 In this video we will trace the history of the Earth's atmosphere from its early beginnings with no oxygen to the present day atmosphere polluted by mankind. By examining rocks laid down over Earth's history we can piece together a story of how our atmosphere has changed. Four and a half billion years ago when the Earth had cooled enough for the surface rocks to solidify, we believe the atmosphere was made mostly of nitrogen with some carbon dioxide, water vapor and hydrogen compounds such as methane and ammonia. Within 800 million years life evolved in the oceans and by 3.2 billion years ago photosynthesis had evolved with the first oxygen producing cyanobacteria similar to today's blue-green algae. At first the oxygen given off simply reacted with the rocks but gradually it built up in the oceans and started to bubble out into the atmosphere. To start with it was a toxic gas harmful to life but soon life evolved to make use of the oxygen through aerobic respiration. By two billion years ago about halfway through the Earth's history there was oxygen in the atmosphere. When scientists look at stars for signs of life on planets that orbit them the presence of oxygen in the atmosphere would be a sure sign that life had taken hold. With the oxygen in place ozone could form in the upper atmosphere so preventing dangerous ultraviolet light from the Sun reaching the land. Before then life could only exist under the water where the UV could not reach. However life did not invade the land immediately the ozone was in place. It was a big evolutionary step for plants which needed water to give them support. About 400 million years ago, 100 million years after the so-called Cambrian explosion, life finally learned to live on land. The atmosphere remained much the same until the last century mostly nitrogen with 21% oxygen and just a tiny amount of carbon dioxide but enough to give plants their carbon during photosynthesis. However a hundred years ago or so another living organism started to change the atmosphere. This organism found seams of coal and pockets of oil and natural gas underground and began extracting them and reacting them with oxygen in the atmosphere increasing the carbon dioxide concentration. Mankind for that was the name of the organism also began to throw away other unwanted material into the atmosphere and you can learn all about these in our related videos. For carbon dioxide see global warming how it happens and global warming the effects on the environment. For the acidic oxides we throw away up our chimneys and in our vehicle exhausts see acid rain and reducing acid rain or its effects. For the CFCs which propelled our aerosols but destroyed the ozone layer see the hole in the ozone layer and for the soot and other particles we throw away into the atmosphere see the effect on health of air pollutants and reducing particulates soot. To summarize it's the presence of life on this planet that has caused the atmosphere to change. First the evolution of oxygen two billion years ago by cyanobacteria. Then very recently mankind is changing the atmosphere as we learned to combine fossil fuels with oxygen to provide energy for our evolving lifestyle.