Evolution of Earth

Evolution of Earth’s Atmosphere:

The early atmosphere

Scientists believe that the Earth was formed about 4.5 billion years ago. Its early atmosphere was probably formed from the gases given out by volcanoes. It is believed that there was intense volcanic activity for the first billion years of the Earth’s existence.

The early atmosphere was probably mostly carbon dioxide, with little or no oxygen. There were smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans.

It is thought that the atmospheres of Mars and Venus today, which contain mostly carbon dioxide, are similar to the early atmosphere of the Earth.

Changes in the atmosphere

So how did the proportion of carbon dioxide in the atmosphere go down, and the proportion of oxygen go up?

The proportion of oxygen went up because of photosynthesis by plants.

The proportion of carbon dioxide went down because:

  • It was locked up in sedimentary rocks, such as limestone, and in fossil fuels.
  • It was absorbed by plants for photosynthesis.
  • It dissolved in the oceans.

The burning of fossil fuels is adding carbon dioxide to the atmosphere faster than it can be removed. This means that the level of carbon dioxide in the atmosphere is increasing. Check that you understand the effects of this by going to Combustion of fuels.

Name of Stage Duration of Stage ( Billions of years ago) Main constituents of the Atmosphere Dominant processes and features
Early Atmosphere 4.4 -4.0 H2O, Hydrogen, Cyanide, ammonia, methane, sulphur, iodine,  bromine, chlorine and agron Lighter gases like hydrogen and helium escaped to space. All water was held in the atmosphere as vapour because of high temperature
Secondary Atmosphere 4.0 -3.3 At 4.0 billion H2o, CO2, and Nitrogen were dominant. Cooling of the atmosphere caused precipitation and the development of the oceans. By 3.0 billion CO2, H2O, N2, were dominant, O2 began to accumulate. Continued release of gases from the lithosphere. Water vapour, clouds common in the lower atmosphere. 

Chemosynthetic bacteria appeared on the EArth some time between 3.9 and 3.5 billion years ago. Life began to modify the atmosphere.
Living Atmosphere 3.3 to present N2=78%, O2=21%, Argon=0.9%, CO2=0.036 % Development, evolution and growth of life increased the quantity of oxygen in the atmosphere from <1% to 21%>. 500 million years ago concentration of atmospheric oxygen levelled off. Human begins started modifying the concentrations of some gases in the atmosphere around the year 1700.
As the Earth continued to cool, the water vapour found in the atmosphere condensed to form the oceans and other freshwater bodies on the continents. Oxygen began accumulating in through photo-dissociation of O2 from, and by way of photosynthesis (life). The emergence of living organisms was extremely important in the creation of atmospheric oxygen and ozone. Without ozone, life could not exist on land because of harmful ultraviolet radiation.
Most of the build-up of oxygen in the atmosphere-occurred between 2.1 and 1.5 billion years ago as a direct result of photosynthesis from ocean based plants like algae. At about 450 billion years ago, there was enough oxygen in the atmosphere to allow for the development of a stratospheric ozone layer that was thick enough to keep terrestrial life protected from ultraviolet radiation. As a result, terrestrial life began its development and expansion at this time. The following table describes the timing of evolutionary development of some of the Earth’s dominant forms of life before and after 450 million years before present.
Approximate Origin of the Major Plant and Animal Group:
Organism Group Time of Origin Organism Group Time of Origin
Marine invertibrate 570 million years ago Fish 550 million years ago
Land Plants 438 million years ago Amphibians 408 million years ago
Reptiles 320 million years ago Mammals 208 million years ago
Flowering Plants 140 million years ago

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