Formation of Earth’s Atmosphere - The early Earth was very different from our Earth today. The early Earth experienced frequent impacts from asteroids and meteorites and had much more frequent volcanic eruptions. There was no life on Earth for the first billion years because the atmosphere was not suitable for life. Earth’s first atmosphere had lots of water vapor but had almost no oxygen. Later, frequent volcanic eruptions put several different gases into the air. These gases created a new type of atmosphere for Earth. The volcanic eruptions spewed gases such as nitrogen, carbon dioxide, hydrogen, and water vapor into the atmosphere but no free oxygen. Without oxygen, there was still very little that could live on Earth.
Slowly, two processes changed Earth’s atmosphere to one that is more oxygen-rich like the one we have today. First, radiation from the Sun caused water vapor molecules to split apart. Remember that a molecule of water is made of the elements hydrogen and oxygen, or H2O. Radiation from the Sun split some of the water molecules into hydrogen and oxygen. The hydrogen escaped back to outer space. The oxygen accumulated in the atmosphere. The second process that changed Earth’s early atmosphere was photosynthesis.
About 2.4 billion years ago, a type of organism called cyanobacteria evolved on the early Earth and began carrying out photosynthesis. Photosynthesis uses carbon dioxide and energy from the Sun to produce sugar and oxygen. The cyanobacteria were very simple organisms but performed an important role in changing Earth’s early atmosphere. They carried out photosynthesis to produce the materials they needed to grow. They gave off oxygen to the atmosphere as they did this.
Oxygen in the atmosphere is important for life for two main reasons. First, oxygen makes up the ozone layer. The ozone layer is in the upper part of the atmosphere, and is made of O3 molecules a particular type of oxygen molecule. It blocks harmful radiation from the sun and keeps it from reaching Earth’s surface. Without an ozone layer, intense radiation from the sun reached the early Earth’s surface, making life almost impossible. Secondly, oxygen in the atmosphere is necessary for animals, including humans, to breathe. No animals would have been able to breathe in Earth’s early atmosphere.
However, there were probably several types of bacteria that lived on Earth during this early time. They would have been anaerobic, meaning that they did not need oxygen to live. Very simple cells lived on Earth for the first few billion years of Earth’s history. Some of the oldest fossils of more complex organisms are from about 2 billion years ago. They are found in Australia.
Besides changes in life and the atmosphere, other changes have also happened since the Earth was first formed. Early volcanic eruptions on Earth released large amounts of water vapor into the atmosphere. The water vapor slowly condensed and returned to Earth’s surface in rainfall. This formed the oceans. Water began to cycle on Earth, and events like rainfall and storms next began to change the Earth’s surface through weathering and erosion. The Earth’s Fresh Water chapter gives more detail on how water cycles on Earth.
The continents were in very different locations than they are now. Scientists do not know how Earth’s land looked exactly after the planet’s first formation. They do know that North America and Greenland formed one giant landmass called Laurentia about 1.8 billion years ago. By about 1 billion years ago, Antarctica may have been close to the equator, even though it now sits at Earth’s South Pole. Today, Earth’s continents continue to slowly shift around the globe.
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