The atmosphere on Earth is extremely complex, and there are millions of reactions taking place all the time, so there is quite a lot to learn in this section! But thankfully it’s not too complex.

The early atmosphere was mainly formed of carbon dioxide, with small amounts of water vapour, methane and ammonia (and possibly nitrogen), and little to no oxygen. This is similar to the atmospheres of Mars and Venus.

It is thought that volcanoes released carbon dioxide, water vapour and nitrogen gas into the early atmosphere. The water vapour then condenses to form the ocean. The early atmosphere eventually stabilised and was made up mainly of carbon dioxide.

As icy comets hit the Earth from space, they may also have added to our water supplies. Many thousands of tonnes of water fall onto Earth from space every year, even today.

Around 3.4 billion years ago, organisms similar to bacteria evolved which could break down chemicals around them to provide energy. Later on, bacteria, algae and plants evolved which could harness the energy of the sun by carrying out a chemical reaction called photosynthesis.

The earliest living organism evolved to survive in conditions of little or no oxygen, so the oxygen produced by photosynthesis was toxic to them.

Approximately 200 million years ago, the atmosphere stabilised and has remained much the same ever since. As well as oxygen, the atmosphere contains approximately 78% nitrogen and small amounts of other gases such as argon (0.9%) and carbon dioxide (0.04%). The early atmosphere had little to no oxygen.

The evolution of plants, algae and bacteria that made their own energy using photosynthesis caused oxygen to be released; this oxygen caused the atmosphere to become richer and richer in oxygen.

Lots of amino acids, also known as the building blocks of life, come together to make proteins, which are found in all living things. Amino acids contain the elements hydrogen, carbon, nitrogen and oxygen.

Miller and Urey carried out an experiment to make amino acids out of chemicals they predicted to have been around at the time when life was formed on Earth. They mixed water, ammonia, methane and hydrogen together in the presence of electric energy to mimic conditions in our early atmosphere. A brown mixture was formed which contained many amino acids.

The early atmosphere contained a lot of carbon dioxide, but carbon dioxide makes up only 0.04% of our modern atmosphere. Most carbon dioxide was stored in living organisms or materials made up of living organisms. Carbon dioxide was taken up during photosynthesis.

Plants were then eaten by animals, so the carbon was transferred to animal bone and tissue. As time passed, the dead bodies of these organisms built up at the bottom of the ocean, where they formed calcium carbonate (limestone) rock. Dead organisms which were crushed and heated by the Earth’s crust turned into fossil fuels, all of which caused a reduction in carbon dioxide levels.

However, there is now evidence that the sea cannot cope with the additional carbon dioxide we are producing from emissions. Coral reefs are dying due to acidic conditions.

As we burn fossil fuels, stored carbon dioxide is released into the atmosphere. The carbon dioxide is absorbed by the ocean. Reactions within the sea make soluble hydrogencarbonates and insoluble carbonates such as calcium carbonate.

Excess dissolved carbon dioxide decreases the pH of the ocean so that it is more acidic – eventually this reaches a level which can kill important marine organisms.