One change that occurred was the increase in atmospheric oxygen. Oxygen is necessary for respiration and its concentration increased with the evolution or appearance of single celled photosynthetic organisms such as stromatolites. One other change is an increase in the concentration of ozone. This is a from of oxygen that exists as three atoms bonded together in one molecule. Ozone protects life from harmful ultra violet radiation. Once ozone concentration was high enough, life was able to develop and flourish on land. Previously life on earth was confined only to the oceans.
