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Millions and billions of years ago…

4.56 Ga (billion years) – The Sun, the Earth and the rest of the solar system are born. The Earth already has an envelope made up of light gases such as dihydrogen and helium.
4.3 Ga – A primitive atmosphere is formed around the Earth. It is believed to be the result of intense volcanic activity. It consists mainly of water vapour, carbon dioxide and dinitrogen. The water vapour is eventually condensed and creates oceans.
3.5 Ga – The cyanobacteria, also called “blue-green algae” appear. They are the first living organisms to photosynthesize under water. Over the next billion and a half years they will contribute to enriching the atmosphere in oxygen and reducing its carbon dioxide concentration.
1.5 Ga – Oxygen concentrations continue to increase and the ozone appears. Life begins to take form on the ocean surface.
1.0 Ga – The oxygen concentration of our atmosphere takes on its current characteristics.
700 to 600 Ma (million years) – Appearance of the Coelenterata (to which medusae belong) and of the first multicellular organisms.
500 Ma – The ozone layer is formed and enables the development of life on Earth, which is finally protected by the Sun’s ultraviolet rays.
400 Ma – Appearance of the first trees and herbaceousplants.
150 Ma – Appearance of the first flowers, which are a quickly renewable source of food.
65 Ma – Beginning of the mammal era.
3 Ma – Appearance of the first Hominoidea walking on two feet.