Among the options provided—photosynthesis, release from CaCO₃ (calcium carbonate), escape of CO₂ to the environment, and escape of oxygen from internal sources—photosynthesis is the primary source of Earth’s atmospheric oxygen.

1. Photosynthesis

• Process:

  • Oxygenic photosynthesis is performed by cyanobacteria, algae, and plants. These organisms use sunlight to convert carbon dioxide and water into glucose and oxygen.

• Impact:

  • The oxygen released as a byproduct accumulates in the atmosphere, gradually increasing its concentration over time.

• Evidence:

  • Geological records, such as banded iron formations and isotopic signatures in ancient rocks, provide clear evidence that the rise of atmospheric oxygen coincided with the proliferation of photosynthetic microorganisms.

• Role in Evolution:

  • The increase in oxygen enabled the evolution of multicellular organisms and complex life forms, including animals and, eventually, humans.

2. Release from CaCO₃ (Calcium Carbonate)

• Process:

  • Calcium carbonate (CaCO₃) is a mineral found in limestone, marble, and chalk. It does not release oxygen under normal Earth conditions.

• Impact:

  • Weathering or heating of CaCO₃ can release carbon dioxide (CO₂), not oxygen.

• Conclusion:

  • This process is not a significant source of atmospheric oxygen.

3. Escape of CO₂ to the Environment

• Process:

  • Carbon dioxide (CO₂) is a greenhouse gas, not a source of oxygen.

• Impact:

  • The release of CO₂ into the atmosphere does not increase oxygen levels.

• Conclusion:

  • This process does not contribute to atmospheric oxygen.

4. Escape of Oxygen from Internal Sources

• Process:

  • Some oxygen is released during volcanic activity and other geological processes, but the amounts are negligible compared to biological sources.

• Impact:

  • Geological oxygen release is far too small to account for the dramatic rise in atmospheric oxygen observed during the GOE.

• Conclusion:

  • This process is not the main source of Earth’s oxygen.

The Role of Photosynthetic Organisms

Cyanobacteria, also known as blue-green algae, were the first organisms to perform oxygenic photosynthesis. Their ability to use sunlight to split water molecules and release oxygen as a waste product was a revolutionary innovation in the history of life. Over time, these microorganisms became abundant in the oceans, producing vast amounts of oxygen that gradually accumulated in the atmosphere.

The rise of land plants about 450 million years ago further increased oxygen production, leading to the high levels we experience today. This final surge in oxygen enabled the evolution of large, complex terrestrial animals.

The Consequences of Oxygenation

The rise of atmospheric oxygen had far-reaching consequences for life on Earth:

• Extinction of Anaerobic Organisms:

  • Many early microorganisms could not survive in the presence of oxygen and went extinct or retreated to oxygen-free environments.

• Evolution of Aerobic Respiration:

  • Oxygen allowed organisms to extract more energy from organic molecules, supporting the evolution of larger and more complex life forms.

• Formation of the Ozone Layer:

  • Oxygen in the upper atmosphere formed ozone, protecting life from harmful ultraviolet radiation.

• Changes in Climate and Chemistry:

  • The oxidation of methane and other greenhouse gases contributed to global cooling and the formation of new minerals.

The Stability of Atmospheric Oxygen

Today, atmospheric oxygen is maintained at about 21% by a balance between oxygen production (mainly from photosynthesis) and oxygen consumption (by respiration, combustion, and geological processes). The residence time of an oxygen molecule in the atmosphere is about two million years, meaning that the oxygen we breathe is constantly being replenished by photosynthetic organisms.

Key Takeaways

• Early Earth’s atmosphere contained almost no free oxygen.

• The main source of Earth’s atmospheric oxygen is photosynthesis, performed by cyanobacteria, algae, and plants.

• The Great Oxygenation Event, about 2.3 to 2.4 billion years ago, marked the beginning of significant oxygen accumulation.

• Other sources of oxygen, such as release from CaCO₃, escape of CO₂, or internal geological sources, are negligible compared to photosynthesis.

• The rise of oxygen enabled the evolution of complex life and the formation of the ozone layer.

• The correct answer to the question is:

  (1) Photosynthesis

Summary Table

Conclusion

The main source of Earth’s atmospheric oxygen is photosynthesis. This process, performed by cyanobacteria, algae, and plants, transformed the planet’s atmosphere from an oxygen-free environment to one that could support complex, oxygen-dependent life. The Great Oxygenation Event marked a turning point in Earth’s history, paving the way for the evolution of multicellular organisms and the rise of life as we know it.

In summary, the correct answer is:

(1) Photosynthesis