Question

why do scientists believe that life first emerged from the ocean and onto land well after the ancestors of modern day cyanobacteria populated the earth

Answer 1

These bacteria used visible light energy to create chemical energy, expelling oxygen that was converted to ozone. The ozone blocked the Sun's harmful UV radiation from reaching the ground

Answer 2

Answer:

Since the ancestors of modern day cyanobacteria added oxygen to the primitive reducing atmosphere of earth.

Explanation:

The primitive atmosphere of the earth did not have oxygen. So, life was not possible on land. The ancestors of modern-day cyanobacteria had the ability to carry out oxygenic photosynthesis which in turn releases oxygen as a by-product. Hence, these ancestors of modern-day cyanobacteria populated the land and added oxygen to the atmosphere of the earth.  

Once enough oxygen was added to support the other life forms, aquatic life forms gradually started occupying the land habitats.