2. The first oxygen evolving forms among the following were-
   (1) Anaerobic autotrophs (2) Cyanobacteria
   (3) Protists (4) Algae

The Early Earth: A World Without Oxygen

For the first two billion years of Earth’s existence, the atmosphere was devoid of free oxygen. The earliest life forms were simple, single-celled microbes that thrived in an environment rich in methane, ammonia, and other reducing gases. These organisms were anaerobic, meaning they did not require oxygen for their metabolism and, in fact, would have found oxygen toxic. The energy they used was derived from chemical reactions involving minerals and sunlight, but not through the process of oxygenic photosynthesis.

The Advent of Photosynthesis

Photosynthesis is the process by which certain organisms use sunlight to synthesize food from carbon dioxide and water, releasing oxygen as a byproduct. However, not all photosynthesis is the same. Early forms of photosynthesis, such as those carried out by anaerobic autotrophs, did not produce oxygen. Instead, they used other substances, such as hydrogen sulfide, to drive their metabolic processes.

The real breakthrough came with the evolution of oxygenic photosynthesis, a process that splits water molecules to release oxygen. This innovation had a profound impact on Earth’s atmosphere and biosphere.

The Rise of Cyanobacteria

Cyanobacteria, also known as blue-green algae, are the earliest known organisms to perform oxygenic photosynthesis. Genetic and fossil evidence suggests that cyanobacteria evolved at least 2.4 to 2.7 billion years ago, with some estimates placing their origins even earlier. These microbes were able to use sunlight, water, and carbon dioxide to produce energy, releasing oxygen as a waste product.

The ability of cyanobacteria to produce oxygen set off a cascade of environmental changes. Initially, the oxygen they released reacted with minerals in the oceans and atmosphere, preventing it from accumulating. However, as cyanobacteria proliferated, the rate of oxygen production eventually outpaced the ability of the environment to absorb it, leading to the gradual buildup of oxygen in the atmosphere.

The Great Oxygenation Event

The accumulation of oxygen in the atmosphere, known as the Great Oxygenation Event (GOE), occurred around 2.3 to 2.4 billion years ago. This event marked a turning point in Earth’s history, as the presence of free oxygen made the environment hostile to many anaerobic organisms but opened up new opportunities for oxygen-dependent life.

Evidence for the GOE is found in ancient rocks, such as banded iron formations, which formed when iron in the oceans reacted with the newly available oxygen. The rise of oxygen also led to the formation of the ozone layer, which protected life from harmful ultraviolet radiation and further enabled the evolution of complex organisms.

Why Cyanobacteria, Not Other Groups?

Among the options given—anaerobic autotrophs, cyanobacteria, protists, and algae—only cyanobacteria are recognized as the first oxygen-evolving life forms. Here’s why:

• Anaerobic Autotrophs:

  • These organisms do not produce oxygen. They use other substances, such as hydrogen sulfide, for photosynthesis and cannot split water molecules to release oxygen.

• Cyanobacteria:

  • These are the first known organisms to perform oxygenic photosynthesis, releasing oxygen as a byproduct. Their emergence is directly linked to the rise of atmospheric oxygen.

• Protists:

  • Protists are a diverse group of eukaryotic organisms that evolved much later, after the rise of oxygen in the atmosphere. They were not present during the early stages of oxygenic photosynthesis.

• Algae:

  • While modern algae are capable of oxygenic photosynthesis, they are eukaryotic organisms that evolved after cyanobacteria. The earliest algae appeared hundreds of millions of years after the first cyanobacteria.

The Legacy of Cyanobacteria

The impact of cyanobacteria on Earth’s history cannot be overstated. By releasing oxygen into the atmosphere, they transformed the planet’s chemistry and created the conditions necessary for the evolution of aerobic (oxygen-using) life. The rise of oxygen also led to the extinction of many anaerobic organisms, but it paved the way for the emergence of complex multicellular life, including animals and, eventually, humans.

Cyanobacteria continue to play a vital role in Earth’s ecosystems today. They are found in a wide range of environments, from freshwater lakes to the open ocean, and are responsible for a significant portion of the planet’s oxygen production.

Key Milestones in the Evolution of Oxygen-Evolving Life

1. Origin of Life (3.8–3.5 billion years ago):

   • The first life forms were anaerobic microbes that did not produce oxygen.

2. Evolution of Oxygenic Photosynthesis (2.7–2.4 billion years ago):

   • Cyanobacteria evolve the ability to split water and release oxygen.

3. Great Oxygenation Event (2.3–2.4 billion years ago):

   • Oxygen begins to accumulate in the atmosphere, leading to the formation of banded iron formations and the ozone layer.

4. Rise of Complex Life:

   • The presence of oxygen enables the evolution of aerobic organisms, including protists, algae, and eventually animals.

Why Not Protists or Algae?

Protists and algae are both eukaryotic organisms, meaning their cells contain a nucleus and other organelles. Eukaryotes evolved after the rise of oxygen in the atmosphere, making it impossible for them to have been the first oxygen-evolving life forms. The ability to perform oxygenic photosynthesis in eukaryotes was acquired through a process called endosymbiosis, in which ancestral eukaryotic cells engulfed cyanobacteria, giving rise to the chloroplasts found in modern algae and plants.

The Role of Anaerobic Autotrophs

Anaerobic autotrophs were important pioneers in the early biosphere, but their metabolic processes did not produce oxygen. They relied on alternative sources of energy, such as hydrogen sulfide, and were eventually outcompeted or driven to marginal environments by the rise of oxygenic photosynthesis.

Evidence from the Rock Record

The transition from an anoxic to an oxygenated Earth is recorded in the geological record. Banded iron formations, which are layers of iron-rich sedimentary rock, provide evidence for the reaction of iron with oxygen in ancient oceans. The presence of these rocks coincides with the timing of the Great Oxygenation Event and the proliferation of cyanobacteria.

The Continuing Influence of Cyanobacteria

Today, cyanobacteria are found in almost every aquatic and terrestrial habitat. They are essential contributors to the global carbon and nitrogen cycles and are a major source of oxygen in the atmosphere. Cyanobacteria also form symbiotic relationships with fungi to create lichens and with plants to fix atmospheric nitrogen.

Key Takeaways

• Cyanobacteria were the first organisms to evolve oxygenic photosynthesis, releasing oxygen into the atmosphere.

• The rise of cyanobacteria led to the Great Oxygenation Event, which transformed Earth’s atmosphere and enabled the evolution of complex life.

• Anaerobic autotrophs, protists, and algae did not produce oxygen or evolved after the rise of atmospheric oxygen.

• The correct answer to the question is:

  (2) Cyanobacteria

Summary Table

Conclusion

The first oxygen-evolving life forms on Earth were cyanobacteria. These remarkable microbes transformed the planet’s atmosphere through oxygenic photosynthesis, setting the stage for the evolution of complex life. While other groups, such as anaerobic autotrophs, protists, and algae, played important roles in Earth’s history, it was cyanobacteria that initiated the rise of oxygen and reshaped the course of life on our planet.

In summary, the correct answer is:

(2) Cyanobacteria