Originally, life only existed in the oceans where the water was deep enough to shield organisms from UV radiation, but shallow enough for photosynthesis to occurl-Around 600 million years ago there was a massive diversification of life:
the Cambrian Explosion. During this time life not only diversified but moved to land. This was made possible by an
important addition to Earth's atmosphere. What was that addition?
A)
ozone layer
B)
water vapor and clouds
C)
particulate matter from volcanic eruptions
D)
increased amounts of oxygen and carbon dioxide

The formation of the ozone layer was the addition that helped life to diversify and get out on land.

Explanation:

Earth's atmosphere was not always the same. Initially, it was an atmosphere that had very little oxygen, which prevented the development of organisms that are more complex. This started to change when the cyanobacteria started to use the process of photosynthesis in order to create their own energy, as this led to separating the oxygen from the carbon during the process and the oxygen was let out in the water and atmosphere.

With the levels of oxygen increasing, a sublayer of ozone started to form in the stratosphere. This was a key moment that enebled organisms to start coming out on land because the ozone has the property to not allow the majority of the UV radiation to pass through, thus protecting the organisms from its harmful effect.