20. The first living beings on earth were anaerobic because
    (1) There was no oxygen in air
    (2) oxygen damages proteins
    (3) oxygen interferes with the action of ribozymes
    (4) they evolved in deep sea

The Early Earth: An Oxygen-Free World

Billions of years ago, Earth’s atmosphere was vastly different from today. It was composed mainly of gases such as hydrogen, methane, ammonia, water vapor, and carbon dioxide—but it lacked free oxygen (O₂). This “reducing” environment was crucial for the formation and stability of organic molecules, which are the building blocks of life.

The absence of free oxygen meant that the first living beings had to survive without it. Oxygen is a highly reactive molecule; in an environment rich in oxygen, most organic compounds—including those essential for life—would be rapidly oxidized and destroyed. Therefore, the earliest organisms evolved biochemical pathways that did not require oxygen and, in fact, were often harmed by its presence.

Why Were the First Living Beings Anaerobic?

Several factors contributed to the anaerobic nature of early life. Let’s examine each of the options provided and determine which is most accurate:

1. There Was No Oxygen in the Air

• Explanation:

  • The most fundamental reason early life was anaerobic is that Earth’s atmosphere lacked free oxygen. Oxygen only began to accumulate later, thanks to the evolution of oxygenic photosynthesis by cyanobacteria.

• Evidence:

  • Geological and chemical analyses of ancient rocks show that free oxygen was absent from the early atmosphere.

  • The Great Oxygenation Event, which marked the rise of atmospheric oxygen, occurred about 2.4 billion years ago—long after the first life forms had evolved.

2. Oxygen Damages Proteins

• Explanation:

  • While it is true that oxygen can damage proteins and other biomolecules by causing oxidative stress, this is not the primary reason early life was anaerobic.

• Evidence:

  • Oxidative damage is a secondary effect. The main reason early organisms were anaerobic was the absence of oxygen in their environment, not its harmful effects per se.

3. Oxygen Interferes with the Action of Ribozymes

• Explanation:

  • Ribozymes are RNA molecules with catalytic activity. Some ribozymes can be sensitive to oxygen, but this is not the main reason early life was anaerobic.

• Evidence:

  • The sensitivity of ribozymes to oxygen is a biochemical detail and not the overarching explanation for the anaerobic nature of early life.

4. They Evolved in the Deep Sea

• Explanation:

  • Some early life may have evolved in deep-sea environments, such as hydrothermal vents, which are often low in oxygen. However, this is not the primary reason early life was anaerobic.

• Evidence:

  • The deep sea is one possible habitat for early life, but the main reason for their anaerobic nature was the global absence of oxygen in the atmosphere and oceans.

The Biochemical Implications of an Oxygen-Free Environment

In the absence of oxygen, early organisms relied on anaerobic metabolic pathways, such as fermentation and anaerobic respiration, to generate energy. These pathways are less efficient than aerobic respiration but are well-suited to environments where oxygen is absent or toxic.

The presence of oxygen would have been detrimental to these early life forms for several reasons:

• Oxidative Damage:

  • Oxygen can react with and damage proteins, lipids, and nucleic acids, leading to cell death.

• Inhibition of Anaerobic Enzymes:

  • Many enzymes involved in anaerobic metabolism are inhibited or destroyed by oxygen.

• Limited Energy Production:

  • Aerobic respiration, which uses oxygen, is much more efficient at producing energy, but the enzymes and pathways required for it evolved later.

The Transition to an Oxygen-Rich World

The rise of atmospheric oxygen, known as the Great Oxygenation Event, was a turning point in Earth’s history. As oxygen began to accumulate, many anaerobic organisms went extinct or were forced into oxygen-free niches. However, some organisms evolved mechanisms to tolerate or even utilize oxygen, leading to the emergence of aerobic life forms.

The ability to use oxygen for respiration provided a significant evolutionary advantage, as aerobic metabolism is much more efficient at generating energy. This allowed for the evolution of larger, more complex organisms and paved the way for the diversity of life we see today.

Key Takeaways

• The first living beings on Earth were anaerobic because there was no oxygen in the atmosphere.

• Oxygen is highly reactive and can damage biomolecules, but its absence was the main reason early life was anaerobic.

• The sensitivity of ribozymes to oxygen and the possible deep-sea origin of early life are secondary factors.

• The correct answer to the question is:

  (1) There was no oxygen in air

Summary Table

Conclusion

The first living beings on Earth were anaerobic because the early atmosphere and oceans lacked free oxygen. This absence of oxygen was essential for the formation and survival of the earliest life forms, allowing them to thrive in a world that would have been toxic to most modern organisms. The rise of oxygen later in Earth’s history transformed the planet and set the stage for the evolution of complex, aerobic life.

In summary, the correct answer is:

(1) There was no oxygen in air