Oxygen

Everything happens there are 4.5 billion years, early in the formation of our solar system. It was at that moment was born the primitive atmosphere of the planets. Venus and Earth are two terrestrial planets, almost the same size, formed at the same time with the same materials, yet its atmosphere has evolved differently.
On the surface of Venus atmospheric pressure is 90 times higher than on Earth, its surface temperature is very high, 460° C on average and clouds consist of sulfuric acid.
Protoplanets on training, material condenses into a hot core. The gases expelled by the molten rock is retained the primitive atmosphere by gravity. The Earth's atmosphere is protected by the magnetic shield the earth's core, the solar wind will never carry away the gases as has happened on Mars. The atmosphere of the red planet is now 100 times less dense than ours. The primitive atmosphere of Earth contains a large amount of CO2. This gas can retain heat still represents 95% of the atmosphere of Venus. Superheated water from our sister planet has evaporated.

Why the Earth has not undergone the same evolution?
If the Earth had retained its original atmosphere, it would now be the same atmosphere as Venus. Chance gave a different gravitational orientation to Earth. A large protoplanet came tearing much of this primitive atmosphere, in a collision giant, who gave birth to the moon. This collision will be good for us. Rid of toxic mixture of origin, the atmosphere will evolve into its current composition. After the collision the atmosphere that is no longer composed of molten rock sprayed and after millions of years it will be composed of nitrogen, CO2 and water vapor, elements that come from the sky and the inside the Earth. Nitrogen and carbon dioxide are discharged from the molten core during volcanic eruptions. As for water, it is not from the fumes emitted during the intense volcanic activity from the beginning of the creation of our planet.
The terrestrial planets are formed by agglomeration of asteroids, a few million years. This constant bombardment from the beginning will be distributed in all directions, asteroids covered with ice and gradually increase the amount of water planets.

The primitive atmosphere of the Earth does not yet contain oxygen.
An event will trigger the production of oxygen there are ≈2.5 billion years.
These are the cyanobacteria that have invaded Earth and have rejected enough oxygen to make it a permanent part of the atmosphere.
It's that time of the great oxidation or the catastrophe of oxygen. Cyanobacteria that have learned to oxygenic photosynthesis, absorb CO2 and use solar energy to crack water molecules, releasing oxygen. The strong oxidizing power of oxygen transforms the iron into iron oxide which is deposited on the seabed. Earth rust, iron oxide is deposited in layers. Once saturated minerals, oxygen begins to accumulate in the atmosphere. Under the pressure of oxygen will change life as we know it today.
Microbial growth is exploding in the shallow waters of the oceans. Cyanobacteria derive their energy from the sun through their photosynthetic activity. They secrete a kind of gelatin to protect themselves from ultraviolet rays. They evolve in different bacterial colonies and spread over much of the planet. Drawing their energy from carbon dioxide, they release more oxygen. Cyanobacteria are exposed to light and release a toxic gas for the time, oxygen, which will boost life on the planet. The Earth is today an oasis of life, a singularity that is the cocoon of gas that surrounds it. This layer of air insulates us from space, feeds us and protects us.

The Earth's atmosphere (from Greek ἀτμός, steam, air and σφαῖρα, sphere), this is miraculous shield coverage of gases surrounding the Earth.
Dry air consists of 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide and traces of other gases. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface, trapping heat from the greenhouse effect and reducing the temperature differences between day and night. There is no clear border between the atmosphere and space, the outer boundary of the atmosphere is defined as the distance where the supposed atmospheric gas molecules continually undergo gravity and the interactions of its magnetic field.
The layer of air varies greatly depending on the latitude and the Earth's magnetic field continually distorted by the solar wind. The air becomes less dense and gradually fades into space. But the altitude of 120 km marks the boundary where atmospheric effects become noticeable during re-entry of an object.
The Kármán line lies 100 km, is considered the boundary between the atmosphere and space, but the last molecules of atmospheric gas that constantly undergo gravity are certainly much further.

NB: Amber flowing trees, stuck in passing a number of small animals like insects, spiders, aquatic organisms and even marine plankton. The resin fossilized, then keeps them for millions of years.