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.

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.