Question

For life to flourish on Earth, several changes occurred that affected the atmosphere, such as _____.

1. evolution of cyanobacteria that produce O2.
2. volcanic outgassing to create a thicker atmosphere.
3. formation of an ozone layer to block harmful radiation.
4. all of the above.
5. only a and c.

Answer 1

The correct answer is A. ie.e, evolution of cyanobateria which possessed chlorophyll, for photosynthesis and could produce oxygens, which could be utilised by other organsims. This was followed by evolution of plants, which further increased the amount of oxygen, and afterwards animals evolved. 

Answer 2

Answer: 4. All of the above.

Explanation:

Previously there was no oxygen in the atmosphere of Earth. with the evolution of cynobacteria, oxygen was produced by photosynthesis. Volcanic out-gassing released huge amount of gases (water vapor, carbon-dioxide and other hydrocarbons) which created a thick atmosphere.

Ozone layer formed which blocked the harmful radiation of Sun and helped flourishing of life on Earth. It was formed when sunlight strike oxygen molecule which split and single oxygen atom joined another oxygen molecule forming O₃ (ozone).

Hence, correct answer is For life to flourish on Earth, several changes occurred that affected the atmosphere, such as 4. All of the above.

[1. evolution of cyanobacteria that produce O₂.

2. volcanic out-gassing to create a thicker atmosphere.

3. formation of an ozone layer to block harmful radiation.]