Hydrogen, Carbon, Nitrogen, and Oxygen

Oxygen, Carbon and Hydrogen formed from exploding Super Nova, join with Silicon, Calcium, Nitrogen, Iron and other heavy elements to form dust, then sand, then pebbles, then rocks, meteorites, and finally the planet

As the Rocks are pressed together under the force of gravity, they heat, eventually the earth is so large and the gravity so great that the rocks inside the earth melt

This melting allows atoms to interact with each other, Nitrogen atoms can bind together to form molecular Nitrogen. Carbon and Oxygen in the Rocks form Carbon Dioxide, whilst Hydrogen and Oxygen form Water

Convention currents form, keeping things mixing

The convection currents act against the coolest rock, that is at the surface, this cool rock is solid and is called the crust

The convection currents cause the crust to move, sometimes forming cracks where the melted rock can emerge on the surface.

As the melted rock emerges on the surface some of it is liquid, but some of it, with the reduced pressure that is on the surface, becomes a gas.

The gas leaves the earth, but is still attracted due to gravity, it slowly forms the atmosphere

This gas includes Carbon Dioxide, Oxygen, Nitrogen and Water

The main gas leaving a volcano is Water, the 2nd is Carbon Dioxide

A lot of the gaseous atoms remain in the earth, in fact the solid earth is 45% oxygen - as part of solid molecules such as silicon oxide

The Cooled Water condenses into clouds and falls back to the earth forming oceans

This water and the nitrogen is added too by the occasional ice comet crashing into the planet

The planet cools, oceans form, and carbon dioxide prevents the planet from getting too cold

The Nitrogen, Carbon dioxide and Oxygen are all water soluble, so they dissolve into and out of the oceans

The Early Atmosphere, before life (prebiotic) was extremely rich in Carbon Dioxide, and had very little oxygen

Volcanoes were the primary form of gas release, releasing all this Carbon Dioxide and water into the Atmosphere

Under the ocean geological thermal vents also poured Carbon dioxide into the ocean, along with other carbon structures such as methane (CH4)

Lots of other chemicals were released from these Geothermal vents - a rich enough soup that it is believed life first formed here.

After millions of years of being confined to geothermal vents, some microbes developed the ability to photosynthesize. These photosynthetic bacteria turned atmospheric Carbon dioxide, as it dissolved into the oceans, into oxygen, this dissolved into then out of the ocean. These Photosynthetic bacteria, and then photosynthetic algae, changed the atmosphere from mainly Nitrogen and Carbon Dioxide, to being what it is today; Nitrogen and mainly Oxygen.

Slowly life became more complex. Bacteria evolved into protists, then into plants, finally animals.