Question

Absence of oxygen in earth, made the atmosphere reducing. But it later became oxidizing in nature due to the emergence of ________.

• (A) Angiosperms
• (B) Gymnosperms
• (C) Cyanobacteria
• (D) Eukaryotic algae

Answer 1

The Correct Option is C

Explanation:
Cyanobacteria also known as Cyanophyta, is a phylum of bacteria and they obtain their energy through photosynthesis. By producing gaseous oxygen as a byproduct of photosynthesis, Cyanobacteria are thought to have converted the early reducing atmosphere into an oxidising one, which dramatically changed the composition of life forms on Earth, by stimulating biodiversity.Most scientists believe that for half of Earth's $4.6$ billion year history, the atmosphere contained no oxygen, perhaps as long ago as $3.5$ billion years.Free oxygen is toxic to anaerobic organisms and rising concentrations may have wiped out most of the Earth's anaerobic inhabitants at that time.Hence, the correct option is (C).