YouTube - 3of5 - Animal Armageddon - The Fall of the Age of Fish - Hell on Earth

A mass extinction which causes 70% of life on earth perish. This pushed the early fish to move on land. Which develop a new kind of fin. The first ice age came which causes the sea level to lower down.

As an example this is how life on earth face each adversity to form a newer stronger species to appear. This a great example video of how evolution works.