Scientists think that the atmosphere we breathe today is nothing like the atmosphere the Earth started with. In fact scientists think that 99% of today's atmosphere is new compared to the original air, which might have been what formed with the Earth out of the cloud shown on page 1 of this tour. Thus scientists call the atmosphere of the Archean a secondary atmosphere, which evolved into the atmosphere we have today.
Scientists think that the secondary atmosphere came from the many
volcanic eruptions which took place in Earth's early history, and which
continue today. All volcanoes are different but in general they release
gases such as H2O, CO2, SO2,
H2S, HCl, N2, NO2. These molecules are
not the Nitrogen and Oxygen which are the majority of gases in the air
today, however. In fact, oxygen, which is what humans and other animals
require to survive , was not a part of the Earth's earliest atmosphere!
This means that Earth's earliest environment was
anerobic, that is without oxygen. Oxygen came into the
This is page 6 of 10