The air we breathe is something we often take for granted. What came about as a perfect set up for life, came in some very significant steps. Here’s a simpler attempt to explain how we got to where we are.
The Earth formed about 4.6 billion years ago. Back then the solar system was it started with a lot of debris floating around the sun. Impacts of these objects came to started to form larger objects that were circling the sun. As they did so, they would hit more and more objects and like a rolling snowball, what became our planet was one of those larger objects. When new impacts occurred, kinetic energy (energy of movement) turned into thermal energy (heat). At one point, another very large object, maybe even as large as the size of Mars, collided with the growing Earth and helped to form what is now the moon.
Each asteroid that struck us brought hydrogen, nitrogen, carbon dioxide and carbon monoxide. Most of the lighter gasses were able to escape as our gravity was not enough to hold them. The collisions and the resultant heat created this big ball of molten rock. As the Earth absorbed most of the asteroids in it’s path around the sun and the number of collisions slowed. At this point the Earth started to cool from the outside to the inside. There is still a core of heat in the center of our planet.
Some of the internal magma would spew out in the form of volcanoes. Each of these eruptions would contain carbon dioxide and water vapor. As time passed, the Earth cooled enough for the water vapor to condense and the oceans were formed. When the oceans first formed, carbon dioxide (CO2) concentrations were a few hundred times what we have today. This greater a greater greenhouse effect and prevented the oceans from freezing. In fact, the Earth was so warm, there weren’t any polar ice camps until 2.5 billion years ago.
The warm water became a nice area for microscopic organisms called cyanobacteria. These organisms supported themselves with photosynthesis. This process uses sunlight and carbon dioxide and outgasses oxygen. Through time there was enough oxygen to support land life and soon our planet became a host to plenty of life.
Today’s concentration of oxygen in the atmosphere is lower than most people realize, sitting at about 21% which plenty for us.