What was the environment from which aquatic vertebrates colonized the terrestrial environment?

Life began inside the water, as it is now scientifically being proved and then after the evolution happened and the land mass appeared there were some abundant form of key energy resources for the different beings to survive and they left water for there terrestrial lives, as different form of animals went on develop the features like having lungs instead of gills, the fore and hind limbs or some of them crawl over the land in order to move from on point to another.  

The first challenge was how to create a protective layer for the body that can protect it from any ray coming from the Sun directly.

The second challenge was that how to breath while being outside the water.

The most important thing was to have a proper body structure to support the organism to live inside the terrestrial environment.

Explanation:

Here, are some of the features which the different animals adapted in order to live on land.

The animals evolved themselves to have a set of lungs instead of having the gills, due to having more atomic form of oxygen inside the free air.

Along, with which the developed the protective covering for them selves to fight and defend them selves from other beings.

Then now had the hind and forelimbs to support there body move from one point on land to another, while having the spinal cord supporting there body.

They developed a new way to haunt by having the exposed bones inside there oral cavity and had the most of its prey by biting them to death.

Along, with which the fertilization process was further improved to provide more options for the beings to cross with each other and have a healthy offspring.

As they had developed there immune system and other senses to fight any pathogen or any type of foreign body that can be hazardous for the whole system.

They now had the  proper way to keep there offspring inside there body while covering it in a more protective layers having all the required amount of nutrients and other materials to develop themselves.

Explanation:

carbon atoms, hydrogen, oxygen, and nitrogen.