Evolution of the Atmosphere

The first atmosphere on Earth was mostly carbon dioxide and water vapour. It was made of up gases given off from volcanoes and comets colliding with Earth. The carbon dioxide had a warming effect and provided “food” for primitive bacteria and algae the first “life”. They consumed carbon dioxide and released oxygen as waste. Colonies of algae formed stromatolites which produced more oxygen.

 By 1800 million years ago, when the first animals appeared, it was 60 percent of its present level. Solar radiation then turned some into ozone in the stratosphere. Insignificant invertebrates, the protochordates, have a body plan very similar to that of larval fish and amphibians, which were probably their successors in the Cambrian.  By the Devonian period the age of the fist was at its height. The Jawless fish such as Pieraspis which had bony plates and shovel like mouths. Many fish were able to adopt to fresh water. The Devonian also saw first sharks. Since their skeletons are made of cartilage, there are fewer fossils, but their sharp teeth often survive.

 During the Devonian, the first fishes came out of the water. Lobe finned fish developed auxiliary lungs and could haul them out of the water if supplies of oxygen ran low. From them evolved the lung fish and an even closer relative that still lives in the Indian Ocean – the coelacanth. By the Carboniferous, amphibious creatures called labyrinthodonts, with fully formed feet, had established themselves on land.