Evolution — BIOLOGY STD 12 Science — Question

2. Cyanobacteria

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.

Cyanobacteria were therefore responsible for one of the most significant extinction events in Earth's history. Eventually, aerobic organisms began to evolve, consuming oxygen and bringing about an equilibrium in its availability.