SSC CPO 2025 PYQ — Question 85

Answer: Prokaryotes → Oxygen Accumulation → First Eukaryotes → Multicellularity.

Explanation:

• The evolution of life on Earth has occurred in a specific sequence. First, prokaryotes emerged; these were simple cells lacking a nucleus (approximately 3.5 – 3.8 billion years ago). Subsequently, organisms such as cyanobacteria began performing photosynthesis, leading to the accumulation of oxygen in the atmosphere. This event is known as the 'Great Oxidation Event.'
• The oxygen-rich atmosphere paved the way for the development of more complex cells, bringing the first eukaryotes into existence (approximately 1.8 – 2 billion years ago); these cells possessed a distinct nucleus and organelles. Finally, the aggregation of these single-celled eukaryotes—and their specialization to perform specific functions—led to the evolution of multicellularity (approximately 600 million to 1 billion years ago). This sequence illustrates the progression of life from simple to complex forms.

Additional Points:

• • Prokaryotes: Their DNA is free-floating within the cytoplasm (e.g., bacteria).
• • Eukaryotes: Their DNA is enclosed within a nucleus, and they possess organelles such as mitochondria.
• • ​​Key Timeframes: The Earth is approximately 4.5 billion years old, and the oldest evidence of life is found in the form of 'stromatolites.'

Potential Exam Questions and Answers:

Question: Which organism was primarily responsible for the abundance of oxygen in Earth's atmosphere?

Answer: Cyanobacteria (or blue-green algae).

Question: Approximately how long ago did the 'Great Oxidation Event' occur?

Answer: Approximately 2.4 billion years ago.

Question: The presence of a nucleus within a cell is a characteristic feature of which type of cell?

Answer: Eukaryotic cell.