After the big bang the earth was a very hot ball of fire and volcanoes erupted frequently releasing carbon dioxide and water vapor into the air
After the earth's surface cooled down, the water vapor condensed and rain fell creating huge buckets of water that we now call oceans
The earth's atmosphere was made up of mostly carbon dioxide. Oxygen was not yet created, all animals were aquatic and respired anaerobically
Algae, an aquatic plant, began to photosynthesize; it took in carbon dioxide and released oxygen
Due to the large amounts of carbon dioxide, algae could photosynthesize often and therefore increase in amount. The increase in algae caused a huge decrease in carbon dioxide concentrations and increasing oxygen. Animals evolved and began to live depending on oxygen and respire aerobically
After the atmosphere mainly consisted of Oxygen, people began polluting the air and releasing a large amount of carbon dioxide leading the amounts of carbon dioxide to increase in the atmosphere but still at a very small percentage