Question

On the earth surface, the reducing atmosphere changed into an oxidizing atmosphere with the release of oxygen took place at this moment due to the
(a)Evolution of oxygenic photoautotrophs
(b)Evolution of anoxygenic photoautotrophs
(c)Evolution of chemo photoautotrophs
(d)Evolution of anaerobic heterotrophs

Answer 1

Hint: On the Earth’s surface, the atmosphere present in the beginning was reducing in nature as there was no oxygen present to oxidize things. The atmosphere was primarily containing methane, hydrogen, ammonia, and water vapor.

Complete Answer:
On the earth’s surface, the reducing atmosphere changed into an oxidizing atmosphere with the release of oxygen taking place at this moment due to the evolution of oxygenic photoautotrophs. Blue-green algae or cyanobacteria (evolved around 2.7 billion years ago) is the name of such photoautotrophs that are responsible for changing Earth’s atmosphere to oxidizing. They did this because oxygen was released by them as a by-product during the process of photosynthesis. They are known as photoautotrophs because they derive their energy from the light rays of the sun.

Additional information:
Let us know about the other organisms mentioned in the options.
Anoxygenic photoautotrophs: Anoxygenic photoautotrophs performed photosynthesis to fulfill their energy demands but they did not release oxygen as a byproduct. This is because a hydrogen donor is such an organism as hydrogen sulfide. Thus sulfur is obtained as a byproduct in such cases.
Chemo photoautotrophs: Chemoautotrophs are organisms that are autotrophic in nature which means they make their own food but they do not derive energy from sunlight. Instead, the source of their energy is chemical reactions hence the name chemoautotrophs.
Anaerobic Heterotrophs: Anaerobic heterotrophs do not make their own food and do not utilize oxygen to carry our metabolism in their body.

So, the correct option is ‘Evolution of oxygenic photoautotrophs’.

Note:
-S.L.Miller was the first scientist to create a reducing atmosphere in a laboratory similar to the one that existed on Earth billions of years ago.
-UV rays from the sun broke the water molecules present in the atmosphere which also led to the release of oxygen.
-It is believed that Earth was formed 4 billion years ago.