Evolution of the Atmosphere

Stages of Evolution of the Atmosphere

The evolution of the atmosphere that we inhabit today can be split into three main stages: the early atmosphere, ocean formation, and the biological era.

The Early Atmosphere

When the Earth first formed, it had no atmosphere. The planet was a sweltering ball of molten rock under constant strike by asteroids and rocks. When the bombardment began to slow down, a thin atmosphere was created from hydrogen and helium, the most abundant elements in the universe.

Volcanoes formed the first true atmosphere. When they erupted, they spewed carbon dioxide, nitrogen, hydrogen and water vapour. Gradually, the atmosphere filled up with these gases.

Fig. 1: In Earth's early days, there were volcanoes everywhere, unsplash.com.

Cooling and Ocean Formation

After a few million years of volcanoes and sweltering heat, the planet gradually began to cool down. The water vapour condensed into a liquid, and rain fell for centuries, filling up the rock basins to form oceans.

Carbon dioxide dissolved into the water, creating carbonate compounds. These compounds were then precipitated to form sedimentary rocks and fossil fuels.

The Biological Era

The oldest pieces of evidence for life on Earth are stromatolites, which appeared 300 million years after the formation of the oceans.

Photosynthesis caused a significant shift in the Earth's atmosphere.

Oxygen (O₂) is a byproduct of photosynthesis. Initially, rocks took up most of the oxygen produced, so it didn't build up in the atmosphere. It wasn't until algae evolved that atmospheric oxygen levels began to increase almost a billion years later. As photosynthesis increased, carbon dioxide (CO₂) was used up and slowly removed from the atmosphere.

Simple aquatic animals began to evolve 800 million years ago. As plants diversified, oxygen levels increased, enabling more complex animals to develop.

The Modern Day Atmosphere

The water vapour cooled down to form oceans. The carbon dioxide got used up and replaced by oxygen. But what about the nitrogen from volcanic eruptions?

Nitrogen is an inert gas, so it didn't react with anything. Its unreactivity allowed it to slowly build up until it became the dominant gas in today's atmosphere.

Evolution of the Atmosphere: Timeline

From the formation of the Earth to the invention of the sewing machine, Earth's atmosphere has changed and affected life on the planet.

• 4.6 billion years ago: Earth was formed. The temperature was too hot to sustain life, and volcanic eruptions were constant.
• 3.8 billion years ago: Earth's temperatures cooled to below 100ºC. Water vapour in the atmosphere condensed, creating oceans. Some carbon dioxide was locked into rocks.
• 3.5 billion years ago: Life first evolved in the oceans, forming stromatolites.

• 2.7 billion years ago: Algae evolved. Atmospheric carbon dioxide decreased, replaced by oxygen.
• 1.8 billion years ago: The first eukaryotes evolved. These organisms are more complex than bacteria.
• 800 million years ago: The first simple animals evolved.
• 540 million years ago: Increased oxygen levels enabled rapid animal evolution. This period is known as the Cambrian Explosion.
• 200,000 years ago: Homo sapiens (modern humans) evolved on the continent of Africa.
• 200 years ago: Industrialisation began affecting the Earth's atmospheric composition and climate.

The Gas Percentages of Earth's Early Atmosphere

It's impossible to know the atmospheric composition billions of years ago, but experts have made educated guesses. Constant volcanic eruptions suggested that Earth's atmosphere was mainly carbon dioxide, with some water vapour and small amounts of nitrogen. Methane and ammonia were present in trace amounts.

Evolution of the Atmosphere: Facts

Find below a list of facts about the evolution of the atmosphere!

• 3 billion years ago, the sun was only 70% as bright as today. But greenhouse gases in the atmosphere trapped enough heat to keep the planet warm.

• When oxygen was first produced, it reacted with iron in rocks and soil. Red iron oxide was formed, creating huge expanses of redbeds.

• Whilst the increase in oxygen benefitted some organisms, others struggled. Oxygen gas was poisonous for many organisms, so they moved into extreme anaerobic habitats or went extinct.

• Despite rising temperatures, we currently live in an ice age! This ice age began approximately 3 million years ago and peaked around 20,000 years ago. We are experiencing an interglacial period of warmer temperatures within the ice age.

Fig. 2: Several thousand years ago, most of the UK was covered by ice, unsplash.com.

• The Earth's atmosphere is divided into five layers - the troposphere, the stratosphere, the mesosphere, the thermosphere and the exosphere. Temperatures in the different layers can vary drastically.

  The mesosphere can fall to -90ºC whilst the thermosphere can reach 2000ºC.

To recap, Earth's early atmosphere was dominated by carbon dioxide and water vapour from volcanoes. As temperatures cooled, the water vapour condensed. The newly formed oceans were the birthplace of life. Photosynthesis replaced carbon dioxide with oxygen, enabling a wide variety of plants and animals to evolve.